Variable Power Supply

[Mati256]

Hello. I'm looking to build a variable power supply to replace the "wall wart" I'm currently using. I have build one before, but my notes are lacking some specifications for some reason.

I have googled around and I can't find a good schematic to build one, I don't know what I'm doing wrong.
Finding components where I live is no easy task, they usually stock some common components, nothing too fancy.
For example, I wanted to build this Supply from Make Magazine: http://makezine.com/projects/0-24-Volt-2-Amp-Bench-Top-Power-Supply/
But I can't find a BR805D Bridge Rectifier in the electronics store. 
I want to know with what individual diodes I can replace this bridge?
What about that mini volt meter? I have never used one, would any voltmeter be useful in this situation?

Also, if you happen to know a better, simpler VPS, please point me to the schematics.
What I'm actually looking for is a 12V or 20V 1A VPS with a voltmeter display.

Thanks

[liquibyte]

BR805D = 2A 50V bridge rectifier.  Over thinking leads to madness.  1N4001 - 1N4007 gets you there.  Mini Voltmeter on Ebay.

Sometimes you just have to experiment with component values based on what you have on hand or is readily available.  If you toggle your country flag, you'll get much more useful information from the various folks that are more local to you.

[Mati256]

Thanks liquibyte I was guessing I could use a 1n4007, but looks like the BR805D Max. Average Rectified Current is 2A and the 1n400X is 1A? Is that ok?

[denelec]

I built a similar power supply more than 25 years ago.  It still works.
You can use any 2A, 50V or more rated diodes or bridge.
Use a big heatsink on the LM317. And make sure your case has ventilation holes.
And add a fuse for safety.

I think any voltmeter should do.  I made mine with a recycled needle VU meter.  Same for the ammeter.

[liquibyte]

Thanks liquibyte I was guessing I could use a 1n4007, but looks like the BR805D Max. Average Rectified Current is 2A and the 1n400X is 1A? Is that ok?

Since the max of the 317 is 1.5A I'd say on average you'd probably never want to draw more than 1A continuous as a small supply anyway.  Others with more experience than I will probably give you much more detailed info but I've used the 1N400x diodes many times in cases like this without issue.  If you really need a 2A diode, try using the parametric search on any of the electronics parts supply houses like Digikey or Mouser.  There are almost too many choices with discrete components and the best way to learn is to learn to read the datasheets.  It's daunting for a newbie but eventually you'll get to the point where you'll understand things enough to make informed choices.  Of course, you could always use a 1N540x series rated at 3A.

[homebrew]

http://makezine.com/projects/0-24-Volt-2-Amp-Bench-Top-Power-Supply/

That sir, is a VERY CRAPPY build instruction!
It is dangerous for various reasons:

(1) No fusing whatsoever, neither on the primary side, nor on the secondary side. I have my doubts that the transformer shown even has a temperature fuse in it nor that it would be inherently short circuit proof.

(2) Ridiculous case! Wood is definitively not recommended to build enclosures that house components operating on the mains...

(3) Standard LM317 are only rated for 1.5A not 2.0 (http://www.ti.com/lit/ds/symlink/lm117.pdf)

It has severe functional limitations:

(4) Minimum voltage would be 1.25V

(5) The pisspoor heatsink shown would run red hot on some 100mA! (And set the wood on fire  )

There are however tons of good DIY projects out there.
For example: http://hpm-elektronik.de/nt30-4-netzteil.htm

Or just use the google image search to find tons of good schematics. (google for variable voltage current power supply schematic)

Play safe!

[macboy]

100% agree with homebrew. That  design, and especially the construction used by the author, are complete garbage. The heatsink is a bad joke, the lack of fusing is dangerous, and the over-rating of the LM317 shows that the author knows less than he purports to know.

[liquibyte]

Yeah, sorry I didn't even look at the article but did a bit of searching based on OP's first post.  There are good and bad 317 circuits out there and this is one of the poorly thought out ones.  To be honest, there's way too many 317 versions out there because they're so easy to make and very few of them center around safety concerns or even good explanations as to why what is being done is why the way it is.  Not only is fusing important but earthing the chassis and transformer the right way are as well.  The ubiquitous LM317 is a great chip in my opinion though.  I keep a stock of them on hand for breadboarding stuff up but to be honest, they don't make great variable supplies for the most part but that doesn't mean that it's not possible.  No one here has ever really put forth a great hobby design to be honest even though bad ones flow through all the time.  Perhaps we need to put together a low part count, solid performing first build for the new folks here and point them to that build when these things get proposed.  I've yet to see anything like that offered even though it gets mentioned rather often.

As far as this build goes, isn't that fuse for the variable side a little underrated at 1A for a 5A transformer?

[homebrew]

As far as this build goes, isn't that fuse for the variable side a little underrated at 1A for a 5A transformer?

Why should that be the case? 1A at 230V gives approx. 230W. That should be plenty of power for the main transformer's output of 28V @ 5A ...

[homebrew]

No one here has ever really put forth a great hobby design to be honest even though bad ones flow through all the time.  Perhaps we need to put together a low part count, solid performing first build for the new folks here and point them to that build when these things get proposed. 

