cap question

[Wild E]

I've noticed that some circuits have a large value electrolytic cap (say 100uF) across the power supply. What is it's purpose? Also are electrolytic caps used because they have higher values?

[daverdave]

they're decoupling caps, look up decoupling capacitors on the net and you'll get some good descriptions of their function. As far as I know they keep the different power supply signals seperate from each other for a start.

[R.G.]

It's to make the power supply "stiffer", so that the power supply voltage changes less with changes in loading.

The bigger the capacitor, the more it can store and release electrical charge to keep the power supply constant. Another way of saying this is to note that the various parts of the circuit can't change the power supply as much with what they're doing locally, so the parts of the circuit can't interact through the power supply, which is good in most cases.

Yes, electrolytics are used because they are bigger capacitance per volume, so you can get more stiffening per size.

[Wild E]

How do I know when I need a "stiffer" power supply?

[JKowalski]

When you are loading down the power supply with a large time variant current (like AC) and getting voltage fluctuations in the power line that can affect parts of the circuit that shouldn't be affected by , you need a capacitor. Generally, it's good practice to include a moderate value electrolytic across the power rails of most effects, there's no harm . It also helps filter out some more of the ripple (unregulated/wall wart) or hash (SMPS, though it's best to include a low value as well say 0.1uF in parallel with the electro to help with high frequencies) from different types of power supplies that may be used with it.

When you are loading down the power supply with a large static current (like DC), and getting a large voltage drop in the PSU, then chances are you just need a higher power PSU, because your current one can't give you enough continuous current.

[flintstoned]

So if I'm using a 1spot supply, do I even need these caps on my builds?

[petemoore]

 Only if you're getting AC ripple or less than pure DC supply that is noticable.
 The caps are inexpensive, less than the testing required to find out if it'll ever get the chance to have to do any actual filtering, which a rail filter cap right at the effect usually tends to, but it depends too.
  If you have an LFO, a good thing is to have a BFC [whatever size is bigger than flattens to DC the ripple] with a very short or no wire to the LFO.