Author Topic: First steps testing and troubleshooting bench power supply  (Read 9698 times)

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Offline guymoTopic starter

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Re: First steps testing and troubleshooting bench power supply
« Reply #25 on: August 07, 2017, 08:13:18 pm »
I am now convinced that the oscillation is the main issue with this power supply.

I adjusted the resistor in the compensation loop and was able to reduce the oscillation on both channels to about 1V amplitude. This done, the output voltage can now be adjusted across the full range and is stable under load; "stable" apart from the 1V 1MHz oscillation.

Is there anything else that can be done about the oscillation? The compensation is a trimmer and a 1nF cap in series. Is it worth tinkering with the capacitance? What should I try?
 

Offline bd139

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Re: First steps testing and troubleshooting bench power supply
« Reply #26 on: August 07, 2017, 08:44:28 pm »
Is the output electrolytic capacitor duff? They tend to provide a big chunk of the stability.
 
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Offline guymoTopic starter

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Re: First steps testing and troubleshooting bench power supply
« Reply #27 on: August 07, 2017, 09:35:42 pm »
Is there a way to test in-circuit? So far I've got away without desoldering anything.
 

Offline bd139

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Re: First steps testing and troubleshooting bench power supply
« Reply #28 on: August 07, 2017, 10:06:47 pm »
You can usually spot them with one of three methods in circuit:

1. Temperature. Knackered ones tend to get quite warm if you run them hard. Fire up the supply for 5/10 minutes with no load, then shut it down and feel the capacitor and see if it's warm.
2. Connecting another one across it. At least one order of magnitude larger than the existing one. If it becomes stable then the cap is shot or the design is bad. HP designs usually aren't bad so by elimination, it's the cap. This doesn't work at all if the cap is shorted for obvious reasons. One on the end of a couple of croc clip/minigrabber leads is fine - doesn't have to be soldered in.
3. Ohmmeter across it. Any DVM should be fine. The current source in the meter will charge the cap up which will increase the voltage across it and read a higher resistance over time. If it doesn't go up the cap is shot. check the polarity of the DVM first - some of them have a positive voltage on the negative terminal in ohms mode.

You can trace out shorted ones with a 4.5 digit or better DVM in low ohms mode based on the trace resistance.
 
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Offline guymoTopic starter

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Re: First steps testing and troubleshooting bench power supply
« Reply #29 on: August 08, 2017, 06:18:24 am »
Thanks again -- I'll give all of those a go later today .
 

Offline alm

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Re: First steps testing and troubleshooting bench power supply
« Reply #30 on: August 08, 2017, 10:44:12 am »
And make sure what you are measuring is really conducted from the power supply and not common mode noise. Connecting both probe tip and ground lead to the same output terminal (shorting the probe) and checking if you still see the signal is a way to detect this. Dave also discusses it in this video:
 
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Offline guymoTopic starter

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Re: First steps testing and troubleshooting bench power supply
« Reply #31 on: August 08, 2017, 02:17:19 pm »
Thanks for the pointer. I am concerned that my measurement equipment and skills aren't quite up to the task, but when trimming the compensation network I could clearly see the ripple changing smoothly as I turned the trimmer, first reducing in amplitude and then suddenly jumping up to a full rail-to-rail clipping oscillation when I took the trimmer too far, so I am pretty sure this ripple is coming from the supply. I'll try it on a different mains circuit and try shorting the probe as you suggest to see if anything looks suspicious, though.

(The schematic of the error amp section is attached. R30 is the trimmer that I'm talking about. I thought I'd better include this in case it gives someone an idea or you notice that I have done something appallingly wrong!)
 

Offline Vtile

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Re: First steps testing and troubleshooting bench power supply
« Reply #32 on: August 08, 2017, 03:03:53 pm »
Try to DVM method for the C5 and try to put the another capacitor in parallel. It somewhat sounds that the controller circuit is oscillating?? A couble oscillator screenshots could say something to people who have lots of experience with these.
 

Offline guymoTopic starter

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Re: First steps testing and troubleshooting bench power supply
« Reply #33 on: August 08, 2017, 06:27:51 pm »
Progress!

I checked the output cap using an Ohmmeter as suggested, and it appeared to be ok, reading a rising resistance. It topped out around 11k, but I guess the actual reading is kind of meaningless, isn't it?

Then I tried a 470uF cap in parallel with the output cap (80uF). Bingo! With just that capacitor as load, the scope reads a dead flat line with no noise or ripple visible.

Testing with a resistor in parallel, things looked ok but as I turned up the supply beyond about 10V  it began current limiting and then blew the top off the capacitor, which I guess was a bit underspecified at 35V  :-[  Would things have been better if I had turned up the voltage more slowly?

Brushing aside my poor test procedure (ahem! beginners' forum!) does this mean that the output cap needs replacing? I suppose it is still working as a capacitor but no longer the right value to keep the circuit stable. If so, what should I look for in a replacement? Clearly it should be 8uF 300V as specified for the original, or at least close to that, but is there any other spec to bear in mind or any recommended brand or series for such parts?