Great idea! I would immediately be willing to contribute. But what would be the specs?
Something like 0-15V @ 1.5A linear regulated. Maybe some additional fixed voltages, too like +/- 15V and 5V for analog stuff and logic ...

[liquibyte]

No one here has ever really put forth a great hobby design to be honest even though bad ones flow through all the time.  Perhaps we need to put together a low part count, solid performing first build for the new folks here and point them to that build when these things get proposed. 

Great idea! I would immediately be willing to contribute. But what would be the specs?
Something like 0-15V @ 1.5A linear regulated. Maybe some additional fixed voltages, too like +/- 15V and 5V for analog stuff and logic ...

I'd say that a first build is usually a single variable supply, I know mine were.  Then I started doing things like 317/337 stuff and then graduated into learning ways to increase the amps.  One of the issues that's always run into with the 3 terminal regulators though is heat, one of the often underestimated side effects of doing these designs.  If something that has multiple outputs is proposed, I'm sure it would just be passed over for something simpler for a first build.

I'd like to see a solid 0-20V @ 0-1 or 2A variable voltage and current design with short circuit protection that doesn't suffer from the issues I've been running into with the design I've been trying to fix.  What I often see is that people start out looking at 12,15, or 18 volt at 1 amp and then as research progresses they inevitably graduate to higher voltages anywhere from 20 to 30 or even 50 volts at 2, 3, or 5 amps or more.  More often than not, this is in the belief that bigger is always better and that room to spare is a good thing.

Eventually you end up knowing what you're after and start lowering those expectations back down into something reasonably realistic for working with, say, op amps and the like.  20V @ 2A is usually plenty of room for anyone just starting out but a design that did 2X this with a tracking mode plus a 5V rail might be a good thing if it was reasonably simple to build and cheap enough to put together.

[Vito_R]

You might want to consider going with a LM350/K which can do up to 3 amps.  I built one of those over 25 years ago too still using it today.  You might need a bigger transformer and higher amperage bridge rectifier of course but for the small difference in cost you benefit from the flexibility of having 3 times more current.

[Vito_R]

hmmm im building an upgrade : 0-24v, 0-5A variable current limit ... the PCB is on the way from SEEEDstudio
the 5A is lm317 piggy backed with power PNP. one of those generic 5A types
current limiting is using PMOS, but ... im not sure if the PMOS will work, its a conversion from a simulation
this is the pic of the heatsink to be tested on (10cmx15cmx3cm) ... ribbed fins. this is actually the most important component i think 
the supply comes from a generic 24V 200w PWM power blocks adjusted upwards to about 28v
the worse part of any power supply is sinking heat ... its like vomit ! arrgh

Nice design, are you adding a fan or just sticking with convection cooling?

[liquibyte]

oh def a fan ... @ full draw = more than 125w of very angry electrons
and it will be inside a plastic "bin" of sorts lol
(upgrade from this "bin" of volts not bad for 1.5years of use , lm317 7805 n a 7812 http://3roomlab.blogspot.sg/2013/04/036-most-unlikely-psu-mod-lol.html)

Man, that's some ghetto right there.  Hey it works though right?

[Mati256]

Thanks for all the answers, that schematics proposed is way more complicated than what I'm looking for.
It's great to see my simple question sparked this design idea, I might wait and see what comes out of it and build that.
But, if you happen to come across some neat design, please let me know.

[Mati256]

No, I'm not giving up. But I'm still looking for an approachable Power Supply schematic, the one posted by homebrew is too overwhelming for me. I'm looking for something like the MAKE Powe Supply but safer. 

[liquibyte]

Thanks for all the answers, that schematics proposed is way more complicated than what I'm looking for.
It's great to see my simple question sparked this design idea, I might wait and see what comes out of it and build that.
But, if you happen to come across some neat design, please let me know.

Here, try this one.  It's got a few issues that we're still working on but it works rather well for the most part and should be a good first power supply.  If you really don't want to take the time to have the boards made or don't quite have the confidence to try this out on a perfboard or something similar, I can send you a board I had made.  I keep trying to get someone with the right test equipment to build and test this and help us figure out where the failure on the transient is coming from but I've had no takers so far so I'm willing to give the last two boards I have left away to anyone that wants them.

[Vito_R]

An LM317 or LM350/K design would be a lot simpler then that for him. fuse, transfo, bridge rectifier, a few capacitors, LM 350 or K on a large heatsink, resistor, trim pot to control the voltage and you got yourself a power supply.  I built one when I was a kid and even etched my own very simple pc board at the time.  Was my first electronics project and it's still one of my regularly used power supplies today. The LM350K has built in short circuit protection/current limiting and reliable as hell, never failed on me.

[liquibyte]

what kind of failure on transient? can you illustrate via a ... ermmm graph?

There's a transient that occurs on power on.  Here's a screenshot from the scope of a guy that built one.

An LM317 or LM350/K design would be a lot simpler then that for him. fuse, transfo, bridge rectifier, a few capacitors, LM 350 or K on a large heatsink, resistor, trim pot to control the voltage and you got yourself a power supply.  I built one when I was a kid and even etched my own very simple pcb board at the time.  Was my first electronics project and it's still one of my regularly used power supplies today. The LM350K has built in short circuit protection/current limiting and reliable as hell, never failed on me.

