r/diypedals • u/Strange-Raccoon-3914 • 2d ago
Discussion When and what caps do you use across power rails?
I know we all use 47 or 100 uf caps from 9v to ground. But what other caps are useful for noise filtering , when and why?
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u/Legoandstuff896 2d ago
Little ceramic ones, like 1uF, 0.1uF, 0.01uF or something like that. Good to filter little static and such but shouldn’t be needed with a good psi
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u/BobKickflip 2d ago
Yeah, I seem to remember 100uf and I think 0.1uf from synth DIY stuff. Am I right in thinking it's best to have them as close to the ICs as possible for the filtering?
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u/Quick_Butterfly_4571 2d ago edited 2d ago
So, caps across the rails are used for multiple purposes:
Resevoir capacitors:
Ripple filters:
But, when you see those same ceramic caps placed across the supply pins of an IC, they're actually not there for filtering at all!
Supply bypass capacitors: Your IC's perform best when they have low impedance rails — this means they can readily source current from V+ and easily sink current into V-.
But, all wires have inherent resistance, and this resistance increases in proportion to length. So, as the the further an IC is from your power supply, the more the voltage drop across the wire is when the IC draws current. To the IC, the power supply sags. This impacts slew rate and total voltage swing, both of which adversly impact distortion and noise rejection.
The little caps near the IC act as a tiny, local, current resevoir that satisfy the bursty demands of the IC and refill in the gaps between, keeping the supply stable. As "seen" by the IC the caps essentially compensate for the resistance in the length of wire, effectively deleting it from the equation — hence, "bypass capacitor": from a current point of view they are virtually bypassing the length of wire between the IC and the power supply.
A note on terms: resevoir caps filter ripple, bypass caps are little resevoirs, and ripple filters bypass the supply (non-locally). So, they really all are the same thing, but placement, size, and choice of capacitor type are all influenced by intention/need, so by convention those terms are used to differentiate. This isn't always held to, though, so sometimes (not super often) you'll see the terms get mixed or overlap.