Use of 2 polarised caps in reverse series instead of a single polarised cap?

[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]

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.

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!

[rfengg]

Thanks everyone !!!

[bdunham7]

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.

I had to think about that for a few minutes, and under the situation you describe, you're right--on steady state AC, assuming negligible losses, both capacitors should maintain a correct positive bias that goes to Vp for one and zero for the other, alternatively.  So their 'rest' state is equally charged, but in opposition to each other.  However, below 1 to 1.5 volts per capacitor, the breakdown doesn't occur and the reverse-biased cap just works like a normal cap--which also results in a total capacitance of half the individual values.  In this case, the capacitor charges are aligned, not in opposition.  So what happens if you gradually increase the voltage through this transition?

This transition would be right in the range of audio amplifier outputs, one very common place for NP caps, as quiet music might be well under 1 volt but almost any system can exceed 5 volts if you turn it up.  I've wondered about the linearity of NP caps and many people swap them out now for various non-electrolytic types like the one I posted.  However, an audio engineer once told me that they measured the distortion with NP caps and it really was pretty low.  Actual NP caps are a single unit that has two oxide layers instead of one, in opposed polarity.  I've always assumed that this was superior to the back-to-back arrangement we're discussing, but now I'm not sure.  I have to repair some audio equipment next month so perhaps I'll do some experiments. 

[magic]

It has occurred to me that since bipolar elcos are functionally barely different from two polarised elcos back-to-back, their behavior is going to be the same.
So when the electrically floating electrolyte between the electrodes is discharged to ground potential (due to dielectric leakage), the first application of voltage, and any subsequent application of voltage higher than before, will see double capacitance.

I wonder how many people know about it

[The Electrician]

It has occurred to me that since bipolar elcos are functionally barely different from two polarised elcos back-to-back, their behavior is going to be the same.
So when the electrically floating electrolyte between the electrodes is discharged to ground potential (due to dielectric leakage), the first application of voltage, and any subsequent application of voltage higher than before, will see double capacitance.

I wonder how many people know about it

Those who have read this thread: https://forum.allaboutcircuits.com/threads/polarized-to-non-polarized-capacitor.91406/ know about it.

[Zero999]

At low AC voltages, under 1V, polarised aluminium electrolytic capacitors can be used as is, because it takes a couple of volts to breakdown the electrolyte. There will be slightly more distortion, than with two back-to-back capacitors or a non-polar capacitor, but at very low voltages, under a few hundred mV, it's negligible and not worth worry about.

Here's a paper I found awhile ago which investigates distortion and applying the optimum DC biasing voltage to overcome it.
https://linearaudio.nl/sites/linearaudio.net/files/Bateman%20EW%2012%202002%20mar2003%201uF%20electrolytic%20or%20film.pdf