Capacitor dielectric resistance

[ScottI2R]

Hi all,
I have a hypothetical question about capacitors. This is NOT a problem, but just something my wife and I were discussing after a night out. I thought I had found the solution last night, but I cant recall it. I'm too young to be getting senile! (40)
Anyhow, a theoretically "perfect" capacitor, once charged, and then disconnected from any source or load, should remain charged indefinately. Is this correct? If not then my next question is moot.
I would like to determine how to find the leakage current (actually the dielectric's resistance) in a capacitor charged to X volts. I think if I know the input Z of my meter, I could monitor the voltage decay and apply Ohm's law to figure this out. Obviously the decay is not going to be linear because my meter is going to be a load and thus reduce the voltage over time and hence the current. A quick "snapshot" reading may prove effective.
Hit me with any thoughts you may wish to give. This seems like a neat little thing the wife and I can do at home with some electrolytics I have in the work room. Isn't it great that she is interested in this? BTW, the meter is a fluke 87 model 4.
Thanks,
Scott

In a hundred years, it isn't going to matter anyway.

 

[Skogsgurra]

Pulling our legs. Are you?

Anyhow. Electrolytics are not good for this excersise. The leak heavily. Leakage current is often specified in the data sheets. Also, it increases with temperature.

Select a low leakage capacitor (there is a discussion as to whether polypropylene, polystyrene or polyester has the lowest leakage, why not test them all and tell us about the result?) and keep it dry and clean and at room temperatyre for the best result. Use a FET source follower between capacitor and voltmeter. Charge to desired value (you will have a small voltage drop in the source follower but that error is small relative to all other) disconnect voltmeter and voltage source.

Don't test too often. Make a sanity check after a few minutes. Then read the voltage every hour or so. Make the sampling as fast as you can. A good idea with the 87 is to set it to max/min. That will make a fast sampling and still give you time to read the maximum value. Don't forget to reset the max reading before next sample.

Bob Pease has something to say about low leakage:

http://www.national.com/rap/Story/0,1562,5,00.html

Have fun - and say hello to the Missus!

And you know what? It probably doesn't matter next year either...

 

[Comcokid]

I recently ran some test where I was checking the leakage on large value /low voltage capacitors (130,000uF 20 V electrolytic and 1F 2.5 volt supercaps). I just used a stable powersupply running through a Keithley 2000 to measure the leakage in the uA range.

I just hooked it up, left the power supply on, and would occasionally watch the reading. I had to substitute a precision voltage source as even a good regular power supply wanders enough in it's output setting to cause slowly changing readings. Turns out electrolytics have a lot less leakage compared to supercaps.

 

[IRstuff]

In any case, a perfect capacitor should not leak.

TTFN

 

[ScottI2R]

Thanx IRstuff, just like we thought. So much for "senility!"
And Skogsgurra, no, this is an actual question my wife and I were pondering. Thanks for the advice and it sounds like a great way to go. If I log all this, I will be sure to post the results!

In a hundred years, it isn't going to matter anyway.

 

[Skogsgurra]

It is good to hear that you two share the finer nuances of life. We haven't got quite that far in our family. It is more about life of incandescent lamps and how a door-bell works. But, in hundred years, my wife perhaps tells me about the newest quantum technology.

 

[ScottI2R]

To ALL,
God I LOVE THIS FORUM!!!!!!!!!!

In a hundred years, it isn't going to matter anyway.