Capacitor Leakage Test Fixture

My Simpson 383A Capacohmeter made me nervous, as in its megohm-meter mode it leaves the capacitor under test charged to 300 volts or so.


So, I decided to make a test fixture that would discharge the capacitor after testing, and also allow me to measure the leakage current with a DMM or VOM.


The pushbutton normally leaves the test terminals connected to a 10K ohm 2 watt resistor. Pushing the button temporarily connects them to the Capacohmeter or other source of high voltage. 

It also works well with my McMurdo Silver Model 904 Capacitance Resistance Bridge.


The toggle switch either opens or shorts the current probe test points. When open, you can plug in the test prods of a DMM or VOM to pin jacks to measure the capacitor leakage current. 


The test terminals are banana jacks, so I can connect test leads to them. But I also procured some Grayhill number 02-1 spring clips with banana plugs, so I can use them when convenient. (Here's the spec sheet for Grayhill's line of spring clips.)


The assembled unit is built inside a Keystone #703 phenolic box with a #2042 phenolic panel.


Here's an internal view of the wiring.

Cap leakage test fixture

Here's the schematic. Sorry some of the text isn't rotated correctly; my schematic editor is buggy on the newer OS X Mavericks operating system. I'll have to upgrade the software.

It's quite straightforward. The leakage current test points are on the negative return side because that side of the HV supply is usually grounded, so the meter will stay close to ground rather than being elevated to the full high voltage potential. 

The pushbutton switch is a Switchcraft model 903X. Be sure to use a switch with a high enough insulation voltage rating to keep from being bitten by the high voltage supply. I did check the Switchcraft 903X specification when I selected it, but in the meantime their web-site has changed and the current 903X spec sheet no longer includes insulation voltage ratings :-(

I'm planning to make one improvement: I'm going to add another pair of banana jacks for the capacitor under test, placed adjacent to the existing jacks but spaced wider to better handle the larger components.

© Steve Byan 2011-2019