High value non polarised capacitors

[StephenGiles]

I need a couple of 47uf non polarised capacitors, now correct me if I'm wrong, but can this be done with 2 x 100uf joined by their negative terminals?

[newbie builder]

I'm 99% sure that you would take two polarized 47 uf capacitors and connect them by their negative leads, not 100 uf.

[Alex C]

R.G. posted this in another thread:

You can make a quick and dirty NP cap by tying together the negatives of two equal-sized polar caps.

In the series-NP connection, the capacitance value is funny. Normally caps in series are a smaller capacitance than either cap by itself. If you had two 3.3uF polyester caps, then the expected value for two of them in series is 1.65uF. However, electrolytic caps actually conduct in the reverse direction, so two 3.3uF polarized aluminum electrolytic caps act like they each have a diode in parallel with them that conducts when the voltage is backwards for that one cap. So two 3.3uF caps hooked up as series non polar (i.e. negative to negative) look like a single 3.3uF NP cap.

... except for tiny region near zero volts where they withstand a tiny reverse voltage, so they look like 1.65uF there...

[StephenGiles]

Thanks, sounds good to me.

[R.G.]

There are high value non polarized caps used in the AC motor trades for motor starting and running. If you are using these at high voltage, like for speaker crossovers or something similar, you can get motor start caps, which are purpose-made bipolar electrolytics in big sizes, up to a few hundred uF. For more severe duty, you can get motor run caps, which are designed for continuous high current AC use. They're polypropylene in oil these days, and come up to 100uF.

I use motor run caps in my "Immortal Tube Amp" concept. A tube amp with all of the electros replaced by film caps, and the power supply filters replaced with motor run caps should... never... need a recap job.