Electrolytics in AC circuits

[stkmtd]

I've found a number of circuits here where electrolytics are put in places where an AC current would pass through them. After learning religiously that I should never put reverse current through an electrolytic, I'm wondering if you can make exceptions for the small amount of current/voltage that a guitar pickup outputs. Would this be the case?

Long story short, I've had a hard time finding non-polarized caps at the values I need without ordering online. The shipping would cost more than the parts for what I need. I'm half-tempted to just throw in the electrolytics and pray, but I'm wondering if anyone has experience in this area.

[anchovie]

Electrolytics are fine in the signal path (some frown on them for audio quality but my ears aren't golden enough!). Polarity still applies where there is a DC element to a signal, e.g. an op-amp using a single power supply will have a DC bias on the output pin.

[petemoore]

  9vac is a notorious cap-killer [polarized of course].
  I don't know how much RPolarization potential these electrolytics can resist, ''none'', was my ~understanding..
  Can you show the application/schematic where you see the reverse polarized capacitors.
 

[stkmtd]

It looks like C4 (output of the op-amp) would be subject to AC signals. This could be a misinterpretation, but it still seems like a bad idea to me.

[R.G.]

It looks like C4 (output of the op-amp) would be subject to AC signals. This could be a misinterpretation, but it still seems like a bad idea to me.

It is subject to AC currents, which is fine. However, one side is at a few volts positive, the other at 0V, and the non-grounded side can never be driven below ground, so the voltage can never be reversed. In this application, the cap is being operated as intended.

For electrolytic capacitors, it's the *voltage* that has to be unidirectional, not the current. As long as the *voltage* on the cap is never reversed, it's fine with currents reversing direction. Of course, if the current reverses for along enough, the voltage will reverse too, but that's not how we use them.

Another way to look at it: With AC voltage signals, as long as the capacitor is held with a DC polarizing voltage in the proper direction and the peak AC signal opposing the DC polarizing voltage is less than the DC polarizing voltage, the capacitor voltage is never reversed, so it's fine.

Yet another way to look at it: that circuit works. So do similar circuits. So if they work, something is being done correctly, whether we think it should be that way or not.  Mother Nature has her little jokes.

[stkmtd]

Thanks R.G.

Your explanation makes sense.

I wonder then, if I needed a ceramic cap, but only have an electrolytic version, would it be possible to substitute the electrolytic with the negative end fed to ground? I realize this may not be possible in all situations, but if I can substitute electrolytics for non-polarized caps somehow, I'd be interested. If it's not possible, I may just have to hit up DigiKey and buy a bunch of common capacitors. I'm getting tired of hitting up the local shop buying inappropriate capacitors in a limited selection at a dollar each in some cases.

Is there a set of capacitor values that are common for making stompboxes? I'd like to keep these at hand.