Question on Power Supply Diodes

[Electro Fan]

Attached are a schematic and a photo of a PCB is used in a simple power supply kit.

The kit included five diodes - all five look pretty similar.  Three of the diodes measure 0.638 V and two measure 0.762 V.  The PCB labels refer to three diodes as 4148, and to two diodes as 5V1.  I'm pretty sure the 4148 diodes are the three that measure 0.638 V.  What I can't quite figure is why the other two diodes, the "5V1" diodes, measure 0.762 V.  Maybe this is normal and it's a non-issue or maybe the kit included the wrong diodes?  What are the normal distinctions between a 4148 and a 5V1 diode?

Thx

[bdunham7]

You said 'kit'--are the diodes loose and not installed?  There are more than 5 diodes in that power supply...

5V1 is a 5.1 volt zener diode.  It looks like maybe D7 and D8 are zeners in that schematic, although it isn't very clear.  A 1N4148 is a very common small rectifier diode.

[MK14]

Attached are a schematic and a photo of a PCB is used in a simple power supply kit.

The kit included five diodes - all five look pretty similar.  Three of the diodes measure 0.638 V and two measure 0.762 V.  The PCB labels refer to three diodes as 4148, and to two diodes as 5V1.  I'm pretty sure the 4148 diodes are the three that measure 0.638 V.  What I can't quite figure is why the other two diodes, the "5V1" diodes, measure 0.762 V.  Maybe this is normal and it's a non-issue or maybe the kit included the wrong diodes?  What are the normal distinctions between a 4148 and a 5V1 diode?

Thx

My understanding, is that, in order to make zener diodes (things also change, depending on what voltage the zener is designed for, e.g. <= 6.3V (approx), but I'll leave that for another discussion), rather than normal diodes, such as the 1N4148, they have to change the doping of the semiconductors. That (and maybe other process changes), results in the higher Vf (forward voltage drop), you are measuring.

tl;dr
Yes, it sounds normal, to find slightly higher voltages, in those circumstances.
So, 0.638 V for the 1N4148's, and 0.762 V for the zeners, sounds about right to me.

[george.b]

A Zener diode won't normally be used in forward bias anyway, so...

[Electro Fan]

You said 'kit'--are the diodes loose and not installed?  There are more than 5 diodes in that power supply...

5V1 is a 5.1 volt zener diode.  It looks like maybe D7 and D8 are zeners in that schematic, although it isn't very clear.  A 1N4148 is a very common small rectifier diode.

Yes, there are more than 5 diodes in the kit - the others were clearly marked so it was just these five that seemed somewhat confusing.  All the components started out loose in a bag.

Here is a link to the kit:
https://www.amazon.com/HiLetgo-2mA-3A-Adjustable-Regulated-Protection/dp/B01NBE97WH/ref=cm_cr_arp_d_product_top?ie=UTF8

Instructions:
https://img.banggood.com/file/products/201505080459530-30Vinstall.pdf

[Electro Fan]

Attached are a schematic and a photo of a PCB is used in a simple power supply kit.

The kit included five diodes - all five look pretty similar.  Three of the diodes measure 0.638 V and two measure 0.762 V.  The PCB labels refer to three diodes as 4148, and to two diodes as 5V1.  I'm pretty sure the 4148 diodes are the three that measure 0.638 V.  What I can't quite figure is why the other two diodes, the "5V1" diodes, measure 0.762 V.  Maybe this is normal and it's a non-issue or maybe the kit included the wrong diodes?  What are the normal distinctions between a 4148 and a 5V1 diode?

Thx

My understanding, is that, in order to make zener diodes (things also change, depending on what voltage the zener is designed for, e.g. <= 6.3V (approx), but I'll leave that for another discussion), rather than normal diodes, such as the 1N4148, they have to change the doping of the semiconductors. That (and maybe other process changes), results in the higher Vf (forward voltage drop), you are measuring.

tl;dr
Yes, it sounds normal, to find slightly higher voltages, in those circumstances.
So, 0.638 V for the 1N4148's, and 0.762 V for the zeners, sounds about right to me.

Thanks, that sounds encouraging.  Still curious though why the 3 would be referred to as 4148's and the other 2 as 5V1's and how the markings would help distinguish which is which... maybe the difference in measurements is the only way to differentiate?

[Electro Fan]

A Zener diode won't normally be used in forward bias anyway, so...

So.... don't leave me in suspense, plz
What's the conclusion?  Thx

[bdunham7]

Yes, there are more than 5 diodes in the kit - the others were clearly marked so it was just these five that seemed somewhat confusing.  All the components started out loose in a bag.

OK, the ones you measured as having a higher V_f should be your zeners, but you also should be able to see markings on them with a magnifier.  You might want to test their reverse voltage just to make sure--just use a 9 volt battery and a 4.7 to 10K resistor for a current source if you don't have a meter that can put out 5+ volts on a diode check.

Not a bad kit for NINE BUCKS! 

[Doctorandus_P]

The Zener diodes are indeed for [Edit] D7 and D8.
Look at the extra dash on the cathode in the schematic.

It's a quite usable power supply but it is in need of some adjustments.
There is a (about 6 page) long thread about this power supply on this forum but I'm having some trouble in finding it.

D7 is used to limit a negative voltage for the power supplies of the opamps to be able to regulate the output down to 0V.

D8 is used to generate a reference voltage together with U1 and surrounding resistors.

[MK14]

The Zener diodes are indeed for D8 and D8.
Look at the extra dash on the cathode in the schematic.

It's a quite usable power supply but it is in need of some adjustments.
There is a (about 6 page) long thread about this power supply on this forum but I'm having some trouble in finding it.

These ones (I seem to be missing, an even earlier thread though) ?

https://www.eevblog.com/forum/beginners/0-30v-0-3a-psu-audiogurus-version/

https://www.eevblog.com/forum/beginners/diy-0-30v-0-3a-again/

[Electro Fan]

I've tried to review the other discussions of this general project (thanks again for posting them).

It's kind of hard to follow the discussions well enough to know for sure what was a fluky problem and what turned out to be a good fix, and what the current best parts list might actually be.

fwiw, attached is the current PCB shat ships from Amazon in the link I posted above in case anyone sees any obvious values that should/could be replaced with better values.

also, fwiw, while I've read about using a 20V transformer, unless someone wants to make a case otherwise, I'm still inclined to wire this up to a 24V transformer....

https://www.jameco.com/z/112512-R-Jameco-Reliapro-Power-Transformer-24VAC-2A-115VAC-Wire-Leads_112513.html
(Jameco 112512 is referenced a lot but I happen to have a Jameco 112513)

Only other change is that in addition to trying the small green potentiometers supplied with the kit I'm also going to try these 10 turn pots
https://www.amazon.com/gp/product/B07D7YH9N2/ref=ppx_yo_dt_b_asin_title_o08_s01?ie=UTF8&psc=1

we'll see what happens.....

[Electro Fan]

Yes, there are more than 5 diodes in the kit - the others were clearly marked so it was just these five that seemed somewhat confusing.  All the components started out loose in a bag.

OK, the ones you measured as having a higher V_f should be your zeners, but you also should be able to see markings on them with a magnifier.  You might want to test their reverse voltage just to make sure--just use a 9 volt battery and a 4.7 to 10K resistor for a current source if you don't have a meter that can put out 5+ volts on a diode check.

Not a bad kit for NINE BUCKS! 

Just confirming, your test is pretty much the same as what is suggested here in the Testing Zener Diode/Using Voltmeter section?
https://allabouteng.com/led-zener-diode-test/
Thx

Update:  tried the test with both a 9V battery and DC power supply, got about 4.9V but (not quite 5V)

[Electro Fan]

does 5V1 mean 5 volts with a 1 watt max (I'm guessing not), or a leakage current of 1 volt (probably the answer), or that the zener is set for 5.1 volts, or something else... what does the 1 signify in 5V1?

also, when testing with a power supply rather than a 9V battery does this spec sheet mean the test should be run with a setting of 49mA?
https://solarbotics.com/product/dz51/

I think this does a good job of describing zener operation

but I'm still not sure about the nomenclature of 5V1....

found this too, but still not sure what the 1 in 5V1 is referencing....

 

[MK14]

Update:  tried the test with both a 9V battery and DC power supply, got about 4.9V but (not quite 5V)

That sounds quite reasonable. If you had the exact zener part number and hence its datasheet. You would probably find, it needs something like 5 mA to give the nominal 5.1 V, as less current, can lower the actual voltage.
Also, zeners have tolerances (just like resistors), which would also be in the datasheet, and be something like 5%.
So, 5.1 V +/- 5% and the relatively low current from the 4K7 or 10K resistor you (presumably) used, explains the slightly less than 5 V, you just measured.

N.B. The figures I just gave are very approximate ball park figures. You really would have needed to check the actual datasheet (probably difficult, as you don't seem to be supplied with the full/exact part numbers).

[MK14]

does 5V1 mean 5 volts with a 1 watt max

Not really.

5V1 = 5.1 Volts

A shorthand way of saying 5.1V, and '.' are so tiny, it makes it easier to see (arguably).

[george.b]

A Zener diode won't normally be used in forward bias anyway, so...

So.... don't leave me in suspense, plz
What's the conclusion?  Thx

Sorry - what I meant to say is, the fact that the Zeners' forward voltage is higher than the others is irrelevant, as Zeners aren't used for their forward characteristics; they're used in reverse bias instead.

does 5V1 mean 5 volts with a 1 watt max (I'm guessing not), or a leakage current of 1 volt (probably the answer), or that the zener is set for 5.1 volts, or something else... what does the 1 signify in 5V1?

when testing with a power supply rather than a 9V battery does this mean the test should be run with a setting of 49mA?
https://solarbotics.com/product/dz51/

I think this does a good job of describing zener operation

but I'm still not sure about the nomenclature of 5V1....

It's 5.1V. Same kind of notation as 2K2 meaning 2200.

[Electro Fan]

does 5V1 mean 5 volts with a 1 watt max

Not really.

5V1 = 5.1 Volts

A shorthand way of saying 5.1V, and '.' are so tiny, it makes it easier to see (arguably).

Thx MK14 and george.b, glad to have that cleared up

.. same kind of notation as 2K2 and similar to george.b

[bdunham7]

The solarbotics info didn't seem to make complete sense to me.  To get 'the word' on the specs for a particular device, you need the actual part number and the manufacturer data sheet.  However, a 1mA test current will get you close enough for almost any application that uses a zener as they aren't that precise anyway.  I wouldn't worry anymore about that.  If you do want an example datasheet for a similar zener, look here:

https://www.mouser.com/datasheet/2/427/bzx55-1767760.pdf

The transformer you chose should be good if you are going to derate the power supply as you probably should.  If you use that xformer, put a nice big heat sink on the output transistor and then consider it a 1 amp power supply, you'll probably be happy.  If you try to use it as a 3 amp power supply, that transformer is insufficient but something else will likely go up in flames before it does.  I didn't try to figure out the circuit, as other have apparently analyzed it to death, but you might want to change the short circuit protection to a lower value.  Is that governed by R7?

I'd wait on the 10-turn pots until you see how stable and load-regulated the supply is. 

[Electro Fan]

Thanks.  Derate is a key concept for this project.

I'll start with the included pots and try to get things running before moving to the 10 turns.

I'm going to use the fan and heat sink shown in the attachment below.

Any idea how much current the PCB itself might handle?

My plan is to start slowly with a few milliamps and a volt or two.

Any thoughts on where the most obvious weak link in the chain might be that could be easily upgraded to handle more load?

One other thing that would be great to get some feedback on is this:
https://www.amazon.com/URBEST-Socket-Module-Switch-Terminals/dp/B07M9DJG53

I'm most interested in getting off to a good start on the AC to DC side.  This comes with a 5A fuse but maybe I could cut that down to 2-3A or so for starters?


Thx

[bdunham7]

Any thoughts on where the most obvious weak link in the chain might be that could be easily upgraded to handle more load?

No.  The whole thing, including your transformer, looks good for 1 to 1.5 amp or so.  Anything more and you are starting to push things beyond what can be called 'conservative'.  As it is, there may be components that are marginal at the voltages they are exposed to now, thus the recommendation of using an 18-20VAC transformer and calling it a 24 volt supply.  That part will depend on what parts they have supplied and for $9 delivered, you can't reasonably expect too much.  I'm not even sure you will get 30VDC @ 1A.

I'm most interested in getting off to a good start on the AC to DC side.  This comes with a 5A fuse but maybe I could cut that down to 2-3A or so for starters?

That looks fine and I think a 1 amp fuse might be appropriate--it should be able to handle a few seconds of inrush.  But for safety, fire prevention and damage mitigation, I'd recommend putting a fuse on the output of the transformer.  Perhaps a 2A normal or a 1-1.5 amp slow blow version.  That way you can blow the rest of it up as often as you like without risking any more than $9.

[tunk]

I'm no expert, but comparing to CPU fans and heat sinks,
at 2-3A your fan and heat sink may be on the small side.