Many many thanks to you all for getting me this far. I think we are almost there  -- let me know if you disagree!



 

Offline alm

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Re: First steps testing and troubleshooting bench power supply
« Reply #34 on: August 08, 2017, 07:01:30 pm »
A 35 V cap should be fine up to at least 35 V ;). Not sure why it died around 10 V. If it was an old cap (as in decades old), it might have been leaky. Did you monitor the voltage and do you know if the supply was oscillating?

The 300 V cap sounds way overrated to me. I would not hesitate to replace it with anything over 60 V. But if I was ordering, I might look for something close to 300 V.

Do you have anything in the range 80-100 µF with a voltage rating of at least 40 V that you could put in?

Offline bd139

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Re: First steps testing and troubleshooting bench power supply
« Reply #35 on: August 08, 2017, 07:11:52 pm »
This cap is overrated so that if you're floating the supply at +250v, no horrible accidents take place.  10uF oxide layer may have been popped by the sudden potential difference between ground / negative rail (I think there's a small cap between there) and after that, kaboom on lower voltages! I've done that.

WEAR SAFETY GLASSES!

That's hopeful :)

You probably won't get one close to original spec. Choosing capacitor:

1. Measure pin pitch and note if it's radial / axial.
2. Hit RS: http://uk.rs-online.com/web/c/passive-components/capacitors/aluminium-capacitors/
2. 80uF doesn't exist any more. Try 82uF or 100uF (these things are usually +/- a ton so it usually doesn't matter terribly).
3. Try closest voltage. 250-350 is probably fine.
4. Narrow to lead spacing required.

Good brands: Vishay (BC / Sprague come under this brand), Rubycon, Nichicon.

This isn't necessarily a particularly special capacitor but try and look out for one with a long life rating (2000h+) and 105oC temperature rating instead of 85oC. Hour rating is at full temperature, which it won't get near unless something is seriously wrong. It'll last a lot longer at lower temperatures.

Side note: if it's got three pins you will require a bodge most likely. Pick the nearest thing and fudge it in with some thick wire. There's not much else you can do now about this.
 
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Offline guymoTopic starter

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Re: First steps testing and troubleshooting bench power supply
« Reply #36 on: August 08, 2017, 07:41:46 pm »
A 35 V cap should be fine up to at least 35 V ;). Not sure why it died around 10 V. If it was an old cap (as in decades old), it might have been leaky. Did you monitor the voltage and do you know if the supply was oscillating?

When I was looking at the scope I didn't see any oscillation. Then I turned my attention to the voltmeter and turned up the supply. Once I noticed it topped out at 10V I took a look at the ammeter and could see about 750mA there, slowly falling. I experimented with this a bit, turning the voltage up and down, and then the cap popped. It wasn't old.

I wonder if my tinkering with the error amplifier compensation has slowed down the transient response leading to these current spikes? I'll try putting the trimmer back to roughly where it was and play around some more (with appropriate safety gear).

And then it seems like the next move is to get the board out so I can remove the output caps and try out some replacements. Great -- I'll get on with it.

 

Offline bitseeker

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Re: First steps testing and troubleshooting bench power supply
« Reply #37 on: August 08, 2017, 11:11:36 pm »
Hey guymo. For a beginner you're doing really well. These are good little supplies and it sounds like yours is well on its way to being healthy again. :-+
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Offline guymoTopic starter

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Re: First steps testing and troubleshooting bench power supply
« Reply #38 on: August 09, 2017, 09:01:27 am »
Hey guymo. For a beginner you're doing really well. These are good little supplies and it sounds like yours is well on its way to being healthy again. :-+

Thanks! I'm enjoying and finding it interesting too. The real tricky part is yet to come, though, because I am much better at thinking and learning than I am at physically interacting with things, so my removing those output caps poses a significant risk to the circuit. I'll be ok soldering new ones in, though.
 

Offline alm

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Re: First steps testing and troubleshooting bench power supply
« Reply #39 on: August 09, 2017, 01:47:39 pm »
Connecting a cap to the output that does not have six times as much capacitance as the original cap and that does not die when exposed to > 10 V might still be a useful test.

Offline guymoTopic starter

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Re: First steps testing and troubleshooting bench power supply
« Reply #40 on: August 09, 2017, 02:04:47 pm »
Connecting a cap to the output that does not have six times as much capacitance as the original cap and that does not die when exposed to > 10 V might still be a useful test.

:)

My choice of capacitance was driven by bd139's suggestion to use a cap an order of magnitude larger than the existing one. Well, I got close.

I'm still not sure what happened to that capacitor. If the ammeter's readings were anything like correct then something weird was going on. The voltmeter read ~10V, the ammeter 750mA, and the load was 470uF in parallel with 133R. You can't put that much current through that for any period of time without considerably more voltage, can you?

But yes, before I start taking things apart, I'm going to get hold of a 100uF cap with a decent voltage rating, and see what happens when I connect it across the output after also returning the error amplifier feedback trimmers to their original positions.



 

Offline bd139

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Re: First steps testing and troubleshooting bench power supply
« Reply #41 on: August 09, 2017, 02:14:16 pm »
Probably a silly question but did you put it across it the right way?

