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....