[Electro Fan]

I'm no expert, but comparing to CPU fans and heat sinks,
at 2-3A your fan and heat sink may be on the small side.

Roger on that - I figured it was better than no fan or heat sink but to your point if I can size it up I'll give it a try.  Thx

[Electro Fan]

Any thoughts on where the most obvious weak link in the chain might be that could be easily upgraded to handle more load?

No.  The whole thing, including your transformer, looks good for 1 to 1.5 amp or so.  Anything more and you are starting to push things beyond what can be called 'conservative'.  As it is, there may be components that are marginal at the voltages they are exposed to now, thus the recommendation of using an 18-20VAC transformer and calling it a 24 volt supply.  That part will depend on what parts they have supplied and for $9 delivered, you can't reasonably expect too much.  I'm not even sure you will get 30VDC @ 1A.

I'm most interested in getting off to a good start on the AC to DC side.  This comes with a 5A fuse but maybe I could cut that down to 2-3A or so for starters?

That looks fine and I think a 1 amp fuse might be appropriate--it should be able to handle a few seconds of inrush.  But for safety, fire prevention and damage mitigation, I'd recommend putting a fuse on the output of the transformer.  Perhaps a 2A normal or a 1-1.5 amp slow blow version.  That way you can blow the rest of it up as often as you like without risking any more than $9.

Thx

Are you saying that in addition to the IEC receptacle put another fuse on the other side of the transformer before the PCB?

Also, this whole thing (from the AC outlet to the transformer) looks to be using a two wire connection with no third conductor ground.... I guess that's the way it is with this elementary level project?

Thx again

[bdunham7]

Are you saying that in addition to the IEC receptacle put another fuse on the other side of the transformer before the PCB?

Also, this whole thing (from the AC outlet to the transformer) looks to be using a two wire connection with no third conductor ground.... I guess that's the way it is with this elementary level project?

Yes, a fuse on one transformer lead before the PCB.  It is tricky to get a primary fuse just the right size to sufficiently protect the transformer yet not blow unnecessarily.  I would suspect that a 0.315A slo-blo primary might be perfect and would eliminate the need for a secondary, but you wouldn't know without some testing.  The secondary fuse just gives you an easier to calculate protection against something going wrong.  You should adjust the short protection (R7?) to limit the current so that the secondary fuse only blows if something goes wrong with the power supply circuit itself--which isn't all that unlikely. 

The yellow-green wire should be connected to the center pin on the IEC connector and the other end goes to your chassis or case, if it is metal, and to an external ground connector if you have one.

[Electro Fan]

Are you saying that in addition to the IEC receptacle put another fuse on the other side of the transformer before the PCB?

Also, this whole thing (from the AC outlet to the transformer) looks to be using a two wire connection with no third conductor ground.... I guess that's the way it is with this elementary level project?

Yes, a fuse on one transformer lead before the PCB.  It is tricky to get a primary fuse just the right size to sufficiently protect the transformer yet not blow unnecessarily.  I would suspect that a 0.315A slo-blo primary might be perfect and would eliminate the need for a secondary, but you wouldn't know without some testing.  The secondary fuse just gives you an easier to calculate protection against something going wrong.  You should adjust the short protection (R7?) to limit the current so that the secondary fuse only blows if something goes wrong with the power supply circuit itself--which isn't all that unlikely. 

The yellow-green wire should be connected to the center pin on the IEC connector and the other end goes to your chassis or case, if it is metal, and to an external ground connector if you have one.

Thanks for the help with this.  This is turning into a very good learning project.

So far my likely smallest slow blow fuse is a 0.5A

Regarding the yellow-green wire from the IEC connector.... so far this is just going to be a "prototype" with the transformer resting on the the bench next to the PCB.... so no metal case or any case.... so I guess I just have to be careful about what I touch and hope it all works when plugged in....

[MK14]

so no metal case or any case.... so I guess I just have to be careful

I assumed you were going to case it up and do it all safely.

In that case, rather than messing with a 'live' exposed transformer, please consider the fully pre-cased version of apparently the same rated transformer, from the same supplier.

Here:
https://www.jameco.com/z/GPU572402000WA00-Jameco-Reliapro-24VAC-2A-AC-to-AC-Wall-Adapter-Power-Supply_2197548.html

[floobydust]

A 24VAC transformer could be too much for the board and push the op-amps too far.
At no or light loads, a transformer will have a higher output voltage. 24VAC is the absolute max. and measuring that AC with a multimeter doesn't always work because it's a distorted sine-wave when feeding a rectifier.

With say +35VDC and -5.1VDC that is over 40V on TL081 which is spec'd 36V or 40V typical, 42V max. TL081H
Lots of threads about this problem, so keep an eye out for what you get across the big cap.

[Electro Fan]

A 24VAC transformer could be too much for the board and push the op-amps too far.
At no or light loads, a transformer will have a higher output voltage. 24VAC is the absolute max. and measuring that AC with a multimeter doesn't always work because it's a distorted sine-wave when feeding a rectifier.

With say +35VDC and -5.1VDC that is over 40V on TL081 which is spec'd 36V or 40V typical, 42V max. TL081H
Lots of threads about this problem, so keep an eye out for what you get across the big cap.

Would this make you more comfortable?
https://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?langId=-1&storeId=10001&catalogId=10001&productId=149957&CID=EPIPRODUCTLINK

[Electro Fan]

so no metal case or any case.... so I guess I just have to be careful

I assumed you were going to case it up and do it all safely.

In that case, rather than messing with a 'live' exposed transformer, please consider the fully pre-cased version of apparently the same rated transformer, from the same supplier.

Here:
https://www.jameco.com/z/GPU572402000WA00-Jameco-Reliapro-24VAC-2A-AC-to-AC-Wall-Adapter-Power-Supply_2197548.html

Just use a barrel to barrel to wires connection to the same PCB terminal block (or just cut the barrel off)..... seems easy enough and likely much safer.  What do you give up when it gets shrunk down to fit in the case?

I'm guessing there might be a 20V version that might address floobydust's concern too.

[MK14]

Just use a barrel to barrel to wires connection to the same PCB terminal block (or just cut the barrel off)..... seems easy enough and likely much safer.  What do you give up when it gets shrunk down to fit in the case?

I'm guessing there might be a 20V version that might address floobydust's concern too.

They do, an 18 volt version, of what I linked to. But, another one, has caught my eye, which seems to be even better, overall, cheaper and more suitable for experiments.

Here:
https://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?langId=-1&storeId=10001&catalogId=10001&productId=2230635&CID=EPIPRODUCTLINK

It is 16.5 V (Don't forget that you will get around +50% that voltage in practice, minus some losses, after it is turned into DC (Root 2, to be exact, x1.414 etc, minus diode drops).

But, it has a higher current rating, of 2.4 Amps (Don't forget that you have to divide that AC current by root 2, to get the DC current rating, then derate it even further, because the output is capacitive, rather than purely resistivity, which it is rated for).

So, probably around a rated 1.5 Amps DC, (from 2.4 Amps AC).

But, in addition, it actually has on its fully enclosed (safe) case, it has screw terminal output connectors. Making it even easier to experiment with, and possibly more useful in the future. Without needing to get the right barrel connectors, or cut off connectors, or solder etc etc.

Although 18V at 2 Amps (AC), sounds bigger, the higher current one, will tend to NOT drop its voltage so much, as the output current increases. Also, it will keep the output transistor cooler (less dissipation), which will allow higher output currents, for the same waste heat in that transistor.

[MK14]

I'm guessing there might be a 20V version that might address floobydust's concern too.

Consider the 16.5 V, I linked to above.

Unloaded, it is probably 20 V or more, because transformers tend to increase their output voltage, when they have little/no output current (Transformer's Load Regulation).

(Rough rule of thumb, before voltage losses, such as diodes), gives approaching 30 V, but the circuit also has 5.1 V (see original schematic) on the other rail, giving around 35 V, minus diode drops.

But the op-amps (depending on what you have exactly), may only be specified to 36 V.

So, maybe 16.5 V (AC), is around the practical limit, for this exaggerated spec power supply kit.

(Including diode drops, could increase that to 18 V AC, but on the other hand, mains voltage, can fluctuate, so best to have a safety margin).

tl;dr
Even 20 V AC, could be too high, when considering transformer load regulation, and possible mains voltage fluctuations.

On the other hand, you could be very lucky, and have the 44 V rated op-amps, people talk about, but it is not clear, what you have.

EDIT:
I can believe/agree, that if you carefully checked the components you have in the kit (especially what op-amps they really are), accurately calculated all the voltages (including diode drops etc), that some AC figure of around 20 V (AC), would be ok (with the possible exception of mains voltage fluctuations).
But, I feel more comfortable, with a bit of safety margin, hence the lower transformer V AC recommendations, plus your supplier doesn't seem to do ones of 20 V AC, that I could see (pre-cased ones).

[floobydust]

For that 18V 2A transformer, its design is 230VAC but you have say 240VAC almost 5% higher mains. I estimate a 36VA part has 12% regulation so 21VAC at no load to give around 27VDC.
Using the same math, a 24VAC transformer would give almost 37VDC. I usually get about 35VDC with them.
A 20VAC transformer would give about 30VDC, for total 35V to the op-amps. These are just rough math values.

It's hard to know the quality of the imported transformer, it might be worse (than 12%) for regulation. The datasheet says "Voltage mobility rate: below 10%, Voltage deviation: within 5%" so I'm not sure what to expect.

[Electro Fan]

Just to confirm, I'm on US AC (~120V) in case this effects any of the derating calcs.

[george.b]

With say +35VDC and -5.1VDC that is over 40V on TL081 which is spec'd 36V or 40V typical, 42V max. TL081H

I wonder if a good old 741 could be used here instead. It is rated up to 44V after all.

[Electro Fan]

fwiw, the OpAmps look to be TI TL081CP's.

https://www.ti.com/store/ti/en/p/product/?p=TL081CP

I've mounted them in sockets so if they needed to be changed out it's easy to do.

Attached are photos of the partially assembled PCB and a few parts waiting installation.

The total investment in this has not been huge so if I need to experiment a little with some parts that don't make it or need to be upgraded it's doable.  I'm kind of looking forward to plugging it in to see what happens but I can hold off if something looks like it's a no-op or if something would clearly benefit from some re-doing.

[MK14]

fwiw, the OpAmps look to be TI TL081CP's.

The datasheet seems to specify a recommended maximum supply voltage of 30 V (where it is guaranteed to work).
Also, the Absolute maximum supply voltage is 36 V, above which, the device could be damaged.

Really, the power supply should have been properly designed in the first place, so that the transformer that they include and/or specify for it. DOESN'T violate or break, any of its components, datasheet specifications.

tl;dr
AKA Cheap Chinese Electronic Kits, don't always meet the advertised specifications.

[floobydust]

It's not a chinese design, they just copied it, switched op-amps and added hype and a bad pcb trace to the sense resistor.
The history of this 0-30V 2mA-3A power supply goes back to Practical Electronics 1978 October pg. 1070 and that original used three LM741's (44V max.) with transformer spec 20 to 25V, 2A.
There is mention of an old "Greek kit" and other modern revisits of the design. Czech version added 33V regulator.

Lots of discussion on this kit:
https://www.eevblog.com/forum/beginners/0-30v-0-3a-psu-audiogurus-version/
https://www.eevblog.com/forum/projects/chinky-0-30v-3a-psu-issues/

[george.b]

It's not a chinese design, they just copied it, switched op-amps and added hype and a bad pcb trace to the sense resistor.
The history of this 0-30V 2mA-3A power supply goes back to Practical Electronics 1978 October pg. 1070 and that original used three LM741's (44V max.) with transformer spec 20 to 25V, 2A.
There is mention of an old "Greek kit" and other modern revisits of the design. Czech version added 33V regulator.

Lots of discussion on this kit:
https://www.eevblog.com/forum/beginners/0-30v-0-3a-psu-audiogurus-version/
https://www.eevblog.com/forum/projects/chinky-0-30v-3a-psu-issues/

Ha, that answers my question, then. The original project used a 741. Makes sense.

[MK14]

It's not a chinese design, they just copied it, switched op-amps and added hype and a bad pcb trace to the sense resistor.
The history of this 0-30V 2mA-3A power supply goes back to Practical Electronics 1978 October pg. 1070 and that original used three LM741's (44V max.) with transformer spec 20 to 25V, 2A.
There is mention of an old "Greek kit" and other modern revisits of the design. Czech version added 33V regulator.

Lots of discussion on this kit:
https://www.eevblog.com/forum/beginners/0-30v-0-3a-psu-audiogurus-version/
https://www.eevblog.com/forum/projects/chinky-0-30v-3a-psu-issues/

I hadn't read those threads (to save time), now that I partly have, I better understand the situation. Thanks for letting me know.

So the original 1978 electronics magazine design was reasonable. By the time we arrive at the Chinese kit, they have introduced mistakes, and used inferior parts (especially voltage ratings). So, that it is no long anywhere near a 30 V, 3 Amp, power supply.
In fact, it seems it was originally, a 2 Amp, 30 V design.