The actual circuit isn't all that bad to build but is certainly more complicated than your average linear voltage reg circuit.  Plus, I offered to send a board, just add components.  The most expensive part is the transformer at about $30 or so.  Next up would be the two 10-turn pots I used that were $12 each and then the three trimmers at $4.62 each and then the three TLE2141's at about $2 each.  The output cap was $2.64 but a different cap could be used if wanted.  The 2N3055's were $2.34 and the design needs two.  The rest of the stuff is just bog standard things most people have laying around.

[kxenos]

Have you tried to delay the power up of the op-amps for 40-50mSec?

[liquibyte]

Have you tried to delay the power up of the op-amps for 40-50mSec?

Yep, I even worked out this PFET circuit for it.  I haven't had the chance to try it yet though, too much going on right now.

[homebrew]

Here, try this one.  It's got a few issues that we're still working on but it works rather well for the most part and should be a good first power supply.  If you really don't want to take the time to have the boards made or don't quite have the confidence to try this out on a perfboard or something similar, I can send you a board I had made.  I keep trying to get someone with the right test equipment to build and test this and help us figure out where the failure on the transient is coming from but I've had no takers so far so I'm willing to give the last two boards I have left away to anyone that wants them.

Nice supply!

But I can't wrap my mind around the supply of the current-limiting opamp. Why is the zener in series with the opamp supply? There will be a constant 10V drop over the zener but the supply of the opamp would not be regulated at all?

But apart from that, I also believe that the inrush-current of the main filter-cap is the cause of the glitch. The cap might have a quite low ESR and therefore the input voltage will drop a lot until the cap is charged. Hence the series pass transistors are saturated more under this condition. But a few ms later the cap is fully charged and the regulation circuit is not fast enough to compensate ...

My suggeststion: Insert some delay to turn on the output. Maybe as simple as an RC timing element and one FETto clamp the set voltage to zero or something the like ...

[richard.cs]

http://makezine.com/projects/0-24-Volt-2-Amp-Bench-Top-Power-Supply/

That sir, is a VERY CRAPPY build instruction!
...
(2) Ridiculous case! Wood is definitively not recommended to build enclosures that house components operating on the mains...

While I agree that it's a pretty crappy project for most of the reasons given I would argue against your point about the wooden case. A well made wooden case is less of a fire hazard than most plastic boxes, is mechanically stronger for something like a power supply with heavy transformers and is insulating enough to give good secondary protection against electric shock. That particular case isn't great but good cases can be made from wood. The most important thing to be careful of with insulating cases is to avoid conductive screws, transformer mounting bolts, etc passing through the enclosure unless they are prevented from becoming live by some other aspect of the design (being insulated on the inside or being earthed).

There are however tons of good DIY projects out there.
For example: http://hpm-elektronik.de/nt30-4-netzteil.htm

Agreed, way better. Though maybe a little complex for a beginner.

[liquibyte]

Here, try this one.  It's got a few issues that we're still working on but it works rather well for the most part and should be a good first power supply.  If you really don't want to take the time to have the boards made or don't quite have the confidence to try this out on a perfboard or something similar, I can send you a board I had made.  I keep trying to get someone with the right test equipment to build and test this and help us figure out where the failure on the transient is coming from but I've had no takers so far so I'm willing to give the last two boards I have left away to anyone that wants them.

Nice supply!

But I can't wrap my mind around the supply of the current-limiting opamp. Why is the zener in series with the opamp supply? There will be a constant 10V drop over the zener but the supply of the opamp would not be regulated at all?

But apart from that, I also believe that the inrush-current of the main filter-cap is the cause of the glitch. The cap might have a quite low ESR and therefore the input voltage will drop a lot until the cap is charged. Hence the series pass transistors are saturated more under this condition. But a few ms later the cap is fully charged and the regulation circuit is not fast enough to compensate ...

My suggeststion: Insert some delay to turn on the output. Maybe as simple as an RC timing element and one FETto clamp the set voltage to zero or something the like ...

The zener for the current control op amp is due to the negative rail, otherwise the total voltage would exceed the rated 44V.  I didn't design this one, I'm just trying to fix some of its issues.  I've always been concerned about being so close to the max of the op amps I'm using, especially when it comes to the transient.  I get 43.7V after rectification and filtering and the max input to the TLE2141 is 44V.  Hasn't seemed to damaged anything yet during testing but I always thought that putting all three op amps behind the zener would have been a better idea given the tight output.

I agree that the inrush is caused by the filter cap.  I get the same results as far as the transient goes whether I'm scoping the output of the PS itself or the output of the filter caps.  ESR on my caps according to the datasheet is 44 max at 100Hz, not that I entirely understand that number.

The PFET circuit was my attempt at a delay and regardless if it is a good idea or not it does simulate nicely.  I had initially settled on a soft start on the primaries but given the discussions I've had, I think it would really be better to eliminate the issue than hack around it.  3roomlab has given me a couple of ideas that I want to try out and I'll post if I get good results or not.  Simulations are nice but not always related to reality so some further testing is in order.

