Large Ripple on an ATX PC Power Supply? - Page 1
Forum section: electronics / beginners

author: techie1234
author_rank: Regular Contributor
timestamp: July 29, 2016, 06:37:07 pm
content: Being rather new to electronics and having just bought my first oscilloscope (an HP 1743A), I’ve been trying to measure the things I have on hand with it. One of my recent forays was an ATX PC power supply. So I hooked things up and was rather surprised to see quite a bit (+/- 750mV) of ripple on the 5V output when looking at it with a TPD of 20 uS, even without any load. Reading that PC power supplies often need some minimum level of draw (mine said 1.5A @ 5V) to regulate voltage correctly, I wired up some resistors to create a ~2A flow. The ripple got much worse and I got a high-frequency whine from the supply. So I popped a 4700 uF capacitor in and that silenced the whine and reduced the ripple – but even then I’m still left with nearly 1.5V PP of ripple. It doesn’t look like HF noise from my setup – it’s a nice, solid, steady waveform but just one that dances between 4.x and 5.5V. The ATX spec says the ripple on 5V should be 60mV or less, so eithera) the power supply is bad, b) my equipment is bad, c) my technique is bad, or d) all is fine and this what I should expect. Any thoughts on which it might be?

author: Simon
author_rank: Unknown
timestamp: July 29, 2016, 06:41:12 pm
content: what frequency is the ripple.

author: techie1234
author_rank: Regular Contributor
timestamp: July 29, 2016, 06:56:08 pm
content: Quote from: Simon on July 29, 2016, 06:41:12 pmSince I’ve got TPD set to 20 uS and the deformed waveform repeats about every 2 ½ divisions, it would seem to be about 20 kHz. See attached pic (hopefully it will display) - I’ve got the VPD on the oscope set to 2.0.

author: Simon
author_rank: Unknown
timestamp: July 29, 2016, 06:59:03 pm
content: how old is the supply ?

author: techie1234
author_rank: Regular Contributor
timestamp: July 29, 2016, 07:04:51 pm
content: Quote from: Simon on July 29, 2016, 06:59:03 pmGood question - could be a decade old as I scavenged it out of an old server. Let's just say it had a 3/12" floppy installed. I haven't opened it up yet to see if anything looks amiss - I thought this was going to be an easy exercise...

author: techie1234
author_rank: Regular Contributor
timestamp: July 29, 2016, 07:07:23 pm
content: I also tried a variety of different caps (size and construction) in various combinations - it didn't seem to make much of a difference as long as at least a few uF was included in circuit.

author: Kilrah
author_rank: Unknown
timestamp: July 29, 2016, 07:27:28 pm
content: After 10 years it could well have dead caps.

author: Simon
author_rank: Unknown
timestamp: July 29, 2016, 07:28:58 pm
content: Quote from: techie1234 on July 29, 2016, 07:04:51 pmFloppy ? remind me what's one of those ?

author: rrinker
author_rank: Super Contributor
timestamp: July 29, 2016, 07:33:47 pm
content: You aren't using a 10x probe, are you? That's still too much ripple per spec, even it if really is more like 150mv instead of 1.5v. There always have been crappy power supplies sold for PC use that in some cases did not meet the specs.Also, do you have a load on this? Many computer power supplies poorly regulate with no load, especially older ones. I'm always reminded of the one brand of PC (AST I think it was) that came with a large sandbar ballast resistor mounted to the top of the drive cage - if you ran the computer without a hard drive, it needed this resistor connected in order to provide sufficient load to keep the power supply in regulation.Or, it's just toast, which is also highly likely given the age.

author: techie1234
author_rank: Regular Contributor
timestamp: July 29, 2016, 07:44:45 pm
content: Quote from: Kilrah on July 29, 2016, 07:27:28 pmYeah - I was starting to think about that. I have another scavenged ATX supply sitting at the office - so I'll bring it home on Monday to compare results. But, wouldn't the 4700uF (and other value) cap I put in circuit smooth things out to such a degree that it wouldn't be a problem?

author: techie1234
author_rank: Regular Contributor
timestamp: July 29, 2016, 07:46:28 pm
content: Quote from: rrinker on July 29, 2016, 07:33:47 pmNope - it's a 1X probe. I do have a ~2A load - which I added early on thinking it might be the problem.

author: Simon
author_rank: Unknown
timestamp: July 29, 2016, 07:49:12 pm
content: Quote from: Kilrah on July 29, 2016, 07:32:41 pmSlightly too fast but almost authentic tones

author: Simon
author_rank: Unknown
timestamp: July 29, 2016, 07:49:48 pm
content: Quote from: techie1234 on July 29, 2016, 07:44:45 pmMaybe the old cap is causing an overload as partly shorted ?

author: Kilrah
author_rank: Unknown
timestamp: July 29, 2016, 08:01:59 pm
content: Quote from: techie1234 on July 29, 2016, 07:44:45 pmThere are many caps in a PSU - the primary could be throwing crap down the line as well...Quote from: techie1234 on July 29, 2016, 06:37:07 pmNote that while it is very often the 5V rail that is regulated, it is also not necessarily the case.

author: tggzzz
author_rank: Super Contributor
timestamp: July 30, 2016, 07:17:44 am
content: Quote from: Simon on July 29, 2016, 07:28:58 pmTheonlymass storage device on a computer I bought last week for the sole purpose of (re)running the first freely available Smalltalk implementation.Somewhat slow, as expected, but the worst things are the sheer mass of the mouse and thickness of the keyboard. No wonder RSI became a problem fir many people(Well, free = some money for the 7 distribution floppies).

author: techie1234
author_rank: Regular Contributor
timestamp: August 01, 2016, 02:15:26 am
content: As I think Dave would say, "Winner, winner - chicken dinner!" I don't think those caps are in the same shape as when this left the factory...looks like this one will get de-soldered for parts...

author: mariush
author_rank: Super Contributor
timestamp: August 01, 2016, 02:27:47 am
content: The Fuhj*$^$ capacitors would have to be replaced, the Teapo capacitors may still have some life in them but it wouldn't hurt to replace them.Besides those, you'd still have a problem, that dark yellow glue which goes brown as it ages and dries out and becomes conductive , risking shorts or changed resistor values. It's best to remove it.

author: techie1234
author_rank: Regular Contributor
timestamp: August 01, 2016, 02:37:00 am
content: Quote from: mariush on August 01, 2016, 02:27:47 amYeah...I think I'm gonna just gonna junk this one and use it for components and connectors. Most importantly: looks like my 1743A was tellin' the truth!

author: oldway
author_rank: Super Contributor
timestamp: August 01, 2016, 10:35:49 am
content: It is a very good oscilloscope and it never lies.But you would have to replace his time base knob....