Sadly some purchasers of the kit, buy it without realizing, and can end up attempting to get too much voltage and/or current out of it.

[Electro Fan]

Ok, good to understand the history.  I'm guessing that the current kit iteration is in the ballpark but is probably not spot on, and that it could be just a threshold here or there away from an oops.  No worries - I'm looking at this as a learning project rather than expecting to get a useful power supply out of it. 

Once the power supply is assembled my plan is to start at relatively low voltage and current levels and use the ps as a test bed to measure and better think through the circuity.  I think this will help to glue ( solder ) together the physical and the logical.

There are a bunch of things that I hope to learn more about including the role of the op-amps, but even before I get to that this has caused me to realize I need a better understanding of the the AC transformer specs as they lead to rectification and the DC specs.  The kernels of this are now sprinkled through this thread including some formulas, calculations, and margin estimates but I think the next step for me is to figure out how the AC to AC and AC to DC math should work so I can do it in addition to reading about it.  I'm sure I can find lots of info on this but if anyone has any articles or videos that you think are especially good with respect to the power calcs please post them.

After I better figure out what the AC will/should enable the DC to do I'll settle on a transformer.  After that I'll try to work forward toward the rest of the circuit to see where the potential weak links might be.
 

[MK14]

if anyone has any articles

I've had a look on your behalf, and I like the following. Because it is brief, to the point, and seems to give reasonably accurate formulas.

https://www.sowter.co.uk/rectifier-transformer-calculation.php

BRIDGE

Vac = Vdc x 0.71

Iac = Idc x 1.61

Pac = Pdc x 1.14

To give even more accurate results, you would need to adjust for the 0.7 V to 1 V (load current dependent), voltage drop (figure needs doubling for a bridge), from each rectifier (bridge) diode. Which is around 1.5 V to 2 V, for your full bridge rectifier (4 diodes), in the kit.

[MK14]

Ok, good to understand the history.

The original circuit/magazine, seems to be here:
https://worldradiohistory.com/UK/Practical-Electronics/70s/Practical-Electronics-1978-10.pdf

Towards the middle of it. It has a fairly extensive explanation of how it works (if I remember correctly).

EDIT: I tried to upload it (not sure about ownership rights, either), but it was apparently too big.

[WattsThat]

One comment, based on your build photos. I believe you have the pass regulator rotated 90 degrees, the transistor leads should exit parallel with the heatsink fins. In that way, the mounting holes of the heatsink will line up with the holes in the pcb.

As for improvements, here is a link to a site that provides some fixes, the nasty turn off spike and some better op amps, for starters.

http://www.paulvdiyblogs.net/2015/05/tuning-030v-dc-with-03a-psu-diy-kit.html

[MK14]

One comment, based on your build photos. I believe you have the pass regulator rotated 90 degrees, the transistor leads should exit parallel with the heatsink fins. In that way, the mounting holes of the heatsink will line up with the holes in the pcb.

As for improvements, here is a link to a site that provides some fixes, the nasty turn off spike and some better op amps, for starters.

http://www.paulvdiyblogs.net/2015/05/tuning-030v-dc-with-03a-psu-diy-kit.html

I've had a quick look. The combination of reducing the negative 5 V supply, down to only 1.3 V, and using the 44 V op-amps in the article, allows it to use the 24 V AC transformer, and hence get healthier maximum output voltages, and yet be safe from giving the op-amps too much voltage, which is another good improvement.

[Electro Fan]

One comment, based on your build photos. I believe you have the pass regulator rotated 90 degrees, the transistor leads should exit parallel with the heatsink fins. In that way, the mounting holes of the heatsink will line up with the holes in the pcb.

As for improvements, here is a link to a site that provides some fixes, the nasty turn off spike and some better op amps, for starters.

http://www.paulvdiyblogs.net/2015/05/tuning-030v-dc-with-03a-psu-diy-kit.html

Thanks, but I'm not sure what could be rotated 90 degrees....but apparently I'm missing it....  if you can, plz mark up the photo pointing to where the 90 degree rotate should be.  Thx again

Edit:  I think I know what you might be referring to; I've attached two photos showing similar but different PCB layouts - the top photo is from my partially built board and the other is from somewhere else.  One of the transistors is rotated 90 degrees.  It's a little optically confusing depending on which view of the board you have but it's further confusing because that particular transistor has not yet been installed on my PCB (because I was waiting to maybe find a heat sink that would fit before soldering the transistor to the board).  Somewhere between 1978 and 2020, probably fairly recently, someone did the board layout a little different for that transistor.  I'm pretty sure this explains the situation but thanks again for the QC/QA support!

PS, I just noticed the other photo with the transistor rotated 90 degrees is also using sockets for the op-amps.  My kit didn't include the sockets but when I read the various concerns earlier about stuff blowing up I figured sockets might be good; apparently whoever built the board in the other photo came to a similar conclusion.

I added another photo at the bottom which is the specific item I purchased.  It was a little extra ($10.88 total) because it included the radiator heat sink and the fan.

https://www.amazon.com/gp/product/B01KNSCDNU/ref=ppx_yo_dt_b_asin_title_o03_s00?ie=UTF8&psc=1

In the photo you can see it didn't have the op-amp sockets.

For $8.99 you get everything but the heat sink and fan:

https://www.amazon.com/dp/B01NBE97WH/ref=dp_prsubs_1

[Electro Fan]

Ok, good to understand the history.

The original circuit/magazine, seems to be here:
https://worldradiohistory.com/UK/Practical-Electronics/70s/Practical-Electronics-1978-10.pdf

Towards the middle of it. It has a fairly extensive explanation of how it works (if I remember correctly).

EDIT: I tried to upload it (not sure about ownership rights, either), but it was apparently too big.

Wow, MK14, Amazing Find!  1978 article - you are a very good finder.  I'll read this and add it to the learning process, for sure.  Thx!

[Electro Fan]

This article suggests using a variac to bring up a newly built power supply. 

https://worldradiohistory.com/UK/Practical-Electronics/70s/Practical-Electronics-1978-10.pdf

Any reason not to do this, or any suggestions regarding using a variac for this purpose?  Maybe ramp to about 10V, monitor/check the components, continue on and stop at about 16V to begun operating/testing the power supply?  If all goes well then shut down and replace the variac with a ~16V transformer?

[MK14]

This article suggests using a variac to bring up a newly built power supply. 

https://worldradiohistory.com/UK/Practical-Electronics/70s/Practical-Electronics-1978-10.pdf

Any reason not to do this, or any suggestions regarding using a variac for this purpose?  Maybe ramp to about 10V, monitor/check the components, continue on and stop at about 16V to begun operating/testing the power supply?  If all goes well then shut down and replace the variac with a ~16V transformer?

The Variac doesn't replace the transformer at all. It 'replaces' the mains input connection, to your power supply.
So, instead of connecting to the mains, which would always be around 120 V AC, or whatever your voltage is. Instead, you would plug the Variac into the mains, and the Variacs output (it may have a mains socket on it), is used to power the DUT (Device Under Test = Your newly built prototype power supply).

The Variac, has a dial, which allows its output (socket), to vary between a rather low voltage, and approximately the full mains voltage (maybe a slight percentage more).

I'd suggest doing exactly what it says in the article, which is to use a resistor, of the type/value & current meter, when first testing your PSU out.

[ledtester]

One reason to ramp it with a variac is to check for shorts or other wiring mistakes by monitoring the input current as you slowly increase the voltage. You should first estimate what the current consumption of the circuit should be (say with no load.)

Often power supply circuits are tested by using another current limiting power supply for the input power. In this case, however, an AC input source is needed to create the negative voltage rail (as  voltlog discovered). And, of course, this runs into the chicken-and-egg problem if you're building this to get a current limiting power supply in the first place.

[Electro Fan]

One reason to ramp it with a variac is to check for shorts or other wiring mistakes by monitoring the input current as you slowly increase the voltage. You should first estimate what the current consumption of the circuit should be (say with no load.)

Often power supply circuits are tested by using another current limiting power supply for the input power. In this case, however, an AC input source is needed to create the negative voltage rail (as  voltlog discovered). And, of course, this runs into the chicken-and-egg problem if you're building this to get a current limiting power supply in the first place.

Thanks for posting the VoltLog link - that was very relevant and helpful for this project.  Lots of good learning lessons so far from building this kit.  If I manage to get it turned on and working I am going to recommend the kit highly but even if it doesn’t work the process of building it and discussing it in this thread has been pretty worthwhile.  Thanks again for the link - I’m going to look through the comments on the video to see what other nuggets might be in there.

[Electro Fan]

I'm guessing there might be a 20V version that might address floobydust's concern too.

Consider the 16.5 V, I linked to above.

Unloaded, it is probably 20 V or more, because transformers tend to increase their output voltage, when they have little/no output current (Transformer's Load Regulation).

(Rough rule of thumb, before voltage losses, such as diodes), gives approaching 30 V, but the circuit also has 5.1 V (see original schematic) on the other rail, giving around 35 V, minus diode drops.

But the op-amps (depending on what you have exactly), may only be specified to 36 V.

So, maybe 16.5 V (AC), is around the practical limit, for this exaggerated spec power supply kit.

(Including diode drops, could increase that to 18 V AC, but on the other hand, mains voltage, can fluctuate, so best to have a safety margin).

tl;dr
Even 20 V AC, could be too high, when considering transformer load regulation, and possible mains voltage fluctuations.

On the other hand, you could be very lucky, and have the 44 V rated op-amps, people talk about, but it is not clear, what you have.

EDIT:
I can believe/agree, that if you carefully checked the components you have in the kit (especially what op-amps they really are), accurately calculated all the voltages (including diode drops etc), that some AC figure of around 20 V (AC), would be ok (with the possible exception of mains voltage fluctuations).
But, I feel more comfortable, with a bit of safety margin, hence the lower transformer V AC recommendations, plus your supplier doesn't seem to do ones of 20 V AC, that I could see (pre-cased ones).

Per your earlier analysis, here is some Q&A from the comments section of the VoltLog video:

2 months ago
hi can I use a 24v 5a transformer for the above circuit, since i have it with me. thanks

1 week ago
yes you can, but you should not. .  The opamps have a maximum voltage of 30v.  After filter, a voltage of almost 34 volts apear. Under load, I'm sure it is not a issue, but I let mine powered on over night with no load and burn the opamps. This power supply shoud be ratated maximum to use with a 20v transformer. 12 0 12 or 24 0 is too much for this supply.

Looks like your 16.5V recommendation should be a good starting point.  Thx again for the guidance.

[Electro Fan]

One really good breadcrumb leads to the next.

The VoltLog power supply enclosure:

[Electro Fan]

Still waiting on a couple transformers but the rest of the kit is pretty much ready to go.  Going to start with a 16.5V 2400mA transformer as recommended by MK14.

In the meantime I noticed that the fan on the heat sink is powered by a 24V header.  I tested the fan and found that it would turn on around 6V and about 10mA.  At 12V it draws 41 mA, at 18V it draws 58 mA, and at 24V it draws 77 mA.  I'm hoping their is some current left in this design to power something more than the fan. 

[ledtester]

A TO-220 LM78xx regulator is good for at least 1 amp provided it is adequately kept cool. You can always substitute different linear regulator to drive the fan and other components - like this guy did:

https://www.qsl.net/z33t/dc_0-30v_0-3A_eng.html

[MK14]

Still waiting on a couple transformers but the rest of the kit is pretty much ready to go.  Going to start with a 16.5V 2400mA transformer as recommended by MK14.

In the meantime I noticed that the fan on the heat sink is powered by a 24V header.  I tested the fan and found that it would turn on around 6V and about 10mA.  At 12V it draws 41 mA, at 18V it draws 58 mA, and at 24V it draws 77 mA.  I'm hoping their is some current left in this design to power something more than the fan. 

That is a good idea (if I say so myself).
As others have suggested, it is possible to redesign it (partly), and change various bits, so that it can cope with high voltages. But, it is not so easy to perform all the modifications and get hold of all the components, etc.

You MIGHT get away with higher voltages. But at some point, it would either break straight away, and/or be unreliable and/or have significantly shorter component life.

You will probably get something like 19 V AC from a transformer like that (16.5 V AC), when it is very lightly loaded. Because transformers, have a load regulation voltage parameter, which means it goes up by a percentage (e.g. 10% or 15% etc, see datasheet and/or spec sheets, if available), when lightly loaded (less than rated max current, is used).
It's proportional to the current used, so at near 0 amps, will be at its highest.

[Electro Fan]

Still waiting on a couple transformers but the rest of the kit is pretty much ready to go.  Going to start with a 16.5V 2400mA transformer as recommended by MK14.

In the meantime I noticed that the fan on the heat sink is powered by a 24V header.  I tested the fan and found that it would turn on around 6V and about 10mA.  At 12V it draws 41 mA, at 18V it draws 58 mA, and at 24V it draws 77 mA.  I'm hoping their is some current left in this design to power something more than the fan. 

