Capacitor rating question

The 2200uF would be the better choice, but you would probably get away with using either value. Electrolytics are usually something like +50/-20 per cent tolerance rated, so there can be a large difference between what's in the can and what's written on it.

Cheers, Pete.

That all depends on the type of PSU ! (Given the off-standard value of 1200 uF, you are likely talking about a SMPS.)

In a SMPS (Switched mode PSU) like in a PC or in modern low weight wall chargers, you cannot change 1200 uF into 2200 uF nor into anything else. Even more: you cannot replace a 1200 uF capacitor by just any 1200 uF capacitor as it then probably needs to be a special low ESR type :-)

On the other hand: if it is a classic PSU (one with a rather heavy transformer), it is OK to replace your 1200 uF capacitor by

2200 uF. If the new one fits the available space, that is. Or even replace it by 1000 uF.

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Kind regards,
Gerard Bok

the 2200 will work fine how ever, ESR could be factor. try to get a low ESR type. It will specify low ESR in the parts search.

"

Woops. I stand corrected. Have to admit, when I sent my original reply I was just thinking in terms of a smoothing capacitor in a bog-standard linear PSU. If it's a switcher, that's a whole different ball-game.

Cheers, Pete.

Will either of those values fit OK into the vailable space ?

If the 1200uF failed it may mean it was under-rated for the job and a

2200uF might well be the better choice, but as ever it's never 'quite' as simple as that. Simply a better brand of capacitor may provide as much improvement as you require. You can't go far wrong with Panasonic caps btw. If this was some cheap Chinese piece of kit it's unlikely the original cap would remotely match a Panasonic part for 'ripple current' which is probably what's killed it. Is the cap case 'bulged' too ?

One other thing, is this a swichmode PSU or 50/60Hz with a traditional AC transformer ? Switchmode supplies require 'special' low-ESR caps designed for the job. Don't fit a 'standard' cap into a switchmode supply.

Graham

Cheers for the help everyone. Yeah it was from a slimline PC's PSU which is switched mode. I'll look into getting the proper replacement part although I'm actually still wrangling with the warranty and/or house insurance, but if it turned out to be a cheap fix I thought I could avoid the hassle. Thanks again.

Hey Gerard,

I am also working on a failed power supply, and I *believe* it is an SMPS as it lacks he heavy transformer and other "standard" components I am familiar with. My question: How can I tell what the ESR of a replacement (high temp) cap is? One of the caps in the PS literally exploded, so getting numbers off of it are not possible. Other caps are only swollen however, and I can still read their numbers. There is no schematic for the PS PCB that I can find on the manufacturer's website (Viewsonic), although there are schematics for the other parts of the device. If need be, I can open up the device and read the numbers off of the caps in the replacement PS PCB, but that would be a hassle and I'd rather not go that route. Any idea how I can proceed to refurbish the dead PS? Can I get numbers off of the swollen caps to help me determine what the ESR of these is? *Any* help would be greatly appreciated.

Thanks,

Dave

If an electrolytic capacitor in a PSU leaks, it's probably a production problem (on the part of the manufacturer that produced the capacitor). If a capacitor explodes, it's most probably because of some other fault. (E.g. short circuit in a rectifier.)

If a capacitor exploded, you are facing several problems.

- liquid from inside the capacitor is splattered all over the surrounding PCB. Very hard to clean. And very conductive stuff.

- the capacitor exploded for a reason (see above)

- a good replacement is often very hard to get,

So my advice would be: replace the PSU!

(Which, by the way, is allways the best advice for a failing SMPS. Many enthousiasts don't realise that a quick and cheap 'fix' may cause them dearly. As insurers are very keen to trace home fires back to 'inadequate modification' of electronic equipment.)

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Kind regards,
Gerard Bok

Aaah. Okay. Well, replacing the PS PCB was what I actually did, but I was hoping to refurbish the old one in case the new one went out too. I was planning on replacing the missing/swollen caps with new, higher voltage-rated ones to (in theory at least) give them better heat-dissipating capabilities. If I still wanted to do this, couldn't I just clean the electrolyte from the exploded cap off the PCB with one of the spray-on cleaning solutions? Or would I need something like the now-impossible-to-get Freon TF?

Thanks for your help,

Dave

I think most modern electrolytes are water soluble. Use warm water but onbiously avoid soaking any adjacent components.

How old is your PSU btw ? You may be a victim of the 'bad caps' problem of some years back.

Certain 'off-brand' caps are especially noted for the problem.

Graham

The PS PCB is two or three years old. I half-way suspect this may be the problem, but that is just a gut feeling.

BTW, if the electroloyte is sprayed around by the exploding cap, wouldn't I need to wash the board thoroughly, to get rid of it? And I didn't know electroloyte was conductive. I figured it would be non-conductive, to assiste in keeping the "plates" of the cap separated.

Thanks,

Dave

No. The plates have a very rough surface, increasing their surface area. The electrolyte follows those curves on the surface and acts as the second plate. A thin layer of oxyde, covering the plate forms the insulator. (That's why they sometimes only withstand some 10 volts :-)

What you are looking at is (partially) 'boiled' electroyte, which may not have the same characteristics as the original one. And the aggressive nature of the stuff may have affected the surrounding materials.

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Kind regards,
Gerard Bok

Depends where it went. If 'under' some components, it's best to remove those and clean thoroughly.

It's slightly conductive. Electrolytics caps are very imperfect devices. All electrolytes have some conductivity.

Graham

Wow. Okay, the old PS is toast. Still tempted to "fix" it and give it a burn in of a couple hundred hours, but the possibility of burning down my house makes that a much less desirable idea. :)

Thanks again...

Dave

I like the description of the capacitor:

I shall have to define smell in my next parts list. "C3: 10uF, 16V, a redolent fruity little device with a hint of chives."

I have designed several SMPS's and in every case it was OK to increase the capacitance, in fact you generally made the caps as big as you could afford, to reduce ripple levels. So I would think 2200uF is better, though as someone said, you want a "low ESR" type if possible (to stop it getting too warm from ripple currents heating it). I don't understand why Mr Bok advised against changing the capacitance, but perhaps it is different for multi output PSU's (I did simple non isolated step down or step up types). I'd be interested to learn more on this subject.

Nor do I understand why it is bad to wash the board thoroughly with water. Many PCB's are washed with water to get rid of acidic flux remnants after assembly, and provided you dry it thoroughly afterwards (ie on a radiator or something, fairly rapidly, so nothing rusts) what is going to get damaged by water? Obviously you don't want water on a PCB when it's got mains voltages on it, but there's no reason not to use water for cleaning it.

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Nemo

It will change the loop response and subsequently the stability of the feedback loop.

Graham