Electronics > Beginners
Use of 2 polarised caps in reverse series instead of a single polarised cap?
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rfengg:
On many forums, I have seen that people advocate the use of 2  polarised electrolytic caps  in reverse series instead of a single non-polarised  electrolytic cap.......its mainly in audio applications as a dc decoupler for the speakers, so that there are is no d.c. riding on the speakers.

So if I need a 10uF non polarised cap, then the general advise given  is that if I cannot get hold of a non polarised cap, then to take two 22uF polarised  electrolytic caps and connect them back to back in series (i.e.  the negative terminal of the first  electrolytic to the positive of the second  electrolytic and use the series combination instead)

For the life of me , I cannot understand how would this be allrite?  :-//
When one cap charges in the correct polarity, surely the second one is charged in the wrong polarity as both caps are  connected back to back?
bdunham7:
Reverse biasing the polarized cap isn't what hurts them, it's the overcurrent that results if you do something like put it in a DC power supply backwards.  If you have them in series (and it is negative-to-negative, not negative to positive) the correctly polarized cap will limit the current.  And you will need to use two 10uF caps like this to get a 10uF non-polarized as the usual math doesn't work--the reverse biased cap doesn't contribute.  This seems like it would be pretty non-linear at low levels, but apparently it works well enough. 

If you are really concerned about the quality of the result, I would suggest metal film polypropylene for audio applications. 

https://www.parts-express.com/dayton-audio-dmpc-10-10uf-250v-polypropylene-capacitor--027-428
magic:
You absolutely do need 2x20µF to get 10µF and both capacitors contribute.

The first time you charge them, the forward biased capacitor charges like a normal 20µF cap and the reverse biased capacitor breaks down and conducts like a diode with a minor voltage drop.

When you then discharge, the first cap will be discharging and the other will be charging in reverse polarity. So from now on, they behave as a normal series combination of 2 capacitors.

This state only changes when you apply a voltage higher than any previously applied, in any direction. Or when leakage discharges them.
Zero999:
Yes, an electrolytic capacitor behaves like a poor diode and passes current, in the reverse direction. It only damages the capacitor if the DC high enough and is sustained for long enough to breakdown the oxide layer and boil off the electrolyte.

It also makes no difference which way the capacitors are connected, as long as they're back-to-back.
Gyro:

--- Quote from: Zero999 on July 03, 2019, 10:38:20 am ---Yes, an electrolytic capacitor behaves like a poor diode and passes current, in the reverse direction. It only damages the capacitor if the DC high enough and is sustained for long enough to breakdown the oxide layer and boil off the electrolyte.

--- End quote ---

I've used this approach (at a push) for higher current applications by adding a parallel reverse protection diode across each capacitor. You obviously need to be careful that you stay well below their ripple current rating!
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