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

[Roman Sowa]

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.

Roman