I was asking about very old capacitors and their shelf life on FB vintage test equipment group. The guys tell how good the old capacitor testers are because they have the “magic eye” to check for leakage.
I was thinking that it would be easy to test any capacitor for leakage. All that’s needed is a 1 meg resistor or maybe 100k for higher leakage. And a high impedance meter to measure the voltage across the resistor. Or instead, use a microammeter in series with the capacitor, and a resistor to limit the current.
And then the voltage source, of course. For a voltage source, I’d use a 230 VAC to 12 VAC transformer, secondary connected to the secondary of a 120 VAC to 12 VAC transformer, with a half wave voltage doubler on the output. And plug this in to a Variac. And a high value bleeder resistor on the output. This would give isolation from the AC line.
If a Variac isn’t available, I could use a high resistance, 2 or more watt potentiometer to tap off the required voltage. I think that’s how the capacitor testers do it. The capacitor testers use a magic eye, which doesn’t have the ability to measure the leakage current, it only gives a relative indication of leakage.
I’ll have to look at a schematic of the old capacitor testers to see how they do the leakage test. The big headache is sorting out the signal paths through the rotary switches used in test equipment.
Update – From FB group, Bob Johansen
“Yes, you can check leakage… The eye tube gives a relative indication, the Heathkit is calibrated for the eye to close at approximately 2 uA when checking paper/ mica caps and 2 mA checking electrolytics, this is considered to be the maximum allowable leakage current for each device.”
So thank you, Bob.
