Capacitor dielectric resistance
Capacitor dielectric resistance
(OP)
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
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.





RE: Capacitor dielectric resistance
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...
RE: Capacitor dielectric resistance
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.
RE: Capacitor dielectric resistance
TTFN
RE: Capacitor dielectric resistance
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.
RE: Capacitor dielectric resistance
RE: Capacitor dielectric resistance
God I LOVE THIS FORUM!!!!!!!!!!
In a hundred years, it isn't going to matter anyway.