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I need to use a capacitor in a DC circuit where it would store somewhat higher voltage (hundreds of volts). The cheapest way to do that (in my case) is to connect multiple electrolytic capacitors in series, because their maximum voltage is lower than the voltage I want to store. In theory, it should work well with non-polarized capacitors. I am not sure what would happen if the output gets shorted (by accident – broken off wire etc.). I think that some of them will discharge first because of manufacturing differences and they will be discharged to other capacitors (that will be negatively biased now).

Could this happen with real-world capacitors? Is it sensible to connect a diode in parallel with each capacitor to protect it from being charged negatively?

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  • \$\begingroup\$ If you charge a bunch of series capacitors, and one of them shorts out, how would any remaining capacior become reverse biased? Or do you mean if the output is shorted? \$\endgroup\$
    – Justme
    Aug 19, 2022 at 15:05
  • \$\begingroup\$ I mean shorting the output. \$\endgroup\$
    – jiwopene
    Aug 19, 2022 at 15:18
  • \$\begingroup\$ Does this answer your question? How do I increase the voltage limit by connecting same capacitors? \$\endgroup\$
    – Finbarr
    Aug 19, 2022 at 15:31
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    \$\begingroup\$ It's a good question that deserves an answer, but every consumer switch-mode converter has electrolytics rated at hundreds of volts, they are very common and cheap. There shouldn't be a need to do this unless you're looking at thousands of volts or need a very specific form factor. \$\endgroup\$
    – pipe
    Aug 19, 2022 at 15:38
  • \$\begingroup\$ Don’t! Calculate the maximum difference in leakage current for steady state and max capacitance difference for max current and the voltage difference will be substantial. \$\endgroup\$
    – winny
    Aug 19, 2022 at 16:56

1 Answer 1

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This is quite common. You'll need:

  • Resistors across each cap to balance the voltage, taking into account capacitor leakage current. Without it, if the voltage across the series string of caps is kept constant, and one cap leaks less than the others, its voltage would increase over time. This serves as a bleed resistor too, to avoid murdering the repair tech.

  • If you need quick discharge, diodes can be a good idea to avoid having the cap with the highest capacitance reverse-biasing the others.

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  • \$\begingroup\$ If the capacitors (from tolerance or ageing ?) have different effective values and the V is applied suddenly, then resistors may not be practical. It is unusual that 2 caps in series is lower cost (2 in series is ½ the total effective capacitance) \$\endgroup\$
    – jp314
    Aug 19, 2022 at 15:52
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    \$\begingroup\$ Actually vacuum tube guitar amps do this as a matter of course (modern & vintage). It's not a cost thing, it's a sourcing thing. It's way easier to find 2 400V caps than one 600V cap. The balancing resistors serve two purposes. As mentioned, it ensures the voltage on each cap remains about the same (100k resistors are typical). It also allows for a bleed-off path when power is disconnected.. i.e. it's safer to have them then not. \$\endgroup\$
    – Kyle B
    Aug 19, 2022 at 16:19

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