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Converting polar Electrolytic cap to NP - who is correct?

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bluesdevil:
I did a search on converting polarized electrolytic capacitors to non-polar, but get conflicting info.

The DIY FAQ states:
"You can simulate a non-polarized by using 2 electrolytic caps. Connect them together negative to negative and use the positive leads as the component leads; which is probably why someone notated it as +-||--||-+ Each of the capacitors should be double the value that you need for the circuit because of the series capacitance formula:Ctotal= 1 / (1/C1)+(1/C2)"

R.G. sez in a post:
"You can make a quick and dirty NP cap by tying together the negatives of two equal-sized polar caps.

In the series-NP connection, the capacitance value is funny. Normally caps in series are a smaller capacitance than either cap by itself. If you had two 3.3uF polyester caps, then the expected value for two of them in series is 1.65uF. However, electrolytic caps actually conduct in the reverse direction, so two 3.3uF polarized aluminum electrolytic caps act like they each have a diode in parallel with them that conducts when the voltage is backwards for that one cap. So two 3.3uF caps hooked up as series non polar (i.e. negative to negative) look like a single 3.3uF NP cap.

... except for tiny region near zero volts where they withstand a tiny reverse voltage, so they look like 1.65uF there..."

So will it cut the value in half putting 'em in series (negs connected) or remain the same value of each cap?

R.G.:
I am correct.

An aluminum electro cap conducts in the reverse direction, something like a diode, but with bad diode characteristics. The resistance of the side that is properly polarized is much much more than the reverse biased half, so the charge inside apportions itself across where it's blocked - the properly biased side. So only the properly biased side is acting as a cap, and there's only one cap's worth of capacitance there.

There is a small region around 0V where both are acting like caps but it's best avoided.

Even better, use my rule of thumb: never, ever rely on an aluminum or tantalum electrolytic cap to have a specific value. Use them only where you can make a case for "OK, if I just get a big enough capacitor..." and never where the exact capacitance matters. This is because even if the capacitance is correct today, it will change as the cap ages. And the tolerances on electros is generally bad. As bad as -20% +80%.

If you designed your circuit well, the electro caps are bigger than is absolutely needed, so make each one of the polarized caps be the value of the NP cap you want, and the circuit will work.

If you relied on the value of an NP cap for specific values, like a rolloff frequency - well, may Mother Nature have mercy on you.

aron:
Thank you. I have updated the FAQ

bluesdevil:
Wow R.G., a helluva big thanks for that quick response!!!  Please don't be insulted or offended, but have you ever actually measured the results with a meter or is your statement purely theory? I just realized after posting I have a "CAP" mode on my tester and I'm getting the half reading with two 4.7uf tantalums..... or maybe my multi-meter  tests at the "near zero" voltage you mentioned?
      Like you said the tolerances are rather loose and it'll probably be fine either way, just trying to get to the bottom of it all.

R.G.:
Tantalum will withstand a reverse voltage of a few volts before the leakage comes up on the reverse biased side. So with tantalum the "funny area in the middle" is much bigger, maybe as much as 4-5V depending on the cap. Also, most DMM testers test with a small voltage across the cap so they give better answers when the caps are tested in-circuit. You're probably getting both effects.

In your conditions, I would expect just the results you're getting.

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