[homebrew]

While I agree that it's a pretty crappy project for most of the reasons given I would argue against your point about the wooden case. A well made wooden case is less of a fire hazard than most plastic boxes, is mechanically stronger for something like a power supply with heavy transformers and is insulating enough to give good secondary protection against electric shock. That particular case isn't great but good cases can be made from wood. The most important thing to be careful of with insulating cases is to avoid conductive screws, transformer mounting bolts, etc passing through the enclosure unless they are prevented from becoming live by some other aspect of the design (being insulated on the inside or being earthed).

Interesting point of view ...

Well I'm pretty sure it would not be compliant to any standard anymore (correct me if I'm wrong). But also no one instead of hobbyists are doing it nowadays (except the audio-people building boxes from it ...).
And if it burns ... it burns!

For my project I strictly use Metal enclosures only when it comes to mains stuff ...

[homebrew]

The zener for the current control op amp is due to the negative rail, otherwise the total voltage would exceed the rated 44V.  I didn't design this one, I'm just trying to fix some of its issues.  I've always been concerned about being so close to the max of the op amps I'm using, especially when it comes to the transient.  I get 43.7V after rectification and filtering and the max input to the TLE2141 is 44V.  Hasn't seemed to damaged anything yet during testing but I always thought that putting all three op amps behind the zener would have been a better idea given the tight output.

Hm, I guess I still don't get it ...
The negative rail should only be two diode drops below zero (approx -1.4V). With that zener, the Voltage of U1 is therefore approx. 8.6V below the supply voltage. But U1 (voltage control opamp) always sees the full supply voltage...

However, why not limiting the supply-voltage for all of the opamps to 30V with a standard tree-terminal regulator? 

[liquibyte]

The zener for the current control op amp is due to the negative rail, otherwise the total voltage would exceed the rated 44V.  I didn't design this one, I'm just trying to fix some of its issues.  I've always been concerned about being so close to the max of the op amps I'm using, especially when it comes to the transient.  I get 43.7V after rectification and filtering and the max input to the TLE2141 is 44V.  Hasn't seemed to damaged anything yet during testing but I always thought that putting all three op amps behind the zener would have been a better idea given the tight output.

Hm, I guess I still don't get it ...
The negative rail should only be two diode drops below zero (approx -1.4V). With that zener, the Voltage of U1 is therefore approx. 8.6V below the supply voltage. But U1 (voltage control opamp) always sees the full supply voltage...

However, why not limiting the supply-voltage for all of the opamps to 30V with a standard tree-terminal regulator?

The original circuit for this is rather old and from a Polish magazine if I'm not mistaken.  In the original the components were underrated by a lot so it was redesigned with modern op amps that could handle the 30V output, at least that's what I was told anyway.  The max rating for a TLE2141 is 44V so if you take the reading I get of 43.7 and add the 1.4 negative rail to U1 you'll be exceeding the op amp limit by 1.1V consistently which will lead to early failure. I've always been a little concerned about sitting so close to that limit myself but it works and I've had the thing running for hours at full load with no side effects.  I think the zener was picked as an arbitrary value to give a voltage drop and anything around 5V could also have been used as well to good effect but as I said, I didn't design this.  There have been a few versions that use a regulator both as an input voltage regulator for the op amps and as a voltage reference.  Regardless of that, the circuit still suffers from the transient and I even have a different version that I was sent for testing.

Since David Hess pointed me to the Q2 circuit in an older Tek supply, I've been looking at a few different designs to see how they dealt with things but I'm still rather new at this so it takes me awhile to grasp some of the concepts.  In the redesign, the Q2 circuit was pulled out but the supply then suffered from a turn off transient so I put it back in and, lo, the transient disappeared.  My final task in that area is to deal with the remaining startup transient.  Along the way I've learned a few things which is good but probably not enough.  This design keeps getting posted in many places so I'd like to make it as stable as possible to point people to when they ask about it.  Fun fact: the guy that did the bulk of the redesign has never built this.  To my knowledge, only hobbyists have built these and I'm starting to see why.  For all its faults though it works rather well to be honest.  I don't have enough knowledge or equipment to fully test this as far as it could be but I'm trying, as are several others.  If I could point to a different design that has been vetted I would.  I can't because nothing has ever really been put forth as a definitive go to circuit so I keep working on this one.  It's fairly simple and has current limiting that works well.  True, you won't get down to 0V without a load on the output but 37mV isn't too bad.  If I had to do this over again I'm sure that I'd still end up in the same place because no one has ever put anything else forward as a solid design.

[David Hess]

The absolute maximum ratings for operational amplifiers and other parts are just that and not the same as the operational ratings; running a part at the absolute maximum rating usually compromises reliability.  Typical operational amplifiers with an absolute maximum rating of 36 volts have specifications for +/-15 volt or 30 volt supplies and if you wanted to use a higher supply voltage like +/- 18 volts or 36 volts, then an operational amplifier with an absolute maximum rating of 44 volts should be used.

[AG6QR]

No, I'm not giving up. But I'm still looking for an approachable Power Supply schematic, the one posted by homebrew is too overwhelming for me. I'm looking for something like the MAKE Powe Supply but safer. 

