Dielectric Absorption

Lately I've learned of Dielectric Absorption in capacitors. The effect is caused by the dielectric, which basically can not follow voltage changes quickly enough because of internal friction of its dipoles.

One interesting behavior is that if you completely discharge a capacitor, its voltage will increase again over time as if it was charged.

In the Wikipedia article, a measurement procedure following the IEC/EN 60384-1 standard is described:

1. charge the capacitor to its nominal voltage for 1h
2. discharge it across 5Ω for 10 seconds
3. wait for 15 minutes
4. measure its voltage

Depending on the dielectric material, it will re-charge up to 15%. :-O

I've conducted that experiment with a random 16V 1000µF aluminium electrolytic capacitor and got 0.6706V, which is 4.19%.

The image shows the re-charging curve over nearly 2 weeks (note the logarithmic time scale). The voltage reached 1.083V (6.30%).

Just think of a capacitor in a microwave with 4kV. 6.3% would be 252V.
That is dangerous!