How to make a bipolar electrolytic capacitor from what you have in your junk box.

Late night then, John?

Will it then be a bipolar cap?

In case you missed it, I have an ongoing experiment to see if a 56u

25V polymer aluminum cap will work as a bipolar cap over the range of +15 to -5 volts. I'm running six in parallel at -10 to see what happens there. After about a week, the leakage has dropped by 100:1, sort of like you describe.

Of course, I may have destroyed them as normal-polarity caps; gotta test that soon.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics

Well, I hate to sat this but if you need a bipolar cap in the higher values that is an engineering failure.

But to do it you simply take four of those 50 uF in series in opposite polarity and you get 12.5 uF.

It is rare you would really need a bipolar electrolytic except in like a speaker crossover. Possibly some in instrumentation. However taking them in series like that the ESR adds up. However your idea is not any better.

It is possible that like a 250 volt cap could stand like 5 volts reverse without harm, but that is not good engineering practice. Electrolytics are damn unreliable enough as it is and you are going to do this ? No way. I wouldn't.

Rummage until you find two 50uF caps, connect the (-) terminals together and diode-clamp each one. Now from one (+) terminal to the other, is a 25 uF capacitor. On really big transient swings, it's a 50 uF capacitor and the impedance of a forward-biased diode (or 50 uF) in series, whichever is less.

While I agree in principle, IIRC there's nothing polar about a lytic expect the forming step, so a reverse formed cap should be just as reliable.

So if it goes somewhere in the circuit where it won't get much current, I guess it could go in without forming ;)

NT

Yup. Back-to-back aluminums get complex. I assume a single "bipolar" aluminum electrolytic cap is similarly complex.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics

A polar electrolytic is wound from two differently etched and oxidized foils; cathode and anode. A non-polar cap is wound from two identical foils, I think two anode foils but could be the other way round.

So a polar cap does have a polarity from birth but can as you say be "bent" to some extent.

piglet

Has nobody suggested the Sprague TE series capacitors?

They seem to work very well RP at half voltage rating and as bipolars at half voltage rating.

That was about 40 years ago; maybe they still make that series or equivalent.

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Not at all. 

If you delve into the literature and take a look at how aluminum 
electrolytics work, you'll find that they look like diodes with a huge 
junction area when they're reverse biased and, basically, like 
resistors when they're forward biased. 

From my experiments I've found that oxide can be grown on the normally 
metallic plate with a thickness which will decrease the capacitance of 
the unit, but stand off higher voltages. 

JF

I've not studied the lit on lytics, but I've not encountered one that looks like a resistor.

NT

So not ideally suited for use in the microwave spectrum, then. ;-)

I'm guessing he's referring to the ESR which these caps are known for.

They have ESL too, not just R. More or less all caps do, hence the vee shaped impedance graph.

NT

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What do you think one looks like if you hook it up backwards?  

JF

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Indeed. :)

I did it accidentally years ago. It worked ok. Presumably the circuit limited current, whereupon the cap formed a new oxide layer and stopped leaking.

NT