The Elenco XP-720K is a very simple variable supply offered in kit form.  The instructions contain a schematic.  It has three supply rails.  Two of them are positive and negative variable, using LM317 and LM337 regulators, and the third is fixed 5V, 3A, using LM7805 plus an additional pass transistor.  The design is nothing special, and the positive variable rail (the one using the LM317) is almost straight out of the LM317 data sheet. 

The mechanical construction of this kit is good, with the regulators mounted on a big heat sink that is directly exposed to outside ventilating air.  The case is sturdy metal, and grounded.  The mains input is fused.  Unlike the MAKE supply, there are no obvious safety flaws that I can see.

This supply suffers the problem of only going down to 1.25V, like many LM317-based designs.  Another obvious shortcoming is that it does not have adjustable constant current limiting -- a feature which is extremely useful.

I've built one, and while it's not an innovative design, it is a fairly well-implemented and sturdy version of the standard LM317 type variable supply.  If you're thinking of building a 317-based variable supply of your own, this might be a source of ideas.  If you search around the web, you can see where different people have used this as a base and added modifications, including meters, 10-turn pots, and other enhancements.  The case has a fair amount of room inside for custom additions.

The assembly instructions and schematic are here:

http://www.elenco.com/admin_data/pdffiles/XP720Kweb.pdf

[homebrew]

Regardless of that, the circuit still suffers from the transient and I even have a different version that I was sent for testing.

Maybe a better simulation model might help to debug the circuit. I have attached you a full simulation of your supply in LTSpice. Feel free to mess around :-)

It has quite good regulation performance in voltage mode! However, in current mode it starts to oscillate ...

[liquibyte]

The absolute maximum ratings for operational amplifiers and other parts are just that and not the same as the operational ratings; running a part at the absolute maximum rating usually compromises reliability.  Typical operational amplifiers with an absolute maximum rating of 36 volts have specifications for +/-15 volt or 30 volt supplies and if you wanted to use a higher supply voltage like +/- 18 volts or 36 volts, then an operational amplifier with an absolute maximum rating of 44 volts should be used.

I know but to be honest I think the 2141 is a quality part and the ones I've got in the two supplies I built seem to be chugging along nicely.  Once I figure out how to smooth all this out I plan on fixing that part of it because I'm not comfortable running them at just under the limits.  I do get conflicting advice on that.  Most of the folks over there have said that they've been running these like this for years to no ill effect and just about everyone over here says don't do it.  It may be that a regulator is in order for at least the supply to those.

Maybe a better simulation model might help to debug the circuit. I have attached you a full simulation of your supply in LTSpice. Feel free to mess around :-)

It has quite good regulation performance in voltage mode! However, in current mode it starts to oscillate ...

I've been trying to get that done forever and thought I was just dense.  I really, really appreciate you taking the time to do that and I'll certainly put it to use and see what I can come up with by playing with it.  One question, why did you pulse a mosfet on the output?  So far in all my dealings with LTSpice IV I've only put a load on the output but to be honest I'm not very good with it yet.  I did try an simulate out a version of this but couldn't get it to work so I guess I was doing something wrong.  Good work, this is going to come in rather handy.  Thanks for doing this.

[homebrew]

I've been trying to get that done forever and thought I was just dense.  I really, really appreciate you taking the time to do that and I'll certainly put it to use and see what I can come up with by playing with it.  One question, why did you pulse a mosfet on the output?  So far in all my dealings with LTSpice IV I've only put a load on the output but to be honest I'm not very good with it yet.  I did try an simulate out a version of this but couldn't get it to work so I guess I was doing something wrong.  Good work, this is going to come in rather handy.  Thanks for doing this.

No worries ...
I use the mosfet to switch the output on and off repetitively. The voltage-source at the gate is programmed to do this every 200ms.
You could also use a current source. However you can not test current limiting with such a current source, as it WILL draw its programmed current ...
I also added a minimal load of 1K to discharge the output cap and another 470uF to simulate an average electronic project that might have a bulk capacitor on it. Capacitive loads are often a bit of a thread to regulation as they put another pole into the Bode-diagram and can hence cause instabilities.

One immediate thing I noticed while simulating your supply: The negative rail is very unstable and would really benefit from another 330uF cap after the clamping diodes ...

[liquibyte]

I've been trying to get that done forever and thought I was just dense.  I really, really appreciate you taking the time to do that and I'll certainly put it to use and see what I can come up with by playing with it.  One question, why did you pulse a mosfet on the output?  So far in all my dealings with LTSpice IV I've only put a load on the output but to be honest I'm not very good with it yet.  I did try an simulate out a version of this but couldn't get it to work so I guess I was doing something wrong.  Good work, this is going to come in rather handy.  Thanks for doing this.

No worries ...
I use the mosfet to switch the output on and off repetitively. The voltage-source at the gate is programmed to do this every 200ms.
You could also use a current source. However you can not test current limiting with such a current source, as it WILL draw its programmed current ...
I also added a minimal load of 1K to discharge the output cap and another 470uF to simulate an average electronic project that might have a bulk capacitor on it. Capacitive loads are often a bit of a thread to regulation as they put another pole into the Bode-diagram and can hence cause instabilities.

