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: |