Question about capacitors on the Mid-Fi RNG.

[Bipolar Joe]

This may be kind of a dumb question, but any way...

Does the voltage of a cap matter, pretty much in general, when building? If so, what are they, and how do you figure that out looking at a schematic? Bit of a "Newbie" question.

[Hides-His-Eyes]

For most circuits anything over the 9V supply you'll be using is fine: exceptions are things that have charge pumps, take 12/18v supplies, etc, which you need to watch out for. There's no 'electronic' or sound difference. Generally the bigger the capacitance the more you need to be careful: for example my very small 1uf electrolytics are rated 50V, whereas I have some much bigger 470uf caps rated for only 16V.

[Bipolar Joe]

Ah, OK! Thanks, that's handy to know. I guess it helps to know what parts actually do, as opposed to just flinging a bunch at a breadboard and seeing what happens.

Thanks, again .

[PRR]

A cap's voltage rating is the maximum voltage it can safely tolerate.

Few caps are rated under 10V, and most "9V" circuits won't put more than 9V on any part, so it is rather a non-issue here.

You might use 16V parts because many "9V" power-warts run higher.

There's two broad types of caps: "real" insulators (poly-plastic, paper/oil, ceramic, mica) and "electrolytic" (a nanoscopic layer of oxide on aluminum).

The "real" insulators are hard to shave as thin as 10V, they often start at 50V and classic mica starts at 500V (though now you can get it shaved to 50V).

The electrolytic film can be formed incredibly thin and low-voltage, giving cost and size savings. But it does not make sense to make and stock electrolytic caps in "all" voltages.

1uFd electrolytics can be made for 6V, but they are so small and cheap that usually a 50V is the same price and anything lower is special-order.

A 1,000uFd electrolytic is a big cap, and much bigger at 50V than at 16V. Here you prefer to right-size (20%-100% over the maximum in-circuit voltage) to save pennies and grams and millimeters.

You will often see published plans showing rather higher voltage-rating than the cap probably needs to be. The designer may have used whatever was in the handiest catalog, or had a stockpile of 63V caps. If you don't know better, it might be wise to use the given spec. Or it may be that the design specs could be tightened-up appreciably.

SIDEBAR: There is an old-tech idea that you should not OVER-spec an electrolytic cap. That a 450V cap running at 15V may have short life. That seems to have been a problem with 1950s electrolytic technology. Electrolytics "self-heal", a leaky flaw will re-form the oxide film; apparently some 450V caps did not re-form reliably with 15V applied. However these days we often use 50V caps in 1V (even zero Volt) places and do not have any trouble from that. Caps have gotten better. While I would not use a giant 450V cap in a little 9V stomp for other reasons, reliability should not be a problem.

[R.G.]

SIDEBAR: There is an old-tech idea that you should not OVER-spec an electrolytic cap. That a 450V cap running at 15V may have short life. That seems to have been a problem with 1950s electrolytic technology. Electrolytics "self-heal", a leaky flaw will re-form the oxide film; apparently some 450V caps did not re-form reliably with 15V applied. However these days we often use 50V caps in 1V (even zero Volt) places and do not have any trouble from that. Caps have gotten better. While I would not use a giant 450V cap in a little 9V stomp for other reasons, reliability should not be a problem.

A side effect of the thinning of oxide when un-biased is that capacitor oxides that thin over time do get "repaired" to some extent by low currents going through the thin spots. This re-forms the oxide layer, but can only go up to the voltage that's on the cap. So a high voltage cap drifts down a bit over time in its withstand voltage. A side effect of that is that the capacitance goes up a bit, courtesy of the thin spots. So the sidebar is correct, using a high voltage electro cap in a low voltage circuit works fine, but the tolerance over time is worse. And after long use at low voltage - or no voltage, on a shelf - you need to be careful about putting such a cap back in a high voltage circuit.

Non electrolytic caps do not have this perverse and complex chemistry. Use them at any voltage less than their rated voltage.

Beginners, do not confuse yourself with the fine points. Just use a cap at equal to or less than its rated voltage. It'll be fine.