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Old 9th August 2010, 08:52 PM   #1
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Default series electrolytics....

I have ran film caps in series to up the voltage rating but never done the same with electrolytics. Is it OK to do this? what should the bleeder resistors values be in the case of two 50v 68,000uf caps run in series? I understand this would drop my total capacitance to about 34,000uf but that is OK, it is still about 4,000 more than needed.

What other gains/losses should be expected when running caps in series? i.e. what does this do to losses, ESR, etc?

Thanks,
Jeff
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Old 9th August 2010, 09:41 PM   #2
benb is offline benb  United States
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The ESR will be twice that of each cap (as they're in series), and the ripple current rating will be the same as one cap. There will be twice the heat generated from the ripple current as one cap with the same ESR, but this will be shared by the two caps. Next question: Linear or switching supply? For a linear, ripple current and ESR most likely aren't high enough to worry about. For switching, it depends.

Not sure what value resistors to use - maybe calculate the value that at operating voltage would dissipate 1/4 to one watt, and use resistors rated at five watts (I like to be conservative so things don't get too warm). If you've got more than a pair of these caps, measure their capacitance (after running them up to rated voltage and down again, to make sure they're fully formed), and use the two that are the closest capacitance. This will cause the voltages across them to be most equalized.

It couldn't hurt to put a 1N400x rectifier across each cap, reversed biased for the "regular" polarity of the cap. If something strange does happen, this will keep the reverse voltage under 1 volt, saving the cap from reverse-voltage damage.

Series inductance will be twice that of one cap, and that may be significant. I've seen other threads here about paralleling (this would be across the series pair, as if they were one big cap) with smaller-value caps, just to make sure that's not a problem.
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Old 10th August 2010, 09:42 AM   #3
RJM1 is online now RJM1  United States
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I seem to remember that the bleeder resistor values should be picked to draw at least 10X the leakage current of the capacitors. Although it's been a long time since I've put electrolytics in series.

Last edited by RJM1; 10th August 2010 at 09:55 AM.
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Old 10th August 2010, 10:28 AM   #4
Eva is offline Eva  Spain
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Yes, the balancing/bleeder resistors have to be sized to prevent differences in capacitor leakage current from resulting in heavy voltage unbalance. For leakage current figures check capacitor datasheet, or just pass a few mA through the balancing resistors at the expense of higher than optimum dissipation.
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