That is a good idea (if I say so myself).

LOL - that's funny

As others have suggested, it is possible to redesign it (partly), and change various bits, so that it can cope with high voltages. But, it is not so easy to perform all the modifications and get hold of all the components, etc.

You MIGHT get away with higher voltages. But at some point, it would either break straight away, and/or be unreliable and/or have significantly shorter component life.

You will probably get something like 19 V AC from a transformer like that (16.5 V AC), when it is very lightly loaded. Because transformers, have a load regulation voltage parameter, which means it goes up by a percentage (e.g. 10% or 15% etc, see datasheet and/or spec sheets, if available), when lightly loaded (less than rated max current, is used).
It's proportional to the current used, so at near 0 amps, will be at its highest.

If you can explain more about this or provide a link to something tutorialish I'd like to better understand this.  Thanks

[Electro Fan]

A TO-220 LM78xx regulator is good for at least 1 amp provided it is adequately kept cool. You can always substitute different linear regulator to drive the fan and other components - like this guy did:

https://www.qsl.net/z33t/dc_0-30v_0-3A_eng.html

Thanks!  That's another very good link about this power supply project.  The schematic helped me understand a little more than the previous schematic I had. 

I'm wondering if the op amps are easy low hanging fruit that could be the beginning of an upgrade path?  Ultimately it would be cool to get the whole thing up a notch or two on voltage and current but especially current.

[MK14]

If you can explain more about this or provide a link to something tutorialish I'd like to better understand this.  Thanks

Assuming you mean about the transformers, output voltage changes (regulation):

https://www.electronics-tutorials.ws/transformer/voltage-regulation.html

https://www.allaboutcircuits.com/textbook/alternating-current/chpt-9/voltage-regulation/#:~:text=Voltage%20regulation%20is%20the%20measure,the%20regulation%20it%20will%20provide.

[Electro Fan]

If you can explain more about this or provide a link to something tutorialish I'd like to better understand this.  Thanks

Assuming you mean about the transformers, output voltage changes (regulation):

https://www.electronics-tutorials.ws/transformer/voltage-regulation.html

https://www.allaboutcircuits.com/textbook/alternating-current/chpt-9/voltage-regulation/#:~:text=Voltage%20regulation%20is%20the%20measure,the%20regulation%20it%20will%20provide.

You are an excellent curriculum finding teacher !  Those articles hit the spot.

Just to confirm, the second article indicates that 3% voltage regulation might be relatively "good" and that ~6.9% might be relatively "poor".  Are these reasonable bookends on some scale or for some basic use case?  Thanks again

Edit:
Looks like this (maybe similar to the 7824 used in the PS project) might come in versions that range from 1.5% to 4%, so presumably this is in the good (enough) range for various applications.... ?

https://www.mouser.com/datasheet/2/308/MC7800_D-1773680.pdf

[Electro Fan]

If you can explain more about this or provide a link to something tutorialish I'd like to better understand this.  Thanks

Assuming you mean about the transformers, output voltage changes (regulation):

https://www.electronics-tutorials.ws/transformer/voltage-regulation.html

https://www.allaboutcircuits.com/textbook/alternating-current/chpt-9/voltage-regulation/#:~:text=Voltage%20regulation%20is%20the%20measure,the%20regulation%20it%20will%20provide.

You are an excellent curriculum finding teacher !  Those articles hit the spot.

Just to confirm, the second article indicates that 3% voltage regulation might be relatively "good" and that ~6.9% might be relatively "poor".  Are these reasonable bookends on some scale or for some basic use case?  Thanks again

Edit:
Looks like this (maybe similar to the 7824 used in the PS project) might come in versions that range from 1.5% to 4%, so presumably this is in the good (enough) range for various applications.... ?

https://www.mouser.com/datasheet/2/308/MC7800_D-1773680.pdf

And this one (in version A) is 2%
https://www.mouser.com/datasheet/2/389/l78-1849632.pdf

[MK14]

Unfortunately, there seems to be some confusion. I need to take some/all of the blame, so don't feel bad.
The term "Voltage Regulation", can be confusing, as it seems to be used, for a number of things.

I was referring to Transformer "Voltage Regulation", which in very simplistic terms just means the voltage varies by say 10% (approx), as the output (load) current changes.
My concern was that it could make the transformer output voltage (AC) too high, for that PSU. Hence talking you into using a lower transformer rating (which you have), which solves that problem.

What you have just linked to, is another form of "Voltage Regulation", which is integrated circuit voltage regulator's "Voltage Regulation". Which also can vary with output (load) current changes.
Maybe a few percent or so, on a cheap/simple regulator (varies widely).

The idea is that the final voltage regulator (which is most of the PCB and circuit, you just bought), but can be just a simple 3 lead, integrated circuit regulator, just like you linked to. Will attempt to filter out (ignore), the transformers voltage variations, voltage ripple and things, to give a nice clean, well regulated output voltage.
BUT, it is not going to be perfect, and will also vary by a percentage (hopefully rather small), as the load (current) increases, temperature changes, and other things.

[MK14]

tl;dr (Summary):

Transformer "Voltage Regulation", just the raw transformer, no extra circuitry or ICs. How its AC voltage drops/changes with transformer output current into the load (of the transformer).

(What you seem to be thinking of). "Voltage Regulation" of the voltage regulator(s), which is something else. It is how stable the voltage regulators output voltage is, with changes in output load current.

[Electro Fan]

tl;dr (Summary):

Transformer "Voltage Regulation", just the raw transformer, no extra circuitry or ICs. How its AC voltage drops/changes with transformer output current into the load (of the transformer).

(What you seem to be thinking of). "Voltage Regulation" of the voltage regulator(s), which is something else. It is how stable the voltage regulators output voltage is, with changes in output load current.

Thanks for the clarifying posts.  No worries.

Your links were helpful and I think I get some of the basics:  A transformer's output by itself can vary based on load or no load - perhaps by 10% or so, hence your margin-adding recommendation to reduce the spec'd voltage to around 16V.  Additionally a voltage regulator can improve the performance of the overall circuit by reducing the variation to 2% or less.  It's all good - and between building the PS and doing the Q&A here it's coming into better focus. 

Pretty soon I'm going to plug in the AC and see what happens on the DC output

Thx again

[Electro Fan]

New question:  I now have a 24V 2A transformer.  When plugged into my ~120V AC (which at the moment measures 123.4V with no load - normally I have seen about 121V) the transformer output measures 28.2V with no load.  What do you think, safe to give it a whirl with the PS or not enough margin for error?  Thx

Update:  I have a Dale 50 Watt 50 Ohm 1% resistor (that measures ~50.2 Ohms).  I'm thinking if I power up with the PS loaded by the resistor, maybe it's going to be ok if I put the PS together properly...?

Update 2:  Or are you saying that voltage seen by the op amps is for some reason independent of the voltage reduction that occurs when PS is loaded?

[MK14]

New question:  I now have a 24V 2A transformer.  When plugged into my ~120V AC (which at the moment measures 123.4V with no load - normally I have seen about 121V) the transformer output measures 28.2V with no load.  What do you think, safe to give it a whirl with the PS or not enough margin for error?  Thx

Update:  I have a Dale 50 Watt 50 Ohm 1% resistor (that measures ~50.2 Ohms).  I'm thinking if I power up with the PS loaded by the resistor, maybe it's going to be ok if I put the PS together properly...?

Update 2:  Or are you saying that voltage seen by the op amps is for some reason independent of the voltage reduction that occurs when PS is loaded?

The capacitor(s), immediately after the bridge rectifier, will attempt to reach almost 1.5 (root 2) times the peak AC voltage (28.2 V, peak as in unloaded), so they would reach 28.2 x root 2 - 1.2 volts worth of diodes =
= 38.6 V plus the other 5 V rail = almost 44 volts (peak, when unloaded).

44 V is the absolute maximum voltage on even the much high voltage op-amps, which YOU DO NOT HAVE, you have the 36 V ones, which are only really specified (working), at 30 V.
Too high a voltage, could instantly fry all your op-amps.

Solutions:
1) Just connect it up. Not recommended.

2) Wait for the new, lower voltage transformer. Probably the best thing to do.

3) Drop the voltage, using other ways. Such as diodes in series. Or putting a load on the dropping resistor (50 ohms).

N.B. Once you over-voltage those op-amps, they may instantly break and never work again. Causing major headaches, when you try and fix or get this item to work.

tl;dr
This is getting so complicated (because of various misunderstandings), and trying to rush things.

Best to start at low voltage and low current. Get it to work, then go from there.

[Electro Fan]

Hi MK14,

Thank you very much for your guidance.

Turns out that I went with your Solution 2.  I used an enclosed transformer spec'd for 16V and 2.5A.

I am very happy to report that the power supply started up on the first try and ran through multiple tests - probably for nearly an hour.

Attached below are the measurements.

A few key results:

With a 50 Ohm resistor the maximum current output was 381 mA with a max DC voltage of 19.1

With a 1 Ohm resistor the PS achieved a max current of 2.7 Amps and a max voltage of 2.7; however I noticed that this put a load of 3.9 Amps on the secondary side of the transformer so I didn't run those settings very long.

At approximately 1V and 1A DC (0.98V and 0.98A) the power supply reached 1.75A on the secondary at 16.97V

I scientifically tested temperature with my fingers and found everything ran cool on all tests with the exception of the 5W 47 Ohm resistor right in front of D1047.  That resistor might be my first thing to upsize - maybe to 50 watts.  It was too hot to touch for more than a few seconds, but to be square I think I noticed it when I was pushing the PS at it's max output pf 2.7A and 2.7V (which is when the secondary was beyond it's spec at 3.9A).

Throughout the test the small fan on the big heat sink at D1047 operated like a champ, adjusting speed based on the V and A potentiometer settings.  Most of the time it sounded like a small jet engine (this is not something anyone who complains about commercial test equipment fans would find acceptable - it's louder than all the fans on my bench combined - but it seemed to do it's job admirably).

I managed to get an on-off switch inline between the transformer secondary and the PS PCB.  I'd recommend this to anyone testing as an initial step.  Longer term I'll get a power switch on the primary side of the transformer.

Your suggestion to use the enclosed transformer and clearly your overall sizing recommendations were very sound advice.  Thanks!

All in all, for about $11 including the PCB and components with the big heat sink and fan, plus another $13 for the transformer it's a highly recommended project as a learning experience.  I wouldn't recommend this as an alternative to an off the shelf PS for home lab work but for entry level learning about power supplies I'd rate it an A if not an A plus.

Not sure what's next.  Probably some more tests,  Maybe building in some metering (vs 4 DMMs).  And maybe/possibly an enclosure.

[MK14]

Congratulations!   

I'm glad and pleased, that worked out rather nicely for you.

Thanks for coming back, and giving such a nice, detailed report on your successes.
Many posters, don't do that. But, such feedback is useful for all the contributors to this thread, and a useful source of information, for anyone who reads it in the future.

[Electro Fan]

Ok, back at it.

In case anyone wants to try the same transformer, it's a MG Electronics Model #MGT1640.
https://www.jameco.com/shop/ProductDisplay?catalogId=10001&langId=-1&storeId=10001&productId=2230635
It says 16.5V, 2400mA in the link but on the box it says 16V, 40 VA.  The one I received has a nice green power light.

Questions...

It might make sense to replace the 5 watt 47 ohm resistor with one of these:
https://www.amazon.com/gp/product/B07JN3BFNN/ref=ox_sc_act_title_7?smid=A1OAXIQ154HL11&psc=1

but, assuming I could make it fit/attach, I'm wondering if this would buy a bunch more head room:
https://www.amazon.com/gp/product/B07JN3GF4G/ref=ox_sc_act_title_9?smid=ATNHGLND9W21T&psc=1

Also, I saw there was another thread going on about op amps but just checking to see if anyone has a favorite op amp that would be recommended to gain some more headroom over the TL081CPs?

[Electro Fan]

Not a lot of answers to those last questions, but no worries.... here's a new question (or two):

If the power supply (under discussion in this thread) is set for 1.5 Volts and 1.5 Amps into a 1 Ohm load, a) roughly what temperature might be expected at D1047 and b) according to this spec sheet what would you expect is the maximum safe operating temperature for D1047?

https://www.st.com/resource/en/datasheet/2sd1047.pdf

Thanks

[bdunham7]

The datasheet lists T_{j max} at 150C.  That's the limit and typically you try to stay well under it.

You don't actually have enough info to do the math here--you don't know what the rectified/filtered pre-regulator voltage is, but given a 16V 2.5A transformer and a 1.5A load, I'll guess it is 18 volts.  And you don't know the specification of the heat sink/fan combo, but let's guess and say 3K/W with the fan at 18 volts (meaning the heat sink will be 3 degrees C/K higher than ambient per watt being dissipated).

