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Old 13th Jan 2012, 3:14 am   #38
dinkydi
Retired Dormant Member
 
Join Date: Feb 2011
Location: Sydney, Australia
Posts: 277
Default Re: capacitors in series - pitfalls?

Hi Kalee20, Thanks for your thoughts.

Your reasoning for the use of balancing resistors is very clearly put. However, the model you use is linear, that is, the resistors (100GΩ and 10GΩ) are fixed in size. In reality, the leakage current of an electrolytic capacitor increases, not linearly as with a fixed resistor, but instead, at an ever-rising rate as the capacitor voltage increases. This means that you do not end up with the extreme 10:1 ratio that your linear model predicts. Instead, the ratio becomes close, in line with the capabilities of the actual capacitors employed.

The non-linear relationship between the voltage and leakage current of an electrolytic capacitor means that the magnitude of the current is a strong measure of the voltage stress of the capacitor. That is, if one considers the leakage current, voltage and current stress are not separate issues.

I take your point regarding the sudden failure of film/foil capacitors (the situation with tantalums is more complex and off topic) but my argument only concerns electrolytic capacitors (see post #25, para 1).

As far as contamination and condensation are concerned, these issues should be addressed separately. The effect of such problems can be quite varied, based on a linear model (imbalance may well be corrected!). Non-linear reality is more sanguine. All electronic circuits require special protection from external influences such as contamination and condensation, if these are relevant concerns. Failure to provide this protection, when a circuit needs it, may have serious consequences.

Peter
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