When does my circuit need a power filter capacitor?

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modman
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Post by modman »

Should I put it even if I do not hear, measure or see (on the scope) a difference? And if so, how do I determine the value?
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madbean
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Post by madbean »

You may not see a difference in your test cases, but I think they are generally incorporated to accommodate a variety of power supply situations--batteries, filtered and unfiltered power supplies, badly wire outlets, etc.

47uF or 100uF works...I even see some using 10uF. I also see a 100nF in parallel with some of those.

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earthtonesaudio
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Post by earthtonesaudio »

If you always use a battery, the only reason I can imagine you'd need a power supply filter capacitor is if the circuit oscillates when the battery's internal resistance gets too high. In that case you probably would be better off with a multilayer ceramic cap than an aluminum electrolytic, because in that case ESR actually matters.

If you ever use an unregulated wall wart, you would want filtering to help reduce hum and ripple.

And if your power supply (regulated, unregulated, battery, or whatever) is ever more than a few cm from your circuit, you might want filter caps to reduce noise that is picked up from the surrounding environment on the supply wires.

One other reason might be if you have a low-current power supply and need some bulk capacity to handle transient current demands.
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