So, now you have a voltage drop across the transistor of 16.5 volts and a current of 1.5A, so 24.75 watts, let's round that to 25 watts.  The transistor datasheet lists the thermal resistance as 1.25K/W, so assuming the transistor is firmly attached directly to the heatsink with good thermal compound, the overall package is about 4.25K/W.  25 watts x 4.25K/W = 106.25 degrees C/K of temperature differential.  Assume an ambient temp of 25C and you have T_j of 131.25C.  That's pretty toasty, you certainly don't have much margin left if either of my two guesses is off the wrong way.

[Electro Fan]

The datasheet lists T_{j max} at 150C.  That's the limit and typically you try to stay well under it.

You don't actually have enough info to do the math here--you don't know what the rectified/filtered pre-regulator voltage is, but given a 16V 2.5A transformer and a 1.5A load, I'll guess it is 18 volts.  And you don't know the specification of the heat sink/fan combo, but let's guess and say 3K/W with the fan at 18 volts (meaning the heat sink will be 3 degrees C/K higher than ambient per watt being dissipated).

So, now you have a voltage drop across the transistor of 16.5 volts and a current of 1.5A, so 24.75 watts, let's round that to 25 watts.  The transistor datasheet lists the thermal resistance as 1.25K/W, so assuming the transistor is firmly attached directly to the heatsink with good thermal compound, the overall package is about 4.25K/W.  25 watts x 4.25K/W = 106.25 degrees C/K of temperature differential.  Assume an ambient temp of 25C and you have T_j of 131.25C.  That's pretty toasty, you certainly don't have much margin left if either of my two guesses is off the wrong way.

Hi bdunham7,

Thanks for the excellent reply.  Not only did you provide the answers but also the methods for arriving at the answers - that was great!  (And fast too.) 

I attached the transistor to the heat sink with with I think is pretty good thermal compound.  So far when I keep the AC current below 2.5 Amps (which is easy to do when I keep the DC below 1.5 V and 1.5 A into the the 1 Ohm load) the max temp I've seen on D1047 is about 48 C (in a room with an ambient temp of 23 C) ..... so maybe I can quit worrying about the temp?

I consistently see about 16.5 V on the AC side of the transformer when I've been trying to keep it under 2.5 Amps on the AC side.  The AC voltage doesn't change much based on what happens on the DC side but it's not hard to push the transformer's AC to 3 Amps or more. At 2 Volts and 2 Amps DC the transformer AC Voltage drops to about 15.7 V and the transformer current goes to about 3.05 Amps, which runs D1047 to about 60 C.  At this point the big cap gets to about 40 C but the op amps and the big resistor barely get beyond the ambient temp.   

So far it seems that the main thing I need to keep an eye on as I dial up more DC is the current on the AC side of the transformer.  If that was spec'd for more headroom I think everything else might be able to perform up a notch.  To be square, I don't really understand exactly what's causing what but the more I measure the more I see some of the relationships, and your explanations on the temp helped a bunch as along with the AC current the temp seems to be a very  key thing to keep an eye on.

Thanks again for walking me through the processes!

[bdunham7]

Do keep in mind that you can only measure the case temp (not the black plastic, but the metal tab), not T_j.  So you have to calculate the wattage, which is the voltage dropped across the transistor x the output current, and multiply that by 1.25 and add that number to the measured case temp to get an approximation of T_j.  So if at 2 amps, 2 volts with a pre-regulator 16V, you have 14V x 2A = 28 watts x 1.25 = 35K difference between T_j and T_case.  So if you are measuring 60C at the case, the chip is at 95C.  Your heat sink must be much better than my guess, b/t/w.

And just an additional note, measuring the AC voltage of a transformer that is feeding a rectified/filtered DC supply is problematic because there will be severe waveform distortion.  Yes, a true RMS meter will correctly read the RMS value, but your circuit isn't responsive to the RMS value, it is only responsive to that portion of the cycle that exceeds the voltage on the filter cap by 2V_f of the rectifiers.  That all gets complicated quickly so I'll simply sat that it sounds like you have a nice 1.5 amp power supply.

[MK14]

At 2 Volts and 2 Amps DC the transformer AC Voltage drops to about 15.7 V and the transformer current goes to about 3.05 Amps, which runs D1047 to about 60 C.

The 60 deg C, could easily be too much temperature for the power transistor, let's do a quick rough calculation.

If there is (16.5 x 1.414 - 2V (diodes)) - 2V output = 19.3 volts and 2 amps across the power transistor, then it would be dissipating around 38.6 watts.

If the transistors junction to case is 1.25 deg C per watt + something like 1 deg C per watt of heatsink to case, thermal resistance.

Then (rough back of envelope calculation), = 2.25 Deg C per watt x 38.6 watts = 60 deg C plus 87 deg C = around 147 deg C = on the high side, as it is really/ideally spec'd for 125 deg C (The 150 deg C mentioned earlier, is the absolute maximum, but it is basically spec'd for a recommended max of 125 deg C). But the datasheet does seem to allow use up to 150 deg C internal JUNCTION temperature.

My figures a bit too high, for two reasons. One is that the ripple voltage will tend to reduce the dissipation (I've ignored ripple), and you may have succeeded in getting better than 1 deg C per watt, between the transistors case and the heatsink.

[bdunham7]

My figures a bit too high, for two reasons. One is that the ripple voltage will tend to reduce the dissipation (I've ignored ripple), and you may have succeeded in getting better than 1 deg C per watt, between the transistors case and the heatsink.

I was going to say...

The filter cap voltage might be 21-22 volts no load, but there's no way it is that high at full load and you can't use his AC voltage measurement x 1.414 to determine it--it will be  much lower.  Also, I had understood that he was measuring T_case, so that the case-to-heat sink junction wouldn't be included in the T_j calculation.  And yes, 1K/W would be a crummy installation, should be a third of that or less.

[MK14]

I was going to say...

The filter cap voltage might be 21-22 volts no load, but there's no way it is that high at full load and you can't use his AC voltage measurement x 1.414 to determine it--it will be  much lower.  Also, I had understood that he was measuring T_case, so that the case-to-heat sink junction wouldn't be included in the T_j calculation.  And yes, 1K/W would be a crummy installation, should be a third of that or less.

I agree, the voltages will usually be lower, than I stated, because it will drop a fair bit at full load. In fact, I think he is actually overloading the transformer, at that point. So, the voltages will tend to be even lower, still.

On re-reading it (I incorrectly thought he had specifically said it was the heatsink temperature he was measuring).

[ledtester]

I think it would be a good exercise to plot measurements of the transistor case temperature (at equilibrium) versus various output voltage and loads.

You can start small -- e.g. 16V at 100mA, 200mA, 300mA, 400mA and then move up to higher power dissipation, e.g. 12V and 8V. Record the transistor case temperature after it has plateaued -- this will probably take only a couple of minutes at the most.

The graph of temp vs. output current for each choice of output voltage should be a straight line and from the line you can determine the thermal resistance between the transistor case and ambient air -- i.e. the effectiveness of your heatsink.

[Electro Fan]

Thanks for all the analysis and info.

The temp varies somewhat depending on the specific spot being measured on the transistor or heat sink etc.  The numbers I gave are the hottest temps - generally (but not always) right on or neat the screw that holds the transistor to the heat sink.

[MK14]

The main issue (assuming you are below the transistor's maximum junction temperature), is the hotter the power transistor gets to, the shorter its life time. It is best to not run it too hot, otherwise, it will fail eventually.

As a rule of thumb, the transistor life, drops by a factor of 2, for every 10 deg C, junction temperature rise.

So, if a transistor had a life of (ultra approximate figure, as I don't know how long it would last, really, at 100 deg C) 10,000 hours at 100 deg C, it would be more like 5,000 hours at 110 deg C and 2,500 hours at 120 deg C. Etc. I'm referring to Junction (internal) temperatures.

They (power transistors) also tend to fail short circuit (but not always!), which can put the full output voltage onto your circuit, with potentially unpleasant results (or pleasant firework demonstration, for around 5 seconds, then unpleasantness  ).

[Electro Fan]

I think it would be a good exercise to plot measurements of the transistor case temperature (at equilibrium) versus various output voltage and loads.

You can start small -- e.g. 16V at 100mA, 200mA, 300mA, 400mA and then move up to higher power dissipation, e.g. 12V and 8V. Record the transistor case temperature after it has plateaued -- this will probably take only a couple of minutes at the most.

The graph of temp vs. output current for each choice of output voltage should be a straight line and from the line you can determine the thermal resistance between the transistor case and ambient air -- i.e. the effectiveness of your heatsink.

I’ve done some of this and so far the heat seems the greatest when DC voltage and current (into a 1 Ohm test load) reach a point that causes AC transformer current to rise past 2.5 Amps to 3.0 Amps and beyond.  Don’t know if it’s a straight line but based on what I’ve observed so far I can believe it might be.

One other observation is that as the transistor approaches the ~60 C max the Fluke IR temp shows more fluctuation than I noticed in the lower (40s C) temp (when DC was at 1.5 V and A into the 1 Ohm load).  I’m wondering if the several degree fluctuations around 60 C are reflecting the non-sinusoid AC ?  Just a wild guess.  Or maybe something else is starting to stress with some oscillation.

My interest in this is to learn more along the lines of what you guys describe/explain so well and to see if there is a way to raise the performance a step at a time by finding and upgrading the weakest link.

[Electro Fan]

The main issue (assuming you are below the transistor's maximum junction temperature), is the hotter the power transistor gets to, the shorter its life time. It is best to not run it too hot, otherwise, it will fail eventually.

As a rule of thumb, the transistor life, drops by a factor of 2, for every 10 deg C, junction temperature rise.

So, if a transistor had a life of (ultra approximate figure, as I don't know how long it would last, really, at 100 deg C) 10,000 hours at 100 deg C, it would be more like 5,000 hours at 110 deg C and 2,500 hours at 120 deg C. Etc. I'm referring to Junction (internal) temperatures.

They (power transistors) also tend to fail short circuit (but not always!), which can put the full output voltage onto your circuit, with potentially unpleasant results (or pleasant firework demonstration, for around 5 seconds, then unpleasantness  ).

Roger on heat and life span.  I’m trying to go easy on the PS while measuring and studying it.  It has run so well since it first turned on that I’m starting to develop an affinity for it - especially since this design had some skeptics - it’s kind of become an over performing under dog that you guys helped nurture and train up and I’d like to strengthen it further by finding and upgrading it’s weakest parts - so I would like to avoid fatiguing out it’s useful life.  It’s not my robot friend but it is kind of cute looking and sounding (with the fan). 

[Jwillis]

Congrats on your success .
One of the great things about this circuit is its really easy to switch out the power transistor to a higher power one without many problems. And because it uses dual rail power to the op amps you can change them to higher voltage ones directly with only moderate component changes in the circuit .
It's kind of fun to just experiment with and expand upon. Nice little learning kit.

[Electro Fan]

New finding, and another question.

A big part of the testing and learning process seems to be tracking down what causes heat.  Mostly I have been looking at temps in the PS at various voltage and current settings with various loads, but I've also been measuring load temps.

Along the way I noticed that when I use a 10 Ohm 100 Watt resistor as the load, at about 7 Volts and 700 mA DC (which is about 17.6V and 1.35 A AC), once everything warms up, the ~7 cm long resistor measures about 21C on one end of the resistor and 54 C on the other end of the resistor.  If I had thought about it I could potentially have imagined that the resistor would heat to ~50 C or so, but I don't think I would have guessed a ~33 C difference from one end of the resistor to the other end.

One question that comes to mind is should (for example) an 18 gauge solid copper wire heat to a lower temp than 18 gauge stranded wire for the same current put through both?   (I haven't found a table comparing solid vs stranded temps by AWG, or even a current carrying comparison table by AWG showing solid vs stranded.... in case anyone has a link.)  Thx

[ledtester]

but I don't think I would have guessed a ~33 C difference from one end of the resistor to the other end.

That suggests the resistive element is concentrated on one end rather than distributed evenly throughout.

I am very skeptical of the "100W" rating of those small power resistors. By comparison, here is a 120W 8 ohm power resistor made by Dale in the 70's:

[Jwillis]

One question that comes to mind is should (for example) an 18 gauge solid copper wire heat to a lower temp than 18 gauge stranded wire for the same current put through both?   (I haven't found a table comparing solid vs stranded temps by AWG, or even a current carrying comparison table by AWG showing solid vs stranded.... in case anyone has a link.)  Thx

https://www.engineeringtoolbox.com/wire-gauges-d_419.html

[Electro Fan]

One question that comes to mind is should (for example) an 18 gauge solid copper wire heat to a lower temp than 18 gauge stranded wire for the same current put through both?   (I haven't found a table comparing solid vs stranded temps by AWG, or even a current carrying comparison table by AWG showing solid vs stranded.... in case anyone has a link.)  Thx

https://www.engineeringtoolbox.com/wire-gauges-d_419.html

Thanks - that chart looks even better than what I was looking for....

I'm guessing that Single Core means solid wire and cores means "strands" of wire that add up to the AWG?

If that's the case the chart is pretty/very interesting.  For example, 16 AWG solid copper is rated for a load of 15 Amps but when it's 3 strands it drops to 10 Amps and to 7 Amps if it has 7-24 strands.  Do you think that's what it's saying, that cores = strands?

