What value cap to bypass power supply cap?

Here’s some answers from some of our members

Short answer ...

05-03-05: Herman
Of course different designers have different opinions on this but a good place to start is to bypass with a value 1/100th of the big cap. You can bypass again with another cap 1/100th of the smaller and see if that does anything.

example, original electrolytic 4700 uF bypassed by a 47 uF film cap and a .47 uF film cap.

Long Answer ...

05-04-05: Sadownic
The values of bypassing caps are determined by the amount of ripple rejection desired and the frequency range. Any capacitor will have a self-resonant frequency calculated by
fr=1/2π√(LC)
Above the self-resonant frequency, the capacitor will start to look like an inductor and its impedance will increase. For optimum ripple rejection, you want the shunt impedance to be as low as possible. Impedance is calculated by

Xc=1/2πfC.

Where: f=frequency (Hz) C=capacitance (farads) L=inductance (Henrys).

As you can see from above, the self-resonant frequency will vary depending on the value of capacitor used and its inherent inductance. The physical construction, size and value of the capacitor will determine the amount of inductance. So in order to maintain maximum ripple rejection across a large frequency range you’ll need to add additional (smaller value) capacitors in parallel with large value caps. Find the spec sheets for the capacitors you plan on using to determine their inductance and self resonant frequency and then calculate the values you’ll need for the additional bypass values. It’s better to understand why things are done instead of using “rules of thumb”. I hope this helps.
Sadownic (Answers | This Thread)

Complete Thread

BY PASS POST
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Thanks! That was very helpful.

Dracule1, Take into consideration that bypassing large inductive electrolytic caps with small non-inductive cap crates parallel resonance circuit that will ring in response to rapid current changes. Designer did not put them for a reason.

I would speculate that you might not like the result but it isn't very expensive to try.

Kijanki, thanks for that info. However, all my power supply caps are film caps which according to the manufacturer have ESR and inductance which are a fraction of electrolytic. I know the ESR is 1 mOhm at 10 kHz, but I don't know the inductance. Are film caps considered to be inductive enough to have parallel resonance?

Dracule1, I don't think so. Looks like you got great stuff there. I second 1% minimum recommendation.