You've given me a great tutorial with this one.  I've been playing with it and fixed a few connections and a couple of values and have been inputting ESR values and such to see how it affects the simulation and to see if I can make it as accurate as possible based on the datasheets.  I've also tried changing a few things such as just using R=limit(10,V(n002)**2/90,10) for the load and adding a pulse voltage to simulate the spike.  I'm still working on trying for realistic results but what I'm seeing looks promising.  3roomlab's simulations gave me an idea so I changed the value of C6 from 100n to 2.2u to see what it would do and the output ramped up slowly hitting full voltage after the spike passed.  I'm going to play around with this in the real world to see if it has an effect without being deleterious to the rest of the circuit in unexpected ways but I think it could be a quick fix, at least for now.  I'd still like to figure out why it's doing what it is but this is a great start.  I should apologize to the member that I got snippy with about LTSpice because the more I learn, the more I realize how wrong I was about it.  It's still confusing but it's becoming less so.

Edit: I'm told this will screw up the current regulation.  Can anyone tell me why a non-inductive sense resistor would be better than the wirewound aluminum one I have now?  It has been proposed that this could be the cause of the initial spike but I'd like opinions before I spend $7 / resistor as replacements and still have the same issue.  Is there a way to minimize the effects of the inductance of the sense resistor on the circuit?

[David Hess]

have any circuit in real actually use PMOS to do this? many linear supply circuits are always BJT as limiting component for current, is PMOS thermal run-away the main problem that nobody use it?

The common source configuration shown in your schematic using a p-channel MOSFET (or PNP bipolar transistor) has voltage gain which depends on the load so it is more difficult to frequency compensate the error amplifier resulting in poorer transient response.  P-channel MOSFETs are also more expensive than n-channel MOSFETs for a given performance.

Edit: I'm told this will screw up the current regulation.  Can anyone tell me why a non-inductive sense resistor would be better than the wirewound aluminum one I have now?  It has been proposed that this could be the cause of the initial spike but I'd like opinions before I spend $7 / resistor as replacements and still have the same issue.  Is there a way to minimize the effects of the inductance of the sense resistor on the circuit?

If the inductance is high then it will affect the response of the feedback circuit but that does not seem likely with such a low value of resistance.

If you want to test it, short out the sense resistor and test for the startup surge again.

[liquibyte]

If the inductance is high then it will affect the response of the feedback circuit but that does not seem likely with such a low value of resistance.

If you want to test it, short out the sense resistor and test for the startup surge again.

I shorted the resistor and it had no noticeable effect on the transient so I guess that's not where it's coming from.  I've been playing around with the simulation and I notice that the output of the current regulator oscillates like the input does but that the other two op amps don't.  I haven't scoped this in the real world but it's certainly interesting as an experiment.

[kxenos]

The simulation has a difference with the original schematic in cathode of zener D9 the original has a supply there, the sim doesn't. Also, why do you need that 10k in the non-inv input with op-amp's output?

[liquibyte]

Actually, this is what I've been working with.  I've corrected a few issues and rearranged things to look more like the original schematic I used to build these so I could make sure things were hooked up right.  I'm attaching the .asc file as well if someone wants to play with it.

[David Hess]

I've been playing around with the simulation and I notice that the output of the current regulator oscillates like the input does but that the other two op amps don't.  I haven't scoped this in the real world but it's certainly interesting as an experiment.

This behavior is understandable and the reason I said earlier that I am not a fan of the cascading the current limit loop with the voltage limit loop.  If they are separate, then each can be frequency compensated for best performance.  If the current limit loop controls the voltage limit loop, then the added phase shift requires the current limit loop to be slower to prevent oscillation or instability.

[liquibyte]

This behavior is understandable and the reason I said earlier that I am not a fan of the cascading the current limit loop with the voltage limit loop.  If they are separate, then each can be frequency compensated for best performance.  If the current limit loop controls the voltage limit loop, then the added phase shift requires the current limit loop to be slower to prevent oscillation or instability.

After messing with this more, I understand better what you're talking about how it's not an ideal situation.  Since the only real issue that I can see is the transient issue, how would I go about separating the two and still have the same functionality it has now?  Also, does anyone have an idea on how to inject a spike on the input of this?  I've been trying to introduce a delay but when I use the way I came up with in the mosfet thread, the pulse stretches out in time with the delay which is really frustrating.

[kxenos]

Can you please remove C5 to see what happens?

[liquibyte]

Can you please remove C5 to see what happens?

Looks like things get wonky.  Sorry for the big pic but resizing it makes it look crappy.

[TunerSandwich]

"0 – 24 Volt, 2 Amp Bench Top Power Supply"

"Mini Volt Meter (ID: 460 from Adafruit Industries NOTE: These operate from +3.2V to +30V DC and are powered by the supply being measured) (1)"

sounds like a very accurate instrument...who needs buffering and isolation anyway ?

 

[David Hess]

This behavior is understandable and the reason I said earlier that I am not a fan of the cascading the current limit loop with the voltage limit loop.  If they are separate, then each can be frequency compensated for best performance.  If the current limit loop controls the voltage limit loop, then the added phase shift requires the current limit loop to be slower to prevent oscillation or instability.

