Sal Brisindi Capacitor Challenge

Sal Brisindi (Sal's Capacitor Corner) posed an interesting experiment in a recent thread on the Antique Radio Forum

Two different specifications.jpg

Two different specifications ...

Sal was shipped a bunch of 160 volt rated capacitors and 450 volt rated capacitors which, apart from the markings, appear to be identical. Are the 160 volt electrolytics actually only rated to 160 volts, or did the manufacturer produce one lot and label half as 450 volt and the other half as 160 volt?

But the same capacitor?.jpg

... but identical capactors?

Sal asked for volunteers to test the capacitors and report back the results This sounded like a fun way to spend an evening, so I signed up for the Sal Brisindi Capacitor Challenge.

Experimental setup.jpg

Experimental setup

I used my Power Designs Model 2K-10 High Voltage Regulated DC Supply to provide the polarizing voltage for the experiment. The 2K-10 is a nice supply, mostly transistorized but with a tube series-pass stage. It provides up to 2 KV at up to 10 mA, with the voltage set by a precision voltage divider running off a temperature-compensated Zener diode. It's spec'd for  0.25% accuracy and less than 0.005% drift per hour.

Capacitor test fixture.jpg

Capacitor test fixture

I used my capacitor test fixture, which I usually use with my McMurdo-Silver Model 904 Capacitance-Resistance Bridge and my Simpson Model 303-A Capacohmeter.

I used my Fluke 27/FM set on the µA range to measure the leakage current. I kept forgetting to set it to the mA range when first hitting the cap with HV. The power supply would go into current limit at 10 mA, and the Fluke would sometimes shut down, I suppose as a protection against the extreme overrange. Cycling the power brought it back.

Sal sent 10 capacitors of each flavor. I tested the 160 volt rated capacitors first at 160 volts and then at 450 volts. I recorded the leakage current after two minutes as timed by the timer on my iPhone. I then tested the 450 volt rated capacitors, again recording the leakage current after two minutes. I botched the experiment somewhat – I should have first given the 450 volt capacitors two minutes at 160 volts before measuring their leakage at 450 volts. 

Sal Brisindi capacitors

You can see that the leakage on the 450 volt capacitors is a bit higher than the leakage on the 160 volt capacitors running at 450 volts. I suspect this is due to not having first formed the 450 volt capacitors for two minutes at 160 volts.

Presuming my explanation for the discrepancy in leakage is correct, I conclude that they are all essentially the same capacitor, whether marked for 450 volts or 160 volts.

I'm glad none of the 160 volt labeled caps went "bang", because the rest of the household is sound asleep :-)

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