[Amps] oil filled caps

Peter Chadwick g3rzp at g3rzp.wanadoo.co.uk
Thu Oct 4 08:40:05 EDT 2007

kd4lyh asked:
>I have some oil filled caps that I want to use in a 6500volt_1amp FWB supply. Im going to have to series them to get the voltage and capacitance I need. Are series connected oil filled caps treated the same as electrolytic caps? Do I still have to run equilizing resistors across the caps or just a bleeder resistance across the total string?<

If you put capacitors in series, the voltage divides in inverse proportion to the capacity. So if they were all exactly the same value, you'd get equal voltages across each. Now if they are +/-20% tolerance on capacity, there's two ways round. One is to use equalising resistors: the other is to use enough capacitors that the one that happens to be the lowest capacity doesn't have excess volts across it.
Personally, I'd go for equalising resistors: they need to be of sufficient voltage and power rating, of course, so you may need several in series. Bear in mind that from a safety viewpoint, you also probably want a megohms times microfarads product to be short enough that the caps discharge in a reasonable time. Somewhere around 15: that means that after 75 seconds, they shouldn't have too much charge left in them.

You still need a bleeder resistance across the whole string. Just in case the one of the ones across the caps open up!
Peter G3RZP

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