After messing with this more, I understand better what you're talking about how it's not an ideal situation.  Since the only real issue that I can see is the transient issue, how would I go about separating the two and still have the same functionality it has now?

i did thought about the cascading problem. i dont quite understand how a separation can be done, in my mind, it seems both forms of regulation must happen at the same time and yet are separate entities in circuitry

The usual solution is to wire the outputs of the two error amplifiers together with some diode OR logic; then either amplifier can pull the output down to limit either the current or the voltage.  This causes the amplifier which is not currently controlling the output to saturate but recovery time is generally fast enough for this not to be an issue and the output transitions seamlessly between constant current and constant voltage mode.

[BradC]

(2) Ridiculous case! Wood is definitively not recommended to build enclosures that house components operating on the mains...

Wow, better tell almost every guitarist in the world they are carrying around a dangerous mains powered object. For that matter, most TV's pre late 60's and almost all antique radios and amplifiers.

Wood is perfectly satisfactory. Now the other points I agree with, but this one is lunacy of the highest order.

[homebrew]

(2) Ridiculous case! Wood is definitively not recommended to build enclosures that house components operating on the mains...

Wow, better tell almost every guitarist in the world they are carrying around a dangerous mains powered object. For that matter, most TV's pre late 60's and almost all antique radios and amplifiers.

Wood is perfectly satisfactory. Now the other points I agree with, but this one is lunacy of the highest order.

No, it isn't ...
There is a reason, why these amps are built on a metal chassis. There is also a reason, why domestic installations are not allowed in wood enclosures. And there is also a reason why today's TV's are not housed in wood anymore ...

Of course you can use woodern cases, if the mains components are inside a metal or plastic enclosure within the wood case or at least mounted on a proper chassis.

But hey, you can do whatever you want ... 

[rolycat]

(2) Ridiculous case! Wood is definitively not recommended to build enclosures that house components operating on the mains...

Wow, better tell almost every guitarist in the world they are carrying around a dangerous mains powered object. For that matter, most TV's pre late 60's and almost all antique radios and amplifiers.

Wood is perfectly satisfactory. Now the other points I agree with, but this one is lunacy of the highest order.

No, it isn't ...
There is a reason, why these amps are built on a metal chassis. There is also a reason, why domestic installations are not allowed in wood enclosures.

Then perhaps you should inform companies such as Tivoli Audio who still build domestic radios and hi-fi into wooden enclosures.

And there is also a reason why today's TV's are not housed in wood anymore ...

And that would be because making wooden housings is a labour-intensive, high cost process, and wood is a relatively heavy material unsuitable for large, lightweight flat-panel televisions. It has nothing to do with safety.

[rolycat]

guess what the tivoli EE will say about his PSU?

That it's poorly designed, incompetently constructed, and has pathetically inadequate heat sinking and ventilation?

[liquibyte]

guess what the tivoli EE will say about his PSU?

That it's poorly designed, incompetently constructed, and has pathetically inadequate heat sinking and ventilation?

This!  With a complicated design you're almost guaranteed to make mistakes or odd decisions and then have to work around that to modify things.  With a circuit as simple as this, there's no excuse for building it as sloppily as has been done.  I'm going to guess this guy doesn't do much in the way of electronics though his bio says that he does.

Michael Colombo

In addition to being an online editor for MAKE Magazine, Michael Colombo works in fabrication, electronics, sound design, music production and performance (Yes. All that.) In the past he has also been a childrens' educator and entertainer, and holds a Masters degree from NYU's Interactive Telecommunications Program.

Maybe I should start writing for MAKE, they don't seem to require much in the way of experience or knowledge apparently.

[jlmoon]

(2) Ridiculous case! Wood is definitively not recommended to build enclosures that house components operating on the mains...

Wow, better tell almost every guitarist in the world they are carrying around a dangerous mains powered object. For that matter, most TV's pre late 60's and almost all antique radios and amplifiers.

Wood is perfectly satisfactory. Now the other points I agree with, but this one is lunacy of the highest order.

No, it isn't ...
There is a reason, why these amps are built on a metal chassis. There is also a reason, why domestic installations are not allowed in wood enclosures. And there is also a reason why today's TV's are not housed in wood anymore ...

Of course you can use woodern cases, if the mains components are inside a metal or plastic enclosure within the wood case or at least mounted on a proper chassis.

But hey, you can do whatever you want ...

Last time I checked most appliances today are housed in poly carbonate structures that when heated emit some pretty nasty stuff!  So which is better?.. lol Paper(wood family)  or Plastic ?   I remember the old ("Works in a Drawer" ~ "Quasar"  a division of Motorola) TV's and yes the drawers were made of wood (pressed wood fibers - some call MDF) 

[homebrew]

Then perhaps you should inform companies such as Tivoli Audio who still build domestic radios and hi-fi into wooden enclosures.

Why? I don't have any knowledge how these radios are constructed internally. Maybe the PSU is housed in a metal compartment shielding it from the wooden case. Who knows ...

