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Old 31st Mar 2010, 11:21 pm   #11
Alistair D
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Join Date: Jan 2010
Location: Edinburgh, UK.
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Default Re: capacitors in series - pitfalls?

Ray's description of the voltage ratio across series connected capacitors is pretty well spot on. It is not just the leakage current that decides the voltage split. The charge(Q) on each cap is also controlled by the actual capacitance of each individual device. The formula Q=CV dictates the actual votage each capacitor. I won't bore you with lots of calculations here but if you connect 2 identical 100 uF caps in series each cap will have exactly half the applied voltage across it, if you connect a 100 uF +20% in series with a 100 uF -20% the voltage across the lower value will be greater.
An added advantage of using resistors to equalise the voltages is that the resistors also act as a bleed to discharge the supply at switch off. In the back of my mind there is a way of connecting a neon to act as a high voltage on and confirm that the bleed chain is intact.
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