Heres 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 youll 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 youll need for the additional bypass values. Its better to understand why things are done instead of using rules of thumb. I hope this helps.

Sadownic (Answers | This Thread)

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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 youll 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 youll need for the additional bypass values. Its 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

.