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Recapping question

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ac/dc

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hi, haven't posted on this board for a while, feels nice to be doing electronics again.
I'm repairing a pc psu (switching, 500w), I think I'm ready to order the new capacitors but.. I've tried to match up faithfully the caps, is it enough to match farads/volts/operating temp 105C? and also dimensions? I do have the specs for things like ripple current/impedence, but surely it's enough to just match farads/volts?
Well it would be great to get some of your opinions on here, I'm sure some of you have had to recap things at times, some probably a lot!
Ok, well any help much appreciated.
 
For electrolytic capacitors used in a switcher, it's likely they should be low ESR types designed for the high frequency ripple currents that a high power switcher can generate.
Otherwise it's possible the capacitors could overheat and fail.
 
I would suggest of more importance than low ESR is simply using high quality capacitors - low quality (cheap) ones will fail, even if they are low ESR, high quality ones will probably never fail.

Incidentally, if you're changing the reservoir capacitor, it's highly unlikely that you can find 105 degree ones, but 85 degree ones are fine, as they run at 100/120Hz and not the high switching frequency of the rest of the capacitors.
 
if you're changing the reservoir capacitor, it's highly unlikely that you can find 105 degree ones, but 85 degree ones are fine, as they run at 100/120Hz and not the high switching frequency of the rest of the capacitors.
Not necessarily true.
The switcher is likely drawing pulses of current from the reservoir capacitor equal to the pulses the output capacitor see, thus it will also need to tolerate those pulses.

I recommend a low ESR capacitor for both to minimize internal heating.
 
Not necessarily true.
The switcher is likely drawing pulses of current from the reservoir capacitor equal to the pulses the output capacitor see, thus it will also need to tolerate those pulses.

Stick a scope on the cap and see what you get :D

It's not an issue, and the 85 degree ones rarely fail - far, far less than the 105 degree ones on the secondary do.

By far the most failures I've actually seen have been items returned from countries with poor mains supplies (predominately Spain), where the capacitor has been destroyed by excessive mains voltage.

When you consider the tiny percentage of repairs we used to get from outside the UK, it's puts it in perspective how reliable they are.

I recommend a low ESR capacitor for both to minimize internal heating.

Except they are impossible to find, and they aren't fitted by the original manufacturers either.

As I said above, a good quality 85 degree capacitor, will far out perform a cheap 105 degree one.
 
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