Capacitor question

[lightning]

l've got a vintage Pioneer receiver and it had two faulty electrolytic capacitors on one of the circuit boards (part of the power supply by the looks of it) causing a hum in the speakers.

The capacitors were 330uf 50v
One was open circuit, the other measured about 120uf

l only had 470uf 35v ones so as a temporary measure l fitted those, and it's now working perfectly with no hum from the speakers at all.

However, the voltage across these capacitors is 44v and l would have expected them to show signs of distress in the few hours it's been on test, but they are not even warm.

When an electrolytic capacitor says 35v is that an absolute maximum or a recommendation?

[duncanlowe]

I'd say they won't last very long. They might be OK for hours but weeks / months / years I doubt it. For the silly cheap cost get some that exceed the spec of the old ones, but via a reputable source.

I had a fairly modern DAB clock radio that also had a massive hum. Cost about £3 to get 5 capacitors of which I only needed two and they exceed the spec of the originals. Panasonic, from a reputable source (CPC). Hum has gone.

So from my perspective, you have proved the original fault but your temporary fix won't last. For the pennies involved fix it properly.

[lightning]

Yes l've got some on order, it was just that l wondered why they seemed happy at well over their rated voltage.

[kellymarie]

Hi your capacitors have a continual voltage rating g of 35 volts they will sit at that for ever but there surge rating mite be say 44 volts which they can stand for a short while as said above this is a surge rating not a continuous rating so yes they will stand it but it won't be too long before they go short circuit when you rake them out feel if they are swollen that's a sure sign they've been over stressed

[lightning]

Thank you for the information, l've changed them
this morning for some 63v 330uf that l found in my spares drawer.

[kalee20]

I'd agree with the others!

Prolonged operation at 44V will possibly cause problems.

Manufacturers do have a margin of safety, they'll design a capacitor to cope with more than they rate it at, however, economic considerations dictate that this margin will not be large. It's likely that the forming voltage for a 35V capacitor is in excess of 40V, possibly even 45V, so you've 'got away with it.'

However, at some voltage, leakage current is going to increase, as the weaker areas of oxide start to fail, and then it'll be a race between further oxide forming to decrease the current, and the pressure rising through overheating and blowing the top off.

So, over-stressing a capacitor on voltage is something to do only on a temporary basis, and only if you're really up against it!

[Maarten]

Originally Posted by lightning

When an electrolytic capacitor says 35v is that an absolute maximum or a recommendation?

It's the maximum. Recommendation is to stay between roughly 10-90 or even 20-80 percent of this value for an optimal life span (*).

The capacitors are usually formed at a higher voltage so won't break down immediately but chances are that small breakdowns in the oxide layer will occur that don't reform uniformly and leakage will increase over time until the capacitor wears out prematurely.

(*) older electrolytics were sometimes marked with a peak voltage and a working voltage, indicating that bringing them up to peak voltage for a few seconds each time on switch on wouldn't cause premature wear. That was a deliberate margin that modern capacitors don't have, though they wouldn't immediately break down either.

[broadgage]

I would consider that the rated voltage IS an absolute maximum, for continual or prolonged operation.

Brief or short term overvoltage is probably acceptable, but not for regular operation.

There are two common failure modes.
Firstly the higher voltage increases losses, which produces heat, which increases the losses. This can result in rapid and violent failure. A low ambient temperature reduces the risk.

Alternatively the capacitor may dry out and steadily loose capacity, this is inevitable eventually but will be accelerated at a higher than designed voltage.

Would I use a "12 volt" capacitor in an appliance powered by 10 dry cells ? yes I might, because firstly the actual voltage will be about 12 volts, not 15 volts most of the time, and secondly the operating hours will probably be limited if only due to battery costs.

Would I use a "12 volt" capacitor in an appliance that used 15 volts from a mains PSU ? No I would not, due to the continual presence of 15 volts, perhaps for years on end.