Capacitor voltage ratings

[acorn]

Hi All

What would you lads consider to be a safe margin for a power capacitor rated at 450 volt dcw, would it be 400 volts or could it be safe to use it at 425 volts.

I am concerned as to the safety margin, when I first switch on using solid state rectifiers there is a high voltage surge on the caps until the valves start to coduct and settle down, is there anway around this, also does any one know what determines the working voltage of a cap is it the thickness of the foil or the dialectric.

Regards Acorn

[petercom]

It depends on the foil specification.

If a capacitor is rated at 450v then it will take 450v. We've looked at whether a cap rated at 400v would take 450v and it won't - reliably. It will take around 420 - 425v but that's the limit. So that, I guess, is the margin - around 5% overvoltage and no more.

[IslandPink]

Peter ,

It must be the dielectric thickness that is the main factor in the voltage rating (?)
Perhaps the type of cap - foil or metallised - may have a bearing on how tolerant they are of over-voltage, but that must be a secondarty issue .

With electrolytics, there is the lifetime issue , and I believe they last a lot longer if used at say 80% of voltage spec than if used at 95% .


Mark

[NickG]

I think the point with electrolytics is that the dielectric is formed by a electro-chemical reaction at the surface of the film, so I guess the surface chemistry of the film is the important factor here.

[NealG]

Re cap life there's more to it than just voltage, see: http://www.bhc.co.uk/life.htm. You need to take into account the ripple voltage as well, plus temp etc.

[acorn]

Hi all


Thanks for all your input, I see from the avox info that a 450 cap has a surge margine of 500 vdcw, I surpose to be on the safe side it would be prudunt to use a 500 dcw cap were possible.

Acorn

[Paul Barker]

Deistance is a factor in voltage rating of any dialectric Mark but so is choice of dialectric material as Peter said.

[IslandPink]

Ah,
I had to read this a couple more times before I twigged . I took 'foil' to mean the metal foil or evaporated metal film , I would have used the term 'film' for the plastic bit . I agree, depending on the film and it's breakdown field spec, the thickness for a given voltage rating would be different from material to material .
I believe that in electrolytics, it's a layer of aluminium oxide that forms the insulating layer, and it can only be grown to a limited thickness, which explains the 500V max rating for all electrolytics , btw .

Mark

[NickG]

Woops, I should have used foil in my post, sorry. I was thinking of the oxide coating as the film.