Film Capacitor v.s Ceramic Capacitor Leakage Current Testing

Thread Starter

SethCap

Joined Nov 3, 2022
4
I have a question regarding the differences between leakage current testing a film capacitor and a ceramic capacitor. I have a test setup that outputs leakage current ratings utilizing a picoamp meter. I am measuring down to the nA range for leakage on a multitude of various capacitors. The ceramic capacitors are showing a fairly steady readout on the instrument 20-25nA. When I place a film capacitor inside this setup the meter is very jumpy and not outputting usable figures for leakage.

The thing is, I have tested all of these film caps on an I.R tester (which is what we typically specify these capacitors to) and when converted from I.R to leakage current they should be in the lower 10-15nA range. Why does this test setup show so much jumpiness with the film caps, even though they should be superior in leakage current ratings? Someone else is request leakage current ratings for a capacitor and they are also experiencing this jumpiness when they are testing on their leakage current setup. So we are both scratching our heads.

I have tested different film caps as well. Dielectric material, capacitance value, voltage rating, metalized and non-metalized,. All of them show this jumpiness. I conducted a 3 minute discharge test to verify these conversions as well. They are within 10% of expected voltage drain calculations. 1200V charge and a drop of 15V after 3 minutes. Expected drop calculated at ~12V.

Any insight into why this method of testing leakage is not consistent would be appreciated. Or suggestions on alternate leakage testing setups or equipment would be appreciated as well.
 

Audioguru again

Joined Oct 21, 2019
5,416
The film in a film capacitor is an insulator with no leakage current unless it is a very very cheap capacitor from "over there".
I have not used high voltage vacuum tube circuits since about 58 years ago.
 

Thread Starter

SethCap

Joined Nov 3, 2022
4
The film in a film capacitor is an insulator with no leakage current unless it is a very very cheap capacitor from "over there".
I have not used high voltage vacuum tube circuits since about 58 years ago.
Yes these caps should be very low leakage in the nA range. The issue is that this other person is requesting insight as to why their typical leakage test is not working for film caps over the typical ceramic caps they use. I have pointed out that our I.R readings can be used in substitution for leakage rating, but now they are requesting a response as to why the leakage testing is showing abnormal leakage averaging (over 10-15 minutes). Somewhere in the 200-2000nA range. Or 20-200x the expected rating.

Is there some phenomena or material property with film that makes it hard to measure leakage current?
 

Thread Starter

SethCap

Joined Nov 3, 2022
4
We were suspecting some interference somewhere in the system, initially. But his readings on the same spec ceramic caps are not showing the same jumpiness in readings. Same test setup and test parameters, just a film cap instead of ceramic.

The film cap is a metalized polyester construction, if that helps add some more information for anyone.
 

Thread Starter

SethCap

Joined Nov 3, 2022
4
These are made locally, down the street from me. US made, wound capacitors. These are high quality capacitors being tested. Both mine and the ceramic caps that they are testing. The curiosity is in regards to testing methods; why there are inconsistencies between film and ceramic capacitors on the same testing setup when built to the same specifications?

If anyone has experience with testing leakage currents on capacitors in the 1000V range I would be very interested in their setup and if they experience the same jumpiness in readings.
 

DickCappels

Joined Aug 21, 2008
9,512
It is my understanding that it is the other way around - ceramic capacitors are like piezoelectric transducers and tend to exhibit changes in capacitance with voltage (they tend to be nonlinear) and film capacitors are nice suitable for audio and such.

If I were in your place with a budget I would try a different brand film capacitor, or better yet, call the applications engineers at the company that made the strange capacitors. That is why they are there.
 
Top