Cap self-recharging

[Marko7]

Hi folks,
Ya know, for years I've always wanted to know just how an electrolytic capacitor, especially a Hi-V filter type, can seem to "recharge" itself after a full drain via a temporary short across it's leads...
In other words, what kind of chemical action or re-action- or other electrical phenomena- accounts for, say, a 22uF/ 500 volt filter cap which is *removed* from any circuitry (isolated) developing up to several measurable volts across it's plates (leads) if it's been in service for some time?
An E.E. assistant college professor said it was beyond the scope of the current material when I quized him 1 day during class. I never did follow up on it.
Ah figgered this forum might be the best place to pose the riddle.

[Per Hansson]

Re: Cap self-recharging

Most likely case is that your meter charges the cap when you measure it.

[SteveNielsen]

Re: Cap self-recharging

My understanding is that momentarily discharging a cap does not completely zero-out the chemical imbalances inside the cap that create and maintain the ability to hold a charge.

[budm]

Re: Cap self-recharging

http://mysite.du.edu/~etuttle/electron/elect53.htm
https://cdn.badcaps-static.com/pdfs/...f439032f62.pdf

[goodpsusearch]

Re: Cap self-recharging

I have noticed this on an online type UPS.

[SteveNielsen]

Re: Cap self-recharging

Might gain some insight in this thread:

http://electronics.stackexchange.com...ergy-over-time

[stj]

Re: Cap self-recharging

it's probably charging up from the energy field around it.
we are swimming in RF, i see no reason it cant collect on a suitable antenna such as the foil roll in a cap.
if you can pull current from the air with a crystal radio - why not a cap.

i bet nikola tesla could explain it!

[Marko7]

Re: Cap self-recharging

Steve, You might be the closest so far.
It certainly doesn't charge from the voltmeter, Per, as they "drain" the source- to varying degrees- due to the impedance of their input. (VTVMs or "vacuum tube voltmeters", were the highest input-Z meters out there for a LONG time and presented a load no more than a couple megohms, if that.)
Nope, stj, i think any & all EMF fields "permeating" the cap would both charge & discharge the cap, as it is all essentially A.C., resulting in a cancellation of any added DC charge.

The phenomena can be easily duplicated (viewed) by *carefully* charging up a *new* 450 or 500 volt filter cap, that starts out with a trace charge in the milliviolt range (after a quik discharge)... applying near full voltage (say 430 VDC- Be Careful!!) from a power supply, & let it sit hooked up & floating overnight.
Then disconnect the cap in the morning, carefully DIScharging it to zero with an appropriate resistor, maybe a 470 ohm power Resistor.
Leave the it connected for a few minutes, verify 0 volts, & then disconnect the resistor.

I *guarantee* that you'll measure several volts, maybe even 10 or 20 volts, after letting it sit & "self-charge" for a few minutes. Not sure if non-electrolytic caps like large mylars will do this, but the electrolytics sure do!

[Marko7]

Re: Cap self-recharging

Steve, et al:
On the webpage you linked us to, Tony Stewart posted:
"On large electrolytic caps, like "main-frame" computer grade 100,000uF and TV HV 10uF 25KV doubler Caps, power supplies there is a phenomena like in batteries, known as memory. After you short it out the voltage creeps back. That's all you need to know. Short it long enough to discharge the memory effect."

Hmmm... All I need to know, huh? ("But wwhhyyyyy??", markie cried.)
P.S: I've discharged some old caps in gee-tar' amps for a couple minutes, dead-shorted, & gotten enough juice back across their plates after a half hour to light up a #47 lamp (briefly)!

[stj]

Re: Cap self-recharging

here's a test.
put a resistor across one for 24hours.
then leave it without the resistor for a day or 2.
then check it.

[kc8adu]

Re: Cap self-recharging

http://en.wikipedia.org/wiki/Dielectric_absorption

[joshnz]

Re: Cap self-recharging

http://en.wikipedia.org/wiki/Dielectric_absorption

This was mentioned on eevblog

[Marko7]

Re: Cap self-recharging

Thanx, kc8 ! didn't figger Wiki had an actual page on the phenomena.