Either way, thanks for finding and posting the link.

[Electro Fan]

It looks like changing wires here or there could potentially help the power supply minimize some heat and improve current capacity but I'm thinking the bigger opportunity to improve performance might be in finding a better transformer to get more headroom with the AC current, and thereby enable the power supply to handle more DC current?  So far it seems that the best I can get on the DC side is about 1.5 Amps because if I turn up the DC voltage to get more DC current the AC current is hitting the 40 VAC transformer's spec of 2.5 Amps.

I'm not sure where the threshold is in the rest of the PS and I don't want to blow up the PS but maybe there is room to move upward toward 18-24 Volts and more current handling capacity, or maybe it might also be worth looking at a toroid transformer?

Any further suggestions/education on the transformer selection process?  Thx

Edit:  saw this link in another thread:

https://www.hammfg.com/electronics/transformers/power/266

Looks like pretty much everything listed that is north of 16V 2.5A (40 VA) is going to push the budget for this experiment, but I guess I'll have to deal with that after I figure out the preferred transformer spec and model.

[Jwillis]

If that's the case the chart is pretty/very interesting.  For example, 16 AWG solid copper is rated for a load of 15 Amps but when it's 3 strands it drops to 10 Amps and to 7 Amps if it has 7-24 strands.  Do you think that's what it's saying, that cores = strands?

Either way, thanks for finding and posting the link.

Yes. Strands = cores.  Those amperage ratings are for a 30^oC ambient , for PVC-insulated wire . I believe PVC softening point is around 85 to 100^oC . Other wire will have different  current ratings based on the insulation . The type of wire you use depends on application and design .  Datasheets for the type of wire you use will tell you the except able current ratings and temperature .

You take fewer core losses with a toroid than with other types of core . 5- 10%  where EI cores can be as high as 15% .

The current I in amps is equal to the apparent power S in volt-amps, divided by the voltage V in volts:
I_(A) = S_(VA) / V_(V)

For example if you need     3 amps at 40VAC on your secondary the core will need to be 120VA .  This also applies to the primary as well . A 120VA core at 230VAC will draw about 500mA  . Like volts current is proportional between primary and secondary .
 
Not sure where you live but here's another place you can look for toroid transformers :https://www.antekinc.com/transformers/
I have a few of these and they are built really solid .

[MK14]

I'm not sure where the threshold is in the rest of the PS and I don't want to blow up the PS but maybe there is room to move upward toward 18-24 Volts and more current handling capacity, or maybe it might also be worth looking at a toroid transformer?

Here is one a member did, which does 40 V at 4 amps.

https://www.eevblog.com/forum/beginners/success-prototype-40v-4amp-mc1466l-power-supply-thankyou-everyone-that-helped-!/

I'm pointing it out to you, to hopefully illustrate the work, and changes needed, to cope with higher capabilities.

Note well, that it has FOUR output transistors. That is because a single output transistor (like you have), can only handle a limited amount of power dissipation.

At a quick glance at the circuit, I just linked to, it seems reasonably straight forward (to make).

EDIT: Also, it seems to use 2 separate secondary windings/taps. Which allows for the much higher output voltages, without having to panic about a design which doesn't really want to handle too high an output voltage.

Analogy: You just bought a 5 watt output audio amplifier, and now want it to do 7 watts.
So, some people carefully help you to achieve this.

In 2 weeks , you now want it to go to 10 watts, output. Which turns out to be a very tricky modification and needs lots of calculations and changes.

What I am trying to say, is that you would be better off, buying (or getting the circuit for) a 50 or 100 watt power amplifier. Problem solved. Rather than trying to start with a 5 watt one, and increasing the voltage/current/power, watching what gets too hot, and trying to continually modify it, all the time.

What you have fundamentally, doesn't handle higher voltages, without a number of modifications. Also, the output current is also fundamentally limited, because it only has one output transistor.

Can you tweak it and get a bit more from it ?
Yes.

But really, I would suggest thinking about getting a kit/circuit or whatever, which can handle higher output voltages and currents, that you are happy with.

Opinions can vary. Some may think tweaking it, to get a bit more is worthwhile (theoretically, you could tweak it and tweak it, to very high voltages and currents). But, I don't really agree.
Better, to use a design which is designed for suitably high voltages and currents.

EDIT2: Or read through the various threads about your psu, and perform ALL the higher voltage modifications. There seem to be at least 2 or more. I suspect the higher voltage mods will rule out increasing the output current rating, as it will make the output transistor, significantly hotter.
The output transistor, is already getting rather hot, I'm not sure you would get much more from it, as it stands.

[Electro Fan]

If that's the case the chart is pretty/very interesting.  For example, 16 AWG solid copper is rated for a load of 15 Amps but when it's 3 strands it drops to 10 Amps and to 7 Amps if it has 7-24 strands.  Do you think that's what it's saying, that cores = strands?

Either way, thanks for finding and posting the link.

Yes. Strands = cores.  Those amperage ratings are for a 30^oC ambient , for PVC-insulated wire . I believe PVC softening point is around 85 to 100^oC . Other wire will have different  current ratings based on the insulation . The type of wire you use depends on application and design .  Datasheets for the type of wire you use will tell you the except able current ratings and temperature .

You take fewer core losses with a toroid than with other types of core . 5- 10%  where EI cores can be as high as 15% .

The current I in amps is equal to the apparent power S in volt-amps, divided by the voltage V in volts:
I_(A) = S_(VA) / V_(V)

For example if you need     3 amps at 40VAC on your secondary the core will need to be 120VA .  This also applies to the primary as well . A 120VA core at 230VAC will draw about 500mA  . Like volts current is proportional between primary and secondary .
 
Not sure where you live but here's another place you can look for toroid transformers :https://www.antekinc.com/transformers/
I have a few of these and they are built really solid .

Thanks for the confirmation on strands = cores, and thanks again for the chart.  All very informative and helpful.

Also, thanks for the transformer explanations with the math.

And thanks too for the antekinc link.  Those look like a very good selection of toroid transformers.

 

[Electro Fan]

I'm not sure where the threshold is in the rest of the PS and I don't want to blow up the PS but maybe there is room to move upward toward 18-24 Volts and more current handling capacity, or maybe it might also be worth looking at a toroid transformer?

Here is one a member did, which does 40 V at 4 amps.

https://www.eevblog.com/forum/beginners/success-prototype-40v-4amp-mc1466l-power-supply-thankyou-everyone-that-helped-!/

I'm pointing it out to you, to hopefully illustrate the work, and changes needed, to cope with higher capabilities.

Note well, that it has FOUR output transistors. That is because a single output transistor (like you have), can only handle a limited amount of power dissipation.

At a quick glance at the circuit, I just linked to, it seems reasonably straight forward (to make).

EDIT: Also, it seems to use 2 separate secondary windings/taps. Which allows for the much higher output voltages, without having to panic about a design which doesn't really want to handle too high an output voltage.

Analogy: You just bought a 5 watt output audio amplifier, and now want it to do 7 watts.
So, some people carefully help you to achieve this.

In 2 weeks , you now want it to go to 10 watts, output. Which turns out to be a very tricky modification and needs lots of calculations and changes.

What I am trying to say, is that you would be better off, buying (or getting the circuit for) a 50 or 100 watt power amplifier. Problem solved. Rather than trying to start with a 5 watt one, and increasing the voltage/current/power, watching what gets too hot, and trying to continually modify it, all the time.

What you have fundamentally, doesn't handle higher voltages, without a number of modifications. Also, the output current is also fundamentally limited, because it only has one output transistor.

Can you tweak it and get a bit more from it ?
Yes.

But really, I would suggest thinking about getting a kit/circuit or whatever, which can handle higher output voltages and currents, that you are happy with.

Opinions can vary. Some may think tweaking it, to get a bit more is worthwhile (theoretically, you could tweak it and tweak it, to very high voltages and currents). But, I don't really agree.
Better, to use a design which is designed for suitably high voltages and currents.

EDIT2: Or read through the various threads about your psu, and perform ALL the higher voltage modifications. There seem to be at least 2 or more. I suspect the higher voltage mods will rule out increasing the output current rating, as it will make the output transistor, significantly hotter.
The output transistor, is already getting rather hot, I'm not sure you would get much more from it, as it stands.

Hi MK14, as always you have the right info - just what I seem to be seeking over the next horizon is what you consistently identify. 

When you say "one member" if I followed the link correctly it looks like that might be a reference to non other than Jwillis.  The schematic he posted looks like it is indeed a next up from my present power supply project.  I probably couldn't have appreciated that design until I'd built and studied the present design but now I think I can see the beauty in the design he posted.

I think I'm going to twiddle a bit more with the current design - I'm sure there is more I can learn from it - and then I'm inclined to embark on a new PS project.

As long as I have two excellent power supply teachers here, what would you and Jwillis recommend for a circuit board for the design he posted?  I'm not ready to use a software tool to layout a PCB and send it out for manufacture, so what's the next best approach?

Thanks again!

Edit/Update:  looks like the lineage traces to here:
http://www.learningelectronics.net/VA3AVR/circ/ps4002/ps4002.html

[Jwillis]

As long as I have two excellent power supply teachers here, what would you and Jwillis recommend for a circuit board for the design he posted?  I'm not ready to use a software tool to layout a PCB and send it out for manufacture, so what's the next best approach?

Thanks again!

Edit/Update:  looks like the lineage traces to here:
http://www.learningelectronics.net/VA3AVR/circ/ps4002/ps4002.html

Unfortunately the MC1466L and MC1566L  has been discontinued and is getting very difficult to acquire. There are versions done with discrete components  both  through hole and SMD . You can find both versions with board designs online .

[Electro Fan]

These are not them?

https://www.ebay.com/sch/i.html?_nkw=MC1466L&_trksid=m5467.l1311

[MK14]

I think I'm going to twiddle a bit more with the current design - I'm sure there is more I can learn from it - and then I'm inclined to embark on a new PS project.

That sounds like a good idea, for sure. Learn all you can. 

By learning a lot more about linear power supplies. It has, and will more so (the more you tweak it), make you appreciate and understand what you really need/want, and why.

As regards PCBs. In some cases, the PCB design has already been made, and is available as files, to freely download. Which you can send to the PCB production houses, to make for you.
Some of the PCB production houses, even have designs, which you can just request, and they will make and send them to you.
But, those are things for later.

Although I agree the MC1466L's, are very difficult to get hold of, it can be solved, let me explain.

Many power supplies these days, have moved towards so called, switch mode power supplies (smps), rather than linear power supplies, like you have just built.

So, because linear power supplies are a lot less popular, than they use to be. I can understand why, many of the ICs, that made the job of making them, easier, are not made any more.

But, many popular chips, are no longer available. In fact many of the through hole components, especially ICs, are disappearing.

But you could kindly ask on the buy/sell forum here. Someone might have one or can tell you a reliable ebay seller, or something, who has one.

Warning: Re-the link, you just posted, while I made this post.
Many of the chips are fakes, and either won't work (fake), are poor quality copies, or salvaged second hand parts, that may not work properly.
Rarely, they are genuine/working parts. It is tricky, to get the working/genuine ones, but not impossible.
It is a long and complicated subject, how to solve that problem.

Or use another design, which has parts which are easier to obtain. Don't panic, anyway!

[Jwillis]

They have been discontinued since 2002 . If they look like they have brand new shiny plastic with really bright white labelling I would question authenticity . Also they came in a ceramic package . If they don't have a white band around the perimeter where the pins are then they are not authentic.
Not to say they won't work , just don't expect longevity or accuracy .
Also check for MC1566L

[MK14]

They have been discontinued since 2002 . If they look like they have brand new shiny plastic with really bright white labelling I would question authenticity . Also they came in a ceramic package . If they don't have a white band around the perimeter where the pins are then they are not authentic.
Not to say they won't work , just don't expect longevity or accuracy .
Also check for MC1566L

I've just had a quick look. The supplied photograph looks promising. But, sadly the supplied photograph, and the chips you actually receive, don't always match.

But, these seem to be in ceramic packages, and have the white line, you seemed to talk about.

https://www.ebay.co.uk/itm/1PCS-VOLTAGE-AND-CURRENT-REGULATOR-IC-MOTOROLA-CDIP-14-MC1566L/273461231717

EDIT: On reflection, the name in the photograph, doesn't match the ebay sellers name. So I would be very wary (probably best to avoid). Also some of the feedback, seems to talk about apparently fake chips, which is another bad sign.

[Electro Fan]

Ok, thanks for all the heads-up on clone and fake ICs.

Looks like a couple steps are finding the right regulator, and then finding a schematic that could be turned into a PCB.  Might be doable.

As for the regulator, it appears that the MC1446L and MC1566L are similar but the 15 handles a much larger temperature range and has an instantaneous input voltage max of 35V (vs 30V for the 14).  There are probably some other differences but I'm guessing that if either could be found the 15 would be preferable?