To put it as simple as possible:
Mains wiring and components have to be either in a mains earth grounded enclosure or they have to be double insulated. (At least in most parts of the western world!)

Wood is not an approved insulator as its properties vary with the sort of wood and moisture etc. etc. and are hard to control.

In the described project, I do not see any form of double insulation within the case.

So yes: For that purpose: Wood is definitively not recommended!

For god sake, you can enclose ANY enclosure (conforming to your local norms) inside a wood box if cooling is sufficient. But to the best of my knowledge, wood should never be used to insulate mains voltages (As it has been in this case!). Maybe however, to block access ...

[rolycat]

Then perhaps you should inform companies such as Tivoli Audio who still build domestic radios and hi-fi into wooden enclosures.

Why? I don't have any knowledge how these radios are constructed internally. Maybe the PSU is housed in a metal compartment shielding it from the wooden case. Who knows ...

Because you stated categorically that "domestic installations are not allowed in wood enclosures". And then edited that incorrect statement out of your reply.

Wood is not an approved insulator as its properties vary with the sort of wood and moisture etc. etc. and are hard to control.

In the described project, I do not see any form of double insulation within the case.

So yes: For that purpose: Wood is definitively not recommended!

For god sake, you can enclose ANY enclosure (conforming to your local norms) inside a wood box if cooling is sufficient. But to the best of my knowledge, wood should never be used to insulate mains voltages (As it has been in this case!). Maybe however, to block access ...

I think you will find that nowhere did I suggest that the Make magazine project was in any way an example of good design, or that wood is a safe insulator for mains voltages.

[homebrew]

Because you stated categorically that "domestic installations are not allowed in wood enclosures". And then edited that incorrect statement out of your reply.

What on earth has gone wrong here? I did never "edit something out" of any reply !? 

Yes absolutely "domestic installations are not allowed in wood enclosures". At least where I do live (Europe). Here you cannot install a plug in a woodern cabinet without housing it in a plastic or metal box.

[rolycat]

Because you stated categorically that "domestic installations are not allowed in wood enclosures". And then edited that incorrect statement out of your reply.

What on earth has gone wrong here? I did never "edit something out" of any reply !? 

Yes absolutely "domestic installations are not allowed in wood enclosures". At least where I do live (Europe). Here you cannot install a plug in a woodern cabinet without housing it in a plastic or metal box.

The fact that English is (presumably) not your native language may have caused some miscommunication. However, the fact remains that wood is a perfectly suitable material for mains powered electrical enclosures. It goes without saying that this is only true if it is employed by a competent designer, since this is true of any material.

The last time I checked the United Kingdom was part of Europe, and here at least my understanding is that mains voltages can be present in a wooden cabinet as long as they are safely insulated and any conductive penetrations are properly earthed. This does not mean that the entire cabinet has to be lined with either metal or plastic.

If you have specific information to the contrary I would be grateful if you could supply a reference.

[liquibyte]

I finally got the simulation to behave like I wanted.  I'll have to test this in the real world but this is the kind of thing I was after.  Next up, fixing the CC/CV to be more in line with established designs so I'm off to research and model various solutions to see what I can come up with.

The usual solution is to wire the outputs of the two error amplifiers together with some diode OR logic; then either amplifier can pull the output down to limit either the current or the voltage.  This causes the amplifier which is not currently controlling the output to saturate but recovery time is generally fast enough for this not to be an issue and the output transitions seamlessly between constant current and constant voltage mode.

I did some looking around and found various examples of diode logic and can now say I understand it.  What I can't wrap my head around is how to split this apart to apply that in this circuit.  I tried for several hours last night but I think this would really benefit from this if it was redesigned from scratch.  More study is in order.

[AG6QR]

Then perhaps you should inform companies such as Tivoli Audio who still build domestic radios and hi-fi into wooden enclosures.

Why? I don't have any knowledge how these radios are constructed internally. Maybe the PSU is housed in a metal compartment shielding it from the wooden case. Who knows ...

I own a Tivoli model one radio.  I've opened it up in order to clean a dirty volume pot that became scratchy after a decade and a half, and I can report that it has mains inside the wooden box without a metal shield.  I don't have teardown photos myself, but a quick google search led me to this web page where someone else has photos of it.

http://la3za.blogspot.com/2013/12/scratchy-tivoli-audio-model-one.html

The first photo on that listing is showing the radio with the wooden case removed.  You can see the power transformer.  The mains cable is a two-wire one, with no ground lead.   Note that the front and back panels are plastic, while the top, bottom, and sides (removed in that photo) are wooden.

By the way, that site appears to have been written by LA3ZA, a ham radio operator from Oslo Norway.  So if this radio design is faulty, it apparently got through the regulators of at least two countries (I'm in the USA).  I don't know a great deal about this area of regulations, but there's nothing about this radio that stands out to me as an obvious safety hazard.

But this radio isn't high powered, and doesn't produce a lot of heat in operation.  While I don't see a problem with this particular wooden case, I would be cautious about using wood to enclose a linear regulator that could be running close to its thermal limits. 

[rdl]

Funny, I have a lot of stuff that plugs into a mains outlet in the wall. None of them have metal enclosed power cords.