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Old 29th Aug 2007, 9:12 pm   #2
Ray Cooper
Retired Dormant Member
 
Join Date: Jan 2007
Location: Porthmadog, Gwynedd, UK.
Posts: 199
Default Re: capacitors in series - pitfalls?

People keep asking variants of this question, don't they? Perhaps the answer needs archiving. Try this for size:-

No, two 0.1uF 250VW capacitors in series do not make a 0.05uF 500VW unit. True, they make a total of 0.05uF in capacitance: but you can't claim that they'll work properly at 500V. It's all down to how the available voltage distributes itself between the two units - in fact, any leakage currents present in the capacitors can entirely kipper the desired effect.

Leakage current can be thought of as being produced by a high-value resistance in parallel with each capacitor. Imagine, for example, that one unit has an apparent parallel resistance of 100megohm (not bad), whereas the other unit is better at 500megohm (much better).

In DC terms, what we have here is a potential divider of 100M and 500M: total 600M, but if we put 500V across the combination, you get 500*(500/600) volts across the high-resistance unit, and 500*(100/600) volts across the other. My calculator says that these work out to 416.7volts and 83.3 volts, which add up to 500volts. Good.

So the 'low-leakage' capacitor is being heavily over-stressed and is likely to fail: if it does, and goes short-circuit as is quite likely, the other capacitor will soon follow it.

These are only example figures, of course: modern capacitors are likely to be much better than this, but the point is that the leakage resistances of the two will likely be different, so the voltages will not split evenly.

The usual 'fix' for this problem is to swamp any leakage resistance by connecting equal, but much lower, values of fixed resistor across each. There are places in which you can get away with this sort of trick, and other places where you can't - coupling capacitors between valve stages being an obvious example.
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