Apparently the white line around the regulator for both the 14 and the 15 is a requirement but not a guarantee for authenticity?  Not sure if what's in the photo is real or not.... but it looks like it has the white line.

Other questions on Power Supply Project 2:

It uses two transformers?

Also, in the article labeled "Precision Power Supply © 1986 Doug Bedrosian and 2010 Tony van Roon" it says the PS is good up to 40V and 2A.  But Jwillis' March 2018 PS hit 3.98A and in a more recent project in the same thread (just yesterday) another user (j2lr) apparently reached 5A using a MC1466L.  I'm guessing these are variants of the original 1986/2010 design? 

http://www.learningelectronics.net/VA3AVR/circ/ps4002/ps4002.html

https://www.eevblog.com/forum/beginners/success-prototype-40v-4amp-mc1466l-power-supply-thankyou-everyone-that-helped-!/

Back on the existing power supply (Power Supply Project 1) .... any of the Antek toroids trip your trigger the most?

https://www.antekinc.com/transformers/

[MK14]

I don't know how good a make they are (not experienced them), but assuming they are reasonably good.
This one catches my eye (don't worry, I'll be more careful with those sharp dangly wires next time    )

https://www.antekinc.com/as-2212-200va-12v-transformer/

For the following reasons:

The 100 watt ones look nice, but I can't see any 2 x 12 V ones (but there are circa 20V ones).

But at 200 watts, they do 2 x 12V.

A pair of 12 V secondary windings, means that you can switch between 12 V and 24 V, AC.
24V AC seems to be the limit, if you make the various higher voltage changes to your existing setup, so 2 x 12 V is good.

The switching means that you can halve (approx) the heat dissipation in the output transistor(s). The switching can either be through a switch, or eventually through an automated relay (harder to do). Which automatically switches between 1 or both secondaries, in series, making either 12 V or 24 V.

200 watts gives you plenty of current. Too much current capabilities is usually better than too little. It means you can consider multiple output transistors, for even higher output currents.

But I only had a quick look. Don't take that initial one as set in stone, there are other options.

Also, the reason the PSU design uses 2 transformers, is so that the voltage regulator control can use a lower voltage, without depending on the output voltage. Hence the output voltage stage (second transformer), can be very high, as there is no need for the regulator IC to handle the high voltage. E.g. 125 Volts. So called 'Floating regulator' (technique).

[MK14]

They do, do a 100 watt dual 12 V secondary. Their website, didn't easily find/show it, though.

Here:
https://www.antekinc.com/as-1212-100va-12v-transformer/

The choice 100 watt vs 200 watt, depends on your requirements with the eventual output currents.

Approximately, the 100 watt should give you around 2.5 Amps DC (maybe a little bit more), and hence 5 Amps for the 200 watt model. Both at 24 V AC (2 x 12 V).

In theory, you can actually do more complicated designs, whereby the pair of secondaries at 12 V, are actually paralleled up, at the lower voltage setting. Hence doubling the possible output current, compared to the high/full voltage range. But at this stage, it is probably best to not do that.
E.g. You'd have to double the current capabilities of lots of things, to handle so much current. So, it is not so easy or straight forward. The vast bulk of power supplies, DON'T bother to use this method of increasing the output current.

[Jwillis]

To work out a rough guess on transformer secondary voltage , take your desired output voltage of your PSU and divide by 1.414 . For example if you want 30V at the output  30/1.414 = 21.2  so I would choose a 22V transformer.  Same number works to get a estimate of the filtered voltage after the rectifier and smoothing caps .  Secondary voltage X 1.414 = Filtered Voltage .

Give some over head on the secondary current and voltage on your transformer . This would insure stability at full power.
Its been some time since I looked inside that project and remember now that the transformer I had was inadequate to get the full 4A . Just haven't got around to changing it . It was one I ripped out of an old UPS  . The second  transformer only needs to be 25VA to 30VA . 10VA might be to small.
Also when choosing the power transistor you need to work out the SOA (Safe Operating Area) based on the voltage at the collector and not the voltage at the emitter.  So if the voltage is 40V at the collector and your PSU is set for a maximum of 30V , the transistors SOA must be calculated at 40V not 30V . 

[mikerj]

One question that comes to mind is should (for example) an 18 gauge solid copper wire heat to a lower temp than 18 gauge stranded wire for the same current put through both?   (I haven't found a table comparing solid vs stranded temps by AWG, or even a current carrying comparison table by AWG showing solid vs stranded.... in case anyone has a link.)  Thx

https://www.engineeringtoolbox.com/wire-gauges-d_419.html

Please note that "multicore" in that web page does not mean "stranded".  It means multiple conductors within a cable e.g. like a three phase power cable may have four cores.  The more conductors you bunch together in a cable, the hotter they get for a given current which limits the maximum safe current.

[Electro Fan]

One question that comes to mind is should (for example) an 18 gauge solid copper wire heat to a lower temp than 18 gauge stranded wire for the same current put through both?   (I haven't found a table comparing solid vs stranded temps by AWG, or even a current carrying comparison table by AWG showing solid vs stranded.... in case anyone has a link.)  Thx

https://www.engineeringtoolbox.com/wire-gauges-d_419.html

Please note that "multicore" in that web page does not mean "stranded".  It means multiple conductors within a cable e.g. like a three phase power cable may have four cores.  The more conductors you bunch together in a cable, the hotter they get for a given current which limits the maximum safe current.

Hi, thanks for your post.  I'm trying to understand the distinction between multicore and stranded.  If multicore means something like multiple solid conductors rather than relatively smaller/finer strands, I could see how there might be three or four or several such solid conductors in one cable but the chart refers to "43 and above".  That would seem to be a very thick cable if it was really 43 instances of one normal solid core within the one cable, especially for AWG 18 or AWG 16 or thicker. 

I had assumed, maybe incorrectly, that when a wire is listed as X AWG that X would hold constant regardless of whether it's one conductor or multiple conductors.  So for example, the chart shows 16 AWG as having a diameter of 1.3mm; so then if it had 10 conductors (whether called "cores" or "strands") I took this to mean each conductor would be 0.13mm in diameter; otherwise how could both versions (single and 10x) be considered 16 AWG?  If on the other hand, if it's really 10 x 1.3mm when we are referring to 16 AWG that would be something on the order of 13mm in total diameter (plus any insulation).

Either way, however, the chart is interpreted it seems that as multiple conductors (cores or strands) increase in number the current carrying capacity is reduced.

I'm probably missing some distinction(s).

Any clarifications/thoughts?  Thanks

[Jwillis]

One question that comes to mind is should (for example) an 18 gauge solid copper wire heat to a lower temp than 18 gauge stranded wire for the same current put through both?   (I haven't found a table comparing solid vs stranded temps by AWG, or even a current carrying comparison table by AWG showing solid vs stranded.... in case anyone has a link.)  Thx

https://www.engineeringtoolbox.com/wire-gauges-d_419.html

Please note that "multicore" in that web page does not mean "stranded".  It means multiple conductors within a cable e.g. like a three phase power cable may have four cores.  The more conductors you bunch together in a cable, the hotter they get for a given current which limits the maximum safe current.

OK cool . Thank you for clarifying that . So we can assume that equivalent gauge of stranded and solid wire will have more or less the same current capability. This would account for no actual current rating chart for stranded compared to solid .
But stranded wire of equivalent gauge of solid will have a slightly different resistance . Based on these charts the difference seems insignificant .
https://www.calmont.com/wp-content/uploads/calmont-eng-wire-gauge.pdf

[Electro Fan]

One question that comes to mind is should (for example) an 18 gauge solid copper wire heat to a lower temp than 18 gauge stranded wire for the same current put through both?   (I haven't found a table comparing solid vs stranded temps by AWG, or even a current carrying comparison table by AWG showing solid vs stranded.... in case anyone has a link.)  Thx

https://www.engineeringtoolbox.com/wire-gauges-d_419.html

Please note that "multicore" in that web page does not mean "stranded".  It means multiple conductors within a cable e.g. like a three phase power cable may have four cores.  The more conductors you bunch together in a cable, the hotter they get for a given current which limits the maximum safe current.

OK cool . Thank you for clarifying that . So we can assume that equivalent gauge of stranded and solid wire will have more or less the same current capability. This would account for no actual current rating chart for stranded compared to solid .
But stranded wire of equivalent gauge of solid will have a slightly different resistance . Based on these charts the difference seems insignificant .
https://www.calmont.com/wp-content/uploads/calmont-eng-wire-gauge.pdf

The Calmont chart is very helpful.  Between that and the enginneringtoolbox chart and the posted comments it answers all the questions.
Thanks mikerj and Jwillis.   

[Electro Fan]

To work out a rough guess on transformer secondary voltage , take your desired output voltage of your PSU and divide by 1.414 . For example if you want 30V at the output  30/1.414 = 21.2  so I would choose a 22V transformer.  Same number works to get a estimate of the filtered voltage after the rectifier and smoothing caps .  Secondary voltage X 1.414 = Filtered Voltage .

Give some over head on the secondary current and voltage on your transformer . This would insure stability at full power.
Its been some time since I looked inside that project and remember now that the transformer I had was inadequate to get the full 4A . Just haven't got around to changing it . It was one I ripped out of an old UPS  . The second  transformer only needs to be 25VA to 30VA . 10VA might be to small.
Also when choosing the power transistor you need to work out the SOA (Safe Operating Area) based on the voltage at the collector and not the voltage at the emitter.  So if the voltage is 40V at the collector and your PSU is set for a maximum of 30V , the transistors SOA must be calculated at 40V not 30V .

Some more questions about transformer dimensioning in general, and in particular for the original Power Supply Project #1 in this thread:

The specs that came with the kit called for a 24V transformer.  After some discussion in this thread I went with a 16V 2.5A (40VA) spec'd transformer.

fwiw, I have 4 DMMs monitoring the PS:  1) transformer AC V, 2) transformer AC I, 3) PS DC V out, 4) PS DC I out.  Also, on the basic PS design is a LED that illuminates when the set DC current level is exceeded.  So far, the early warning indicator of threshold problems seems to be when the transformer AC I begins to approach 2.5A - at which point various temps start to rise significantly.

So, assuming that I am monitoring all this pretty closely, plus temperatures, what is the downside in using a bigger transformer, such as 100VA, 24V ? 

For example, this 100 VA 24V transformer might handle about 4A.
https://www.antekinc.com/as-1224-100va-24v-transformer/

So, my thought would be to try the 100VA 24V transformer and make sure the DC A never exceeds the 3A PS spec (maybe keep it at ~2.7A for some margin) and make sure the transformer never gets too close to 4A.  Then theoretically the PS might be happy?

Thx

[bdunham7]

Do you have an oscilloscope?

[Electro Fan]

Do you have an oscilloscope?

Not 19, but yes...

[bdunham7]

Then you should look at the AC transformer voltage and current, then the voltage across the filter cap (call that DC+ preregulator) and the output, obviously not all at the same time unless you have isolated channels or two different scopes.  Also you can look at the output voltage and the DC+ prereg at the same time and superimposed (same zero level) so as to see how close one gets to the other at the low points.  You'll learn a bit more about what is actually going on--just looking at RMS values is quite a bit short of the real story.

[Electro Fan]

Then you should look at the AC transformer voltage and current, then the voltage across the filter cap (call that DC+ preregulator) and the output, obviously not all at the same time unless you have isolated channels or two different scopes.  Also you can look at the output voltage and the DC+ prereg at the same time and superimposed (same zero level) so as to see how close one gets to the other at the low points.  You'll learn a bit more about what is actually going on--just looking at RMS values is quite a bit short of the real story.

Ok
10x probe, 1M Ohm scope settings

Transformer shows 40V peak to peak

Big cap shows 20V peak to peak

But on the DC output I'm somewhere between goofed up and flabbergasted (surprised and confused).  Where the DMM shows the PS is set for 1V (and another DMM is showing 101mA into a 10 ohm load) the scope is showing peak to peak of 20V when probing the DC output.   Maybe the scope is reading the AC instead of the DC - why would it do that at the DC output?

(Need some guidance on the current measurement with the scope and "look at the output voltage and the DC+ prereg" superimposed.)

Edit:  pretty much everything except the transformer reads about 20V peak to peak (actually a little over 21V peak to peak)

[bdunham7]

You'd better show a photo or draw a picture.  What do those waveforms look like?  Where is the ground clip attached for each of these measurements?

[Electro Fan]

Ok, duh - bad grounding clip attachment.

DC V output reads about 1.44V on the scope vs 1V on the DMM - definitely closer to being in the stadium ballpark as opposed to out in the parking lot, or across town at some other venue.

[bdunham7]

Ok, duh - bad grounding clip attachment.

DC V output reads about 1.44V on the scope vs 1V on the DMM - definitely closer to being in the stadium ballpark as opposed to out in the parking lot, or across town at some other venue.

That still isn't what it ought to be.  What scope and what does the picture show?

[Electro Fan]

Ok, duh - bad grounding clip attachment.

DC V output reads about 1.44V on the scope vs 1V on the DMM - definitely closer to being in the stadium ballpark as opposed to out in the parking lot, or across town at some other venue.

