[sdiy] Caps, nerds, New Year's Eve and soakage

Byron G. Jacquot thescum at surfree.com
Wed Jan 3 20:54:30 CET 2018

>As a last activity in the office the last day of 2017 (Friday 29th) I 
>have charged a bulky polypropylene capacitor up to 32.1V with the 
>intention to measure it after going back on first Monday of 2018 (here's 
>the New Year's Eve part of the subject).
>The cap is 1.5uF/400V from year 1987.
>To my surprise, after almost 4 days, or more precisely 88 hours, it was 
>still charged to more or less the same voltage. Unfortunately my meter's 
>input resistance is 10Mohm for ranges above 10V, so it started quickly 
>discharging during measurements. First reading was not too big as it 
>started in the middle of converter cycle, and next one was 31.2329V with 
>the next ones dropping by about 0.5V each. So assuming the meter was 
>connected about 1.5 cycle earlier, that indicates it was really about 
>32V at the start. No drop whatsoever during 88 hours!
>To make more precise measurements, I then charged it to 10V, in order to 
>take advantage 1Gohm loading of the meter, and left it for another 29 hours.
>Yesterday's reading - 10.0004V
>Today's reading - 10.0277V
>Not only it did not drop, but voltage rised by 27mV! I think that's 
>dielectric absorption. After all, it was charged to 32V for 4 days, so 
>it could have soaked a bit. OTOH I wasn't expecting any absorption 
>effect to be aparent in polypropylene capacitor.
>So now I discharged it and let it rest till tomorrow to see how high it 
>can go by itself.

This reminds me of some stuff Bob Pease was doing when the LME49720 was introduced.  His seminar around that time was focusing on audio (and audiophile) topics - distortion in opamps, magic capacitors, etc.

There's a writeup where he discusses testing cap leakage and soakage - one thing he did over you was use a hiZ biffer amp infront of the voltmeter.


In the seminar, he actually wrapped some wax-paper & foil foil caps, and poked and prodded them in contrast with some vintage caps and modern film caps.  The film caps did very well in comparison.

-Byron Jacquot

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