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

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rfengg:
Thanks everyone !!!

bdunham7:

--- Quote from: magic on July 03, 2019, 09:42:34 am ---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.

--- End quote ---

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:

--- Quote from: magic on July 04, 2019, 05:29:35 pm ---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 ;)

--- End quote ---

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

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