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

Started by John Fields October 30, 2016
Let's say you need a >10 microfarad bipolar cap rated to stand off 10
volts, right away, and and all you have in your junk box is a 50
microfarad monopolar aluminum electrolytic rated to stand off 50
volts.

If you connect the cap, backwards, to a DC supply and limit the
current into the cap to something which won't blow it up, the current
into the cap  will eventually fall close to zero as the plates reform.
On Sun, 30 Oct 2016 02:41:24 -0500, John Fields wrote:

> Let's say you need a >10 microfarad bipolar cap rated to stand off 10 > volts, right away, and and all you have in your junk box is a 50 > microfarad monopolar aluminum electrolytic rated to stand off 50 volts. > > If you connect the cap, backwards, to a DC supply and limit the current > into the cap to something which won't blow it up, the current into the > cap will eventually fall close to zero as the plates reform.
Late night then, John?
On Sun, 30 Oct 2016 02:41:24 -0500, John Fields
<jfields@austininstruments.com> wrote:

>Let's say you need a >10 microfarad bipolar cap rated to stand off 10 >volts, right away, and and all you have in your junk box is a 50 >microfarad monopolar aluminum electrolytic rated to stand off 50 >volts. > >If you connect the cap, backwards, to a DC supply and limit the >current into the cap to something which won't blow it up, the current >into the cap will eventually fall close to zero as the plates reform.
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. https://dl.dropboxusercontent.com/u/53724080/Parts/Caps/Polymer_Rev_Test.JPG Of course, I may have destroyed them as normal-polarity caps; gotta test that soon. -- 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. 
On Sunday, October 30, 2016 at 12:41:33 AM UTC-7, John Fields wrote:
> Let's say you need a >10 microfarad bipolar cap rated to stand off 10 > volts, right away, and and all you have in your junk box is a 50 > microfarad monopolar aluminum electrolytic rated to stand off 50 > volts. > > If you connect the cap, backwards, to a DC supply and limit the > current ...
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.
On Sunday, 30 October 2016 18:21:40 UTC, jurb...@gmail.com  wrote:
> 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.
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
On Sun, 30 Oct 2016 15:06:33 -0700 (PDT), whit3rd <whit3rd@gmail.com>
wrote:

>On Sunday, October 30, 2016 at 12:41:33 AM UTC-7, John Fields wrote: >> Let's say you need a >10 microfarad bipolar cap rated to stand off 10 >> volts, right away, and and all you have in your junk box is a 50 >> microfarad monopolar aluminum electrolytic rated to stand off 50 >> volts. >> >> If you connect the cap, backwards, to a DC supply and limit the >> current ... > >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.
Yup. Back-to-back aluminums get complex. I assume a single "bipolar" aluminum electrolytic cap is similarly complex. -- John Larkin Highland Technology, Inc lunatic fringe electronics
On 30/10/2016 23:56, tabbypurr@gmail.com wrote:
> ... IIRC there's nothing polar about a lytic expect the forming step, so a reverse formed cap should be just as reliable. >
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
jurb6006@gmail.com wrote:
> 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.
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.
On Sun, 30 Oct 2016 11:16:52 -0000 (UTC), Cursitor Doom
<curd@notformail.com> wrote:

>On Sun, 30 Oct 2016 02:41:24 -0500, John Fields wrote: > >> Let's say you need a >10 microfarad bipolar cap rated to stand off 10 >> volts, right away, and and all you have in your junk box is a 50 >> microfarad monopolar aluminum electrolytic rated to stand off 50 volts. >> >> If you connect the cap, backwards, to a DC supply and limit the current >> into the cap to something which won't blow it up, the current into the >> cap will eventually fall close to zero as the plates reform. > >Late night then, John?
--- 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