We all know that capacitors have a shunt resistance (leakage) and that leakage resistance should be pretty easy to measure, right? Wrong! I've measured a lot of capacitors for short-term soakage (dielectric absorption) per www.national.com/rap/Application/0,1570,28,00.html.
After the short-term soakage stops, it's possible (not easy) to measure the leakage. For example, if you charge a good cap up to 9 V for a few seconds, it will start discharging shortly for several millivolts. If you wait long enough, you may see leakage slow down to a few millivolts per hour. But you will see the long-term soakage. Is that different from the short-time leakage? Maybe not.
Now I will charge up some of my favorite low-leakage capacitors (such as Panasonic polypropylene 1 µF) up to 9.021 V dc (a random voltage) for an hour. I will read the VOUT with my favorite high-input-impedance unity-gain follower (LMC662, Ib about 0.003 pA) and buffer that into my favorite six-digit digital voltmeter (DVM) (Agilent/HP34401A) and monitor the VOUT once a day for several days.
Why did I choose 9 V? Because that's within the common-mode range of the op amp and the DVM at highest resolution. I keep the input ball hook connected to +8.8 V dc between readings. I also keep my left hand grounded to +8.8 V.
DAY BY DAY
One of my e-mail colleagues had been monitoring some good 0.1-µF polystyrenes, and he was impressed that they got down to a leak rate of better than a year after several months. Well, I could see that my polypropylenes had their leak rate improve even better than that in just a few days. Refer to the list of voltages below:
Day 0: 9.0214 V
Day 1: 9.01870 V
Day 2: 9.01756 V
Day 6: 9.0135 V
Day 7: 9.0123 V
Day 8: 9.01018 V
Day 9: 9.00941 V
Day 11: 9.00788 V
Day 12: 9.00544 V
Day 13: 9.00422 V
The first day after soaking for an hour, their leak rate was as good as 2.7 mV per day. Not bad.
If you had a 1 million-MΩ resistor across a 1-µF capacitor at the 9-V level, it would draw 9 pA, which would pull down the capacitor 778 mV per day. All the capacitor types I tested were better than this, except some "oil-and-paper" caps that supposedly had special qualities for audio signals.
If you had a 10-meg-meg resistance, that would cause the cap to leak down 78 mV/day. With 100 meg-megs, it would be 7.8 mV per day. Several good capacitors soon began to leak slower than that. After a mere week, some of the best caps were leaking at a rate down near 1 mV/day. Quite good. So, what's the big deal?
The big deal is that a time constant of 31.5 meg seconds is one year! So any capacitor leaking less than 2.5 mV per day is leaking at a tau (rate) of 10 years or more. If you had to wait a few months to get this leak rate, well, that's not bad. But achieving this leak rate in less than two weeks is, I would say, quite good. Less than a day? Spectacular.
So I'm finding that good polypropylene caps are better than the best (old) polystyrenes, in terms of soakage or dielectric absorption (early or late) and in terms of leakage, early or late. Are Teflons any better? Not much. I may have to buy a couple to find out.