Cap leakage test

[Randall]

What is your preferred method of testing for a leaky capacitor, assuming you don't have an engineering lab in your shop? Let's include both quick and dirty as well as comprehensive methods.

[R.G.]

What is your preferred method of testing for a leaky capacitor, assuming you don't have an engineering lab in your shop? Let's include both quick and dirty as well as comprehensive methods.

You can measure the leakage. Put a largish value resistor in series with the cap, charge it up to a voltage near the working voltage. Let it settle.

A perfect cap has zero leakage, so the voltage difference across the resistor would be zero. However, real caps leak. So the leakage current causes a DC voltage across the added resistor. Leakage current is equal to the voltage across the added resistor divided by the resistance.

The bigger the resistor you add, the more sensitive the test.

That then brings up the question of how much leakage is too much.

[R.G.]

You can charge the cap up to near the working voltage, remove the charging voltage, and then watch the voltage as it leaks down. Get out your stop watch.

The voltage leaks down at a rate determined by the leakage current. Modelling the leakage as a resistance, a little math leads us to the capacitor voltage Vc being

Vc = V0*e^-t/RC) where V0 is the initial voltage, t is time in seconds, C is the capacitance in farads and R is the resistance in ohms.

Set to V0, let it run down for a measured time, then calculate leakage resistance R. Or time it for how many seconds to reaching 63.2% of the initial voltage makes t/RC be one, so R = t/C, same units.

[Enzo]

You can also watch how it behaves in a circuit. If you have a coupling cap from the plate of a tube down to the grid of the next tube, then there will be plate voltage on the one end, a couple hundred volts or whatever. If ther is some positive voltage at the grid of the next tube, it is probably coming through that cap as leakage.

[Steve A.]

I used to have a tester for HVAC service that was mainly a non-contact voltage tester but there were also 2 leads which could be used for testing continuity. When clipped to a motor run capacitor the pitch would drop and fade out after maybe 10 to 30 seconds. (As I recall it was something like 2 seconds for each mfd.) After testing the cap once you would have to reverse the leads to be able to test it a second time presumably because of the charge imparted by the tester.

Steve Ahola

[Tom Phillips]

What is your preferred method of testing for a leaky capacitor, assuming you don't have an engineering lab in your shop? Let's include both quick and dirty as well as comprehensive methods.

A real quick and dirty way that can be used for checking for DC leakage of coupling / blocking cap applications is:
1) Lift (Disconnect) the low voltage side of the cap from the circuit. This leaves the other side connected to a source of DC voltage.
2) Attach you DC voltmeter +probe to the lifted lead of the cap. Attach - lead to chassis ground.
3) Turn on the amp
4) Monitor DC voltage on cap. The meter reading will jump up and then settle toward zero as the cap charges through the meter’s input impedance. The rate of change will depend on the capacitance value. Fast settling for a small cap (like 250 pF) and slower for a larger one (Such as 0.1 µF)

Evaluation of Result.
You need to become familiar with the behavior because this test depends on the cap under test. That is, there is no green light=good cap / red light=bad cap result.
In general a leaky cap will end up showing more than 0.1V on the meter. A good cap may show much less and, with a digital meter, you often end up seeing the display flipping between small + and - readings.

All the coupling caps can be tested in an old Fender in just a couple of minutes by lifting them all at one and then moving the test lead from one to the next W/O cycling the power.

You asked to include quick & dirty so there it is.
That’s my Q&D method. As always, only people who understand how to safely work on energized circuits containing high voltages should try this. Anyone who dos not understand what’s going on should not try the described procedure.

Cheers,
Tom

[Joe L]

A real quick and dirty way that can be used for checking for DC leakage of coupling / blocking cap applications is:
1) Lift (Disconnect) the low voltage side of the cap from the circuit. This leaves the other side connected to a source of DC voltage.
2) Attach you DC voltmeter +probe to the lifted lead of the cap. Attach - lead to chassis ground.
3) Turn on the amp
4) Monitor DC voltage on cap. The meter reading will jump up and then settle toward zero as the cap charges through the meter’s input impedance. The rate of change will depend on the capacitance value. Fast settling for a small cap (like 250 pF) and slower for a larger one (Such as 0.1 µF)

This is the method I use for all non-electrolytic caps in new builds. If I see any voltage once settled, I trash the cap. All electrolytics get pre-tested on my Sencore LC77 at full rated voltage before installation. The manual for the Sencore has a chart for maximum leakage of electrolytics and good caps usually take less than a minute to go below the max allowable leakage.

The Sencore manual makes for some good reading. I found a pdf for it at this link.

[soundmasterg]

The above tests are great methods and I use them too. If I want to check another way, I use my Sprague Tel-Ohmike which is super useful and can test value and leakage up to 600 VDC. You can also use them to reform caps if you really want to do that since they current limit the device under test. You can find them reasonably priced on ebay.

Greg