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I have read in a few places that if two polar electrolytic capacitors of equal value are connected negative-negative in series (or + to +) then they act like a non polar capacitor with half the capacitance of each one and the same voltage rating. The one that is connected in reverse polarity is basically ignored.
I tried this with a dmm across the capacitor and found that although the capacitor connected correctly charged up more than the one connected in reverse, the one connected in reverse still got charged up in reverse bias. The cap with correct polarity was at about 4V and the other was at 1.85V which is beyond the "safe reverse voltage" for electrolytics. I decided not to continue charging it Does anyone know more about this?
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
There are two ways to protect the cap that would get reverse biassed, depending on whether you have a power or a HiFi application, and a ghetto way
The power way to do it is simply to put a diode in parallel with each cap, of adequate voltage and current rating of course. You can either view it as the diodes protecting the capacitors from reverse volts, or rectifying the applied voltage to charge up the common node. This limits the reverse bias across each cap to 0.7v (if your diode can handle the charging current). If you try to calculate the total capacitance during initial charging, you see not C/2, but C, as the other capacitor voltage is held constant by the diode. Only when neither diode is conducting do you see C/2.
The HiFi way to do it is to put a large resistor to the common node and charge it up to a higher voltage than you expect the signal swing to go. This ensures the caps are always charged with the correct polarity, and never approach reverse bias.
The ghetto way is to just connect the two caps. The reverse biassed one will have a much higher leakage current than the correctly biased one, and will slowly charge the common node. The hope is that it can do this without de-forming the capacitor too much.
Thanks, I did some simulating with QUCS and found that I can keep both capacitors charged correctly in my circuit, although I'll only be able to use 1/4 of the total energy storage capacity of the capacitors. I'm making a thompson induction ring launcher.
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Back to back electrolytic capacitors
Does anyone know more about this?