Author Topic: Swapping old capacitors for new.  (Read 823 times)

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Offline Robmanser11

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Swapping old capacitors for new.
« on: February 16, 2018, 12:20:38 pm »
Hi

I put a post up about an old Farnell power supply, L30d but as yet no responses.

One question I have, which may help is, I have discovered that one of the large caps has leaked, so I need to replace them.

They are old Hunts caps, which then became erie. I have the circuit diagram for the ps, and can identify them as 2200nf 63 volt high ripple. I can. I assume just replace them, with two 2200nf 63 volts or higher caps. I have found some originals on eBay, but at £14 each,  I don't see the point and hopefully can just replace with modern caps. Is that correct, despite there being a massive size difference?

If so, obviously have to remount on something, should I attach them to heat sink, the others, original were not.

Thank you
 

Offline Zero999

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Re: Swapping old capacitors for new.
« Reply #1 on: February 16, 2018, 01:33:06 pm »
I think you mean 6800µF, not nF.

Yes, replace them with modern capacitors, preferably from a reliable brand, purchased from a reputable distributor such as: Farnel, RS Components, Digikey, Mouser etc. There's no point in trying to get originals, especially from somewhere like ebay, because they will be old, possibly well used, without much life left in them.
 

Offline Robmanser11

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Re: Swapping old capacitors for new.
« Reply #2 on: February 16, 2018, 10:15:15 pm »
Thank you
All the caps I have found so far are considerably smaller, I guess this won't matter, as long as I mount them properly
 

Offline Jwillis

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Re: Swapping old capacitors for new.
« Reply #3 on: February 17, 2018, 12:08:42 am »
Are high ripple caps designed to handle the fluctuations in current after rectification since the AC current coming in to your home can very?Is that  what makes them  last longer than standard Capacitors?
 

Offline helius

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Re: Swapping old capacitors for new.
« Reply #4 on: February 17, 2018, 01:04:43 am »
Ripple is AC current through the capacitor. It effectively passes through the ESR of the capacitor, dissipating heat. In order for the cap to stay at a safe temperature and pass that heat into the environment, it needs a surface area and volume sufficient to handle the heat output. Obviously, having lower ESR is better, but a capacitor rated for ripple current needs to safely handle the heat even if ESR increases due to aging.
A capacitor after the bridge rectifier experiences some ripple, but it is limited by the frequency of the input. Switching power supplies use much higher frequencies (like 20 kHz) and so the ripple current through the output smoothing capacitors is much higher.
 
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Offline David Hess

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Re: Swapping old capacitors for new.
« Reply #5 on: February 18, 2018, 08:13:21 pm »
Modern capacitors are much smaller and as helius points out, the smaller surface area lowers the ripple current rating.  Making up for this, modern capacitors have lower ESR raising their ripple current rating.  50/60 Hz input capacitors are selected for holdup time so they have a ripple current rating grossly in excess of the circuit requirements so this is not a problem for them.

Mounting considerations may make it advantageous to use a higher voltage replacement which will be physically larger.  In the example shown below, I deliberately selected replacement capacitors with the same diameter as the old ones so the existing mounting arrangements could be used.
 


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