That still isn't what it ought to be.  What scope and what does the picture show?

Thx for the handholding.

Still not very confident - too many moving parts (measurements) for some reason, but now with the PS set for ~1V on DC output (0.997 on the DMM) I get a repeatable ~1.2V on a Rigol MSO2072A.  When I set the PS to ~2V (1.996V on the DMM) the scope shows ~2.24V.

[bdunham7]

I can't tell from the photo exactly where you are connected, but your ground must be somewhere other than the DC- output, like before the load sense resistor or at the part of the circuit that goes negative.  You should:

1) make absolutely sure that your circuit is totally isolated and not grounded anywhere and
2) connect the scope exactly in parallel to the DMM.

If you've already done that, then something is weird.

You can connect multiple channels safely as long as you only use one ground clip in total.  Just take the others off the probes.  The output, as long as it is clean and flat, is the least interesting part.  Look at the voltage across the filter cap, unloaded and with an increasing load. 

Then you can take a look at AC input current and voltage.  For that, I assume your two meters measuring the AC are set up so that one lead from each is at a common point?  If so, that is your ground, the other transformer lead is your voltage input and the other side of the current-measuring DMM is your current input (you'll have to increase the sensitivity on that channel and probably use a 1X probe).  That will be the most interesting for you to see as you increase load.

[Electro Fan]

I can't tell from the photo exactly where you are connected, but your ground must be somewhere other than the DC- output, like before the load sense resistor or at the part of the circuit that goes negative.  You should:

1) make absolutely sure that your circuit is totally isolated and not grounded anywhere and
2) connect the scope exactly in parallel to the DMM.

If you've already done that, then something is weird.

You can connect multiple channels safely as long as you only use one ground clip in total.  Just take the others off the probes.  The output, as long as it is clean and flat, is the least interesting part.  Look at the voltage across the filter cap, unloaded and with an increasing load. 

Then you can take a look at AC input current and voltage.  For that, I assume your two meters measuring the AC are set up so that one lead from each is at a common point?  If so, that is your ground, the other transformer lead is your voltage input and the other side of the current-measuring DMM is your current input (you'll have to increase the sensitivity on that channel and probably use a 1X probe).  That will be the most interesting for you to see as you increase load.

Thanks, I'm going to study this and then come back with some more questions.

I've watched Dave's video on how not to blow a scope several times but I'm betting I do not have it all properly absorbed.  I think this will be a good test to see how far I have yet to go on figuring out ground.

For starters, fwiw, the three pronged power supply transformer and the scope's three pronged plug are both on the same AC circuit.  The DUT being driven by the PS is just a 10 ohm resistor.  Pass/Fail?

[bdunham7]

As long as there is no continuity between the ground pin of the transformer plug and the secondary connections, you should be good.  As a further test, you should be able to connect the ground clip from the scope to either side of the AC secondaries with no sparkles.  Once that is accomplished, the main remaining issues are covered by using just one ground clip, avoiding the damage that would occur if you connected two ground clips (which are shorted by the connection to the oscillloscope) to two points with voltage between them.

Just remember that on a 4 channel scope, the high side inputs of any two channels are separated by at least 2M of resistance, while the ground connections are all connected by about zero ohms.  You can 'short out' your circuit with 2 megohms and it likely won't care, but if you short it with 0 ohms, it sparkles and crackles.

[Electro Fan]

Thanks for the continued education!

Attached is a photo of the transformer.  You can see two heat shrunk spade connectors - red and black.  I'm assuming these are what you are referring to as the secondaries.  I tested continuity between the ground prong and both of the secondaries (first, ground to one secondary, and then ground to the other secondary) and there is no continuity.  There is of course continuity between the ground prong and the center tap between the two outside secondary taps.

fwiw, as complex and confusing as "ground" is, continuity is the opposite, it is simple and straight forward and very useful - it is one of my favorite tests, and I highly recommend it to other beginners

Back on the transformer.  When I first hooked it up I looked and saw that neither of the outside taps (the secondaries) had any markings for plus or minus.  So, I just figured they were two halves an AC sine wave (or sorta kinda near sine wave).  When I hooked it up with that assumption everything ran well, or seemed to but I noticed that when I put two DMMs on the secondaries to measure voltage, the two DMMs didn't agree very well (maybe a volt apart on ~16-18V).  Later, when I switched the secondaries (just reversed them in how they connected to the PS) the two DMMs came into alignment.  Any chance that one of the secondaries is in fact "hot" and the other is "neutral" or "common" even though they are not marked?  And how would a rookie know which is which?  I'm guessing the answer is that I should have noticed that even though the two flat plug blades look identical (rather than one being slightly larger than the other) that one of them is oriented so as to fit into a hot receptacle (as dictated by the location of the ground prong) and the other is the neutral (ie, common?).  I'm pretty sure that's the answer, and that in fact there is/was something different about how the transformer and/or the PS behaved when I made the swap.

So, I think I now have the 100 level stuff somewhat under control and I'm ready to move on to 101.

Moving on to oscilloscope ground clips.  I understand that if I am probing two different parts of the circuit with two probes you really want only one ground clip to be used.  Understood.  But, just for the record, if two probes from the same scope were used to attach to the exact same points in the circuit (both probe tips on a particular point and both ground clips on the same other point) would it then be safe to use both ground clips?  I think the answer is "yes, but there is no sense in risking it."  Yes?  Or would even this scenario present a problem?

Thanks again for all the helpful Q&A and good teaching.

[Electro Fan]

Ok, getting a little adventurous here when I should probably be sticking with 100 and not trying to learn 101 on my own, but here are three more oscilloscope photos.  Let me know if the third one should be telling me something (like don't do that); otherwise why is it showing 52V Pk to Pk?  Thx!

[bdunham7]

Attached is a photo of the transformer.  You can see two heat shrunk spade connectors - red and black.  I'm assuming these are what you are referring to as the secondaries.  I tested continuity between the ground prong and both of the secondaries (first, ground to one secondary, and then ground to the other secondary) and there is no continuity.  There is of course continuity between the ground prong and the center tap between the two secondary taps.

I hadn't seen the transformer before and I'm just a little worried about the 'center tap' thing.  If it is an actual secondary center tap and is also connected to the ground pin, then you have a problem.  I suspect it is just a ground connection.  So use a higher resistance range, higher than continuity, and measure from each secondary to the center screw.  You should get overload or open or whatever your meter indicates for high resistance.  Ideally this resistance should be higher than your meter is capable of measuring--if not, report back before proceeding!

Back on the transformer.  When I first hooked it up I looked and saw that neither of the outside taps (the secondaries) had any markings for plus or minus.  So, I just figured they were two halves an AC sine wave (or near sine wave).  When I hooked it up with that assumption everything ran well, or seemed to but I noticed that when I put two DMMs on the secondaries to measure voltage, the two DMMs didn't agree very well (maybe a volt apart on ~16-18V).  Later, when I switched the secondaries (just reversed them in how they connected to the PS) the two DMMs came into alignment.  Any chance that one of the secondaries is in fact "hot" and the other is "neutral" or "common" even though they are not marked?  And how would a rookie know which is which?  I'm guessing the answer is that I should have noticed that even though the two flat plug blades look identical (rather than one being slightly larger than the other) that one of them is oriented so as to fit into a hot receptacle (as dictated by the location of the ground prong) and the other is the neutral (ie, common?).  I'm pretty sure that's the answer, and that in fact there is/was something different about how the transformer and/or the PS behaved when I made the swap.

If the transformer is properly isolated, there should be no 'polarity' to the output, or in other words, it shouldn't matter which side is which.  If you observed a difference, the most likely explanation I can see is that your power supply derives the auxiliary negative rail from a half-wave tap and this puts a DC bias on the AC circuit as well as a small distortion.  One of your meters may react differently than the other to this bias (or distortion, although that should be small) and thus the difference.  You could try measuring the AC voltage of the transformer connected as before, but try it both ways with both meters and compare the 4 results.  If this theory is right, one meter will give you the same result both ways, while the other will give you two different results.  Or my theory might not be the right one and there might be some other issue.

Moving on to oscilloscope ground clips.  I understand that if I am probing two different parts of the circuit with two probes you really want only one ground clip to be used.  Understood.  But, just for the record, if two probes from the same scope were used to attach to the exact same points in the circuit both for the probe tip and the ground clip, would it then be safe to use both ground clips?  I think the answer is "yes, but there is no sense in risking it."  Yes?  Or would even this scenario present a problem?

As long as both ground clips are at the same place, there are no sparkles even if the probe tips go to two different spots with very different voltages.  However, having an extra ground clip roaming around is a pointless risk in this case because at the low frequencies involved, there's no benefit from extra grounds. 

[bdunham7]

Ok, getting a little adventurous here when I should probably be sticking with 100 and not trying to learn 101 on my own, but here are three more oscilloscope photos.  Let me know if the third one should be telling me something (like don't do that); otherwise why is it showing 52V Pk to Pk?  Thx!

What are you referring to as 'secondary hot' and 'secondary neutral'?  If the secondaries are not ground referenced, as I hope, neither is neutral.  If there were a grounded center tap, then you would have a balanced (or split phase) system and both outer screws would be 'hot' (but not very hot!).  FWIW, if you don't connect both the tip and at least one ground, all you are reading is capacitively coupled noise and that could be anything.  If you have connected the scope ground to the black wire and the tip to the red one for photo #3, then you are getting what you would expect.  52V_p-p ≈ 18.5V_rms.

[Electro Fan]

Attached is a photo of the transformer.  You can see two heat shrunk spade connectors - red and black.  I'm assuming these are what you are referring to as the secondaries.  I tested continuity between the ground prong and both of the secondaries (first, ground to one secondary, and then ground to the other secondary) and there is no continuity.  There is of course continuity between the ground prong and the center tap between the two secondary taps.

I hadn't seen the transformer before and I'm just a little worried about the 'center tap' thing.  If it is an actual secondary center tap and is also connected to the ground pin, then you have a problem.  I suspect it is just a ground connection.  So use a higher resistance range, higher than continuity, and measure from each secondary to the center screw.  You should get overload or open or whatever your meter indicates for high resistance.  Ideally this resistance should be higher than your meter is capable of measuring--if not, report back before proceeding!

Probably ok - and just being cautious - but reporting back before proceeding:

....for the sake of completeness:

From AC blade to AC blade = ~18.5 ohms
From either AC blade to Ground Pin = .0L
From either AC blade to either secondary tap = .0L
From Ground Pin to center tap = ~0.56 ohms
From center tap to either secondary tap = .0L
From one secondary tap to other secondary tap = ~0.8 ohms

- all measurements with 2 wire; I could do better with 4 wire if you want.

Let me know if the above measurements are ok but it looks like the center tap is just a ground connection.

I'll get going back on your other questions and comments.  Thx 

[Electro Fan]

Ok, getting a little adventurous here when I should probably be sticking with 100 and not trying to learn 101 on my own, but here are three more oscilloscope photos.  Let me know if the third one should be telling me something (like don't do that); otherwise why is it showing 52V Pk to Pk?  Thx!

What are you referring to as 'secondary hot' and 'secondary neutral'?  If the secondaries are not ground referenced, as I hope, neither is neutral.  If there were a grounded center tap, then you would have a balanced (or split phase) system and both outer screws would be 'hot' (but not very hot!).  FWIW, if you don't connect both the tip and at least one ground, all you are reading is capacitively coupled noise and that could be anything.  If you have connected the scope ground to the black wire and the tip to the red one for photo #3, then you are getting what you would expect.  52V_p-p ≈ 18.5V_rms.

Ok, this shows I need to get the multipliers (like .707, 1.414, and 2.828) better at hand.  I should have recognized the relationship between 18.5V and ~52V.   Also, it might have helped me avoid coming up with the theory about a hot and a neutral secondary.  I will try the 4 measurements you suggested regarding your explanation...

~that the power supply derives the auxiliary negative rail from a half-wave tap and this puts a DC bias on the AC circuit as well as a small distortion.  One of the meters may react differently than the other to this bias (or distortion, although that should be small) and thus the difference.  Try measuring the AC voltage of the transformer connected as before, but try it both ways with both meters and compare the 4 results.  If this theory is right, one meter will give you the same result both ways, while the other will give you two different results~

- New question: once the primary AC is converted to the secondary AC is the secondary AC floating (ie, not referenced to ground)?  Or does that depend on whether the center tap on the secondary side is used?

Thx!!

[Jwillis]

According to the data sheet for your transformer  the centre terminal is earth ground  and the secondary is (PTC) Fused . PTC (positive temperature coefficient) thermistor increases resistance as temperature rises. Used as a current limiter to protect transformer secondary from over loading .
ELKproductsPDF Page 30

 Since you have no continuity between the any of the prongs to any of the secondary terminals , the secondary is isolated from mains . The centre terminal is an earth ground not a  centre tap for secondary.
The secondary is isolated or floating from earth ground . With such low voltages you need not reference any terminal to ground.