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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
On Friday, 11 November 2016 19:06:35 UTC, John Fields  wrote:
> 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
I've not studied the lit on lytics, but I've not encountered one that looks like a resistor. NT
On Fri, 11 Nov 2016 14:55:30 -0800, tabbypurr wrote:

> On Friday, 11 November 2016 19:06:35 UTC, John Fields wrote:
>> 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.
So not ideally suited for use in the microwave spectrum, then. ;-)
>> 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.
> I've not studied the lit on lytics, but I've not encountered one that > looks like a resistor.
I'm guessing he's referring to the ESR which these caps are known for.
On Sunday, 13 November 2016 13:07:35 UTC, Cursitor Doom  wrote:
> On Fri, 11 Nov 2016 14:55:30 -0800, tabbypurr wrote > > On Friday, 11 November 2016 19:06:35 UTC, John Fields wrote: > > >> 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. > > So not ideally suited for use in the microwave spectrum, then. ;-) > > >> 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. > > > I've not studied the lit on lytics, but I've not encountered one that > > looks like a resistor. > > 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
On Fri, 11 Nov 2016 14:55:30 -0800 (PST), tabbypurr@gmail.com wrote:

>On Friday, 11 November 2016 19:06:35 UTC, John Fields wrote: >> 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 > >I've not studied the lit on lytics, but I've not encountered one that looks like a resistor.
--- What do you think one looks like if you hook it up backwards? JF
On Sun, 13 Nov 2016 13:07:00 -0000 (UTC), Cursitor Doom
<curd@notformail.com> wrote:

>On Fri, 11 Nov 2016 14:55:30 -0800, tabbypurr wrote: > >> On Friday, 11 November 2016 19:06:35 UTC, John Fields wrote: > >>> 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. > >So not ideally suited for use in the microwave spectrum, then. ;-)
--- Indeed. :) ---
>>> 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. > >> I've not studied the lit on lytics, but I've not encountered one that >> looks like a resistor. > >I'm guessing he's referring to the ESR which these caps are known for.
--- I'm not. If you connect a formed electrolytic capacitor across a DC power supply, backwards, and limit the current through the capacitor to a level which won't damage the cap, the current will eventually drop to the leakage current allowed by the new oxide grown. If the process is allowed to continue until there's twice the voltage across the cap, its capacitance will have been reduced by a factor of two, and it'll be bipolar. JF
On Thursday, 1 December 2016 22:06:10 UTC, John Fields  wrote:
> On Fri, 11 Nov 2016 14:55:30 -0800 (PST), tabbypurr wrote: > >On Friday, 11 November 2016 19:06:35 UTC, John Fields wrote: > >> 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 > > > >I've not studied the lit on lytics, but I've not encountered one that looks like a resistor.
> What do you think one looks like if you hook it up backwards?
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