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Dielectric Absorption

A capacitor which has been charged for a long time and then been completely discharged, has a small voltage on its terminal wires again, within seconds or minutes.
This effect is known as dielectric absorption.

This phenomenon has a particularly unfavorable effect in sample and hold applications in which charges are to be stored for comparision/measuring purposes.

The recharging comes from polarization processes in the insulating material and is largely independent of the capacitance of the capacitor and the thickness of the dielectric.



Measuring of Dielectric Absorption


The standard MIL-C-19978 describes the measuring method of the dielectric absorption.

Circuit diagram:

The capacitor Cx is charged for 15 minutes on a reference DC voltage, e.g. up to the rated DC voltage of the capacitor. The initial current surge may not exceed 50 mA.

At the end of the charging time the capacitor is separated from the charging source and discharged over a resistance of 50. The discharging resistance is removed from the capacitor after 10 sec.

The "regained voltage" is measured after a period of 15 minutes with a high ohmic (Ri > 10000 M) millivoltmeter.

The dielectric absorption DA can be calculated according to the following formula:

DA = U1 / U2 x 100%

DA = dielectric absorption
U1 = regained voltage
U2 = charging voltage


Typical values of some dielectrics in % at T = +23°C:

- Polypropylene
- Polyester
- Mixed dielectric
- Ceramic (X7R)
- Ceramic (Z5U)
0.05 ... 0.10
0.20 ... 0.25
0.12 ... 0.18
0.60 ... 1.00
2.00 ... 2.50