Reason to use a much larger cap is that if the current one has a high leakage then it usually compensates for the reduction in time constant for the dead one. It's a diagnostic tool, not a solution. If it does something, suspect that cap and replace it with a closer to reality one then measure it again.
 

Offline guymoTopic starter

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Re: First steps testing and troubleshooting bench power supply
« Reply #42 on: August 09, 2017, 03:17:20 pm »
Wrong polarity was my first thought (after "what the...?") but a post mortem revealed that I had not been as careless as I thought after all.

I have some 100uF 65V caps in hand now. I will try them out this evening.
 

Offline alm

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Re: First steps testing and troubleshooting bench power supply
« Reply #43 on: August 09, 2017, 04:36:58 pm »
I was not saying using a much bigger cap was a bad diagnostic test, but just that killing completely the bandwidth with a huge cap is not a 100% proof to me that the cap is indeed the problem.

Offline bitseeker

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Re: First steps testing and troubleshooting bench power supply
« Reply #44 on: August 09, 2017, 07:35:03 pm »
The real tricky part is yet to come, though, because I am much better at thinking and learning than I am at physically interacting with things, so my removing those output caps poses a significant risk to the circuit. I'll be ok soldering new ones in, though.

Be sure to check out some videos on desoldering and practice on boards that you don't care about before tackling your power supply. You don't have to buy a vacuum desoldering station to do a good job, solder braid is fine, but if you find yourself doing more and more of it, vacuum makes life easier. I use braid, sucker, or vacuum depending on the situation. They all have their pros and cons.
TEA is the way. | TEA Time channel
 

Offline guymoTopic starter

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Re: First steps testing and troubleshooting bench power supply
« Reply #45 on: August 09, 2017, 08:43:48 pm »
I was not saying using a much bigger cap was a bad diagnostic test, but just that killing completely the bandwidth with a huge cap is not a 100% proof to me that the cap is indeed the problem.

No, indeed -- understood, and thank you.

I've just tried with a 100uF cap and 133R resistor. There seems to be about 30mV peak to peak noise: it looks like a damped wave, not really sine-ish, every 7 microseconds. I wasn't able to get a convincing photograph of my scope -- all reflections.

I haven't been able to reproduce yesterday's flat lines at all, even using the larger capacitor. But then, that cap did pop, so I suspect I was doing something appallingly wrong. I haven't seen any issues with high current flow in today's testing and I haven't exploded anything.

I wouldn't be at all convinced that my measurement setup and skills are up to the job of this high frequency low voltage stuff. Under some conditions I could see some noise when the power supply was not switched on, but the frequency was different.

Is this as good as I'm going to get, do you think, or is there more to look into?



 

Offline alm

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Re: First steps testing and troubleshooting bench power supply
« Reply #46 on: August 09, 2017, 09:08:52 pm »
30 mV is way worse than it should be. But I am skeptical (as are you) about the result. Firstly, did you exclude common mode noise? Did the noise disappear with probe tip and ground connected to either output? When you left the probe connected but turned the supply off (sounds like sometimes it did and sometimes not)?

Are both output terminals of the output you are testing floating (not connected to the ground terminal)?

You should be able to get better pictures of the scope screen if you turn down the lighting on whatever is in front of the scope (e.g. you). If less light falls on you, then less light will reflect of you, via the scope screen, to your camera.

Offline guymoTopic starter

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Re: First steps testing and troubleshooting bench power supply
« Reply #47 on: August 09, 2017, 09:44:51 pm »
I set the power supply up in what the manual calls the normal way: with the positive output connected to ground. I measured at the negative output pin, obviously. I've just tried the other way around with the same results. I've managed to take a photo, not great, but attached. The vertical scale is 10mV per division, horizontal is 2us. The shape and amplitude of this wave changes as the feedback trimmer in the error amplifier is adjusted -- this was the smoothest I could get it.

With the supply floating, the results are much worse. At first I couldn't get the scope to show anything sensible, but after much tweaking of the trigger settings, the second image attached came up. Here the vertical scale is 1V. The trace moved up and down the screen as I changed the trigger level. I have no idea what is going on here.

Yes, I could sometimes see much lower noise levels, 5mV or so, with the supply switched off, but not all the time.

I am becoming deeply confused but still very grateful for all the help!
 

Offline guymoTopic starter

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Re: First steps testing and troubleshooting bench power supply
« Reply #48 on: August 10, 2017, 04:52:25 pm »
I had the idea to take a look at the internal voltage references on the scope. They show similar looking noise when I turn up the sensitivity, with spikes at a similar frequency. The peak-to-peak was pretty large, 300mV or so, which I think is way more than zener noise is meant to be.

Obviously this doesn't tell me whether the supply or my measurement setup is at fault. Not sure what to do next really.

 

Offline bd139

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Re: First steps testing and troubleshooting bench power supply
« Reply #49 on: August 10, 2017, 05:54:03 pm »
It's possible whatever is giving current to the zener ref is shot or something upstream from that.
 


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