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I am making a project that needs a capacitor bank. I have six 250 V rated capacitors, a 120 V - 240 V step-up transformer, and a bridge rectifier. Is it safe to charge these using a wall outlet (I am in the US)?

Also, I want to increase the voltage of the bank by charging pairs of capacitors in parallel separately, then connecting the three pairs in series. Is it safe to wire them in series after they are charged, or do I need to put switches in between the pairs before charging so that, once they're charged, I can connect them without having to wire anything while they are charged?

I am not super experienced with making electrical things, so I am just wanting to double check that what I am doing is safe and what precautions I need to take.

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    \$\begingroup\$ Well this sounds like a nice way of ending up dead or blind, what's the capacitance? \$\endgroup\$
    – bobflux
    Nov 21, 2023 at 18:25
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    \$\begingroup\$ For what you need 750V capacitor bank? Also, 240VAC bridge rectified has a peak of something like 340VDC, so you can't charge a 250V cap to 340V. At least, not safely. \$\endgroup\$
    – Justme
    Nov 21, 2023 at 18:26
  • \$\begingroup\$ Further to Andy’s response, I’d suggest using a step up transformer with the secondary voltage suitable for the target voltage and hard wire the capacitors in the configuration you want. Build it in an earthed enclosure. Put something like 220k 3W resistors in parallel with each capacitor to help with equalizing the voltage. Note that peak DC voltage is 1.4142 times the AC RMS voltage when specifying your transformer. \$\endgroup\$
    – Kartman
    Nov 21, 2023 at 21:59

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Is it safe to charge these using a wall outlet (I am in the US)?

It can be made safe to do it this way. You need to take appropriate precautions to protect yourself and anyone in the vicinity. I'm not just talking about protection from dangerous AC voltages (although this is always necessary) but, once the capacitors are charged up they can be very lethal to handle because they will be able to supply massive levels of current and, are not prevented from doing so by a GCFI (ground fault circuit interrupter) as you would get in most household AC installations.

Is it safe to wire them in series after they are charged, or do I need to put switches in between the pairs before charging so that, once they're charged, I can connect them without having to wire anything while they are charged?

It can certainly be made safe but, it's far safer not to have to do this and use a proper capacitor charging power supply or, one of your own design.

I am not super experienced with making electrical things, so I am just wanting to double check that what I am doing is safe and what precautions I need to take.

Be very, very careful. Concentrate on finding ways of not directly handling the capacitors and especially not coming into contact with any capacitor terminal. From my own personal perspective (of one who designs high-voltage capacitor charging supplies) I would use a specialist circuit that basically acts like a current limited boost converter and charge the capacitors as they are intended to be used i.e. in series and, keep away from those terminals.

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