how to identify capacitor rated voltage?

[justin66]

hi im building a big tesla coil Drsstc and i have got a couple of big caps from a motor controller. i have Nichicon BK0-CA1468-H37 my transistor tester measures 5400uf but i dont want to risk exploding such a huge capacitor connecting it to rectified mains. i cant seem to find any info about this cap. and the only marking is this Nichicon BK0-CA1468-H37 so how do i know the maximum voltage of this cap? numbers dont make any sense either

this is the link of the cap in some chinise sell website https://item.taobao.com/item.htm?id=632143698293

[Terry Bites]

https://www.nichicon.co.jp/english/technical/index.html

[Manul]

I'm sure that it is possible to dig information about this capacitor or the part number coding. If it turns out to be hard, then you can find its aproximate voltage rating by lab testing. Use a series resistor, lets say 100 Ohms (for peace of mind), multimeter on mA range and slowly increase voltage on a lab supply to find where it starts to breakdown with leakage current suddenly increasing. If the capacitor was not used for some time, let it sit for a while at low voltage like 10V, so it rebuilds the oxide layer, otherwise leakage measurement could end up being confusing. It is not dangerous for aliuminum electrolytic capacitor to breakdown if current is small and for short duration. When you observe sharp change in leakage, call the test done, don't allow more then 5mA. The real voltage rating will likely be somewhere 30-50% less then measured breakdown voltage.

Edit: Not for high voltage or very high energy capacitors! Stay safe.

[bdunham7]

The first three characters are letters--BKO, not BK0.  That may have impeded your search a bit, although I cannot find this p/n or even the series actually listed anywhere.  These are found in appliance inverters and motor VFDs and AFAIK, are commonly used on rectified mains in 220-240V countries.  That one is pretty big--how physically large is it? 

[bdunham7]

It is not dangerous for aliuminum electrolytic capacitor to breakdown if current is small and for short duration.

I'm not so sure of that when a capacitor this large is involved.  If a spot in the capacitor breaks down, you have a fully charged capacitor waiting to discharge through that fault.  If it doesn't recover quickly as the voltage drops but runs away instead, you have have pretty big event.  For example if is 5.4mF, rated 420WVDC and breaks down at 600VDC, that's 972 Joules.  Inverter capacitor explosions do happen and they can be pretty exciting.

[Manul]

It is not dangerous for aliuminum electrolytic capacitor to breakdown if current is small and for short duration.

I'm not so sure of that when a capacitor this large is involved.  If a spot in the capacitor breaks down, you have a fully charged capacitor waiting to discharge through that fault.  If it doesn't recover quickly as the voltage drops but runs away instead, you have have pretty big event.  For example if is 5.4mF, rated 420WVDC and breaks down at 600VDC, that's 972 Joules.  Inverter capacitor explosions do happen and they can be pretty exciting.

I agree, my mistake, if we are talking about high voltage capacitors, this should better not be done. With high voltage dissipated power under same leakage current goes way up and also capacitor stored energy goes skyhigh. I think that electrolytic capacitors breakdown more or less uniformly, so breakdown should not produce hot spots and experience runaway, but safety is first - so not for high voltage. I will correct my post.