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

This phenomenon has a particularly unfavourable effect in sample and hold applications in which charges are to be stored for comparison/measuring purposes. The recharging comes from polarisation 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. Referring to the circuit diagram in Figure 1:



The capacitor Cx is charged for 15 minutes on a reference DC voltage, for example, 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 Ω. The discharging resistance is removed from the capacitor after 10 seconds. The 'regained voltage' is measured after a period of 15 minutes with a high-Ω (input resistance greater than 10 000 MΩ) millivoltmeter.

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

DA = U1/U2 x 100%.

Where: DA = dielectric absorption; U1 = regained voltage; U2 = charging voltage.

Typical values of some dielectrics at a temperature of 23°C are: polypropylene, 0,05 to 0,10; polyester, 0,20 to 0,25; mixed dielectric, 0,12 to 0,18; ceramic (X7R), 0,60 to 1,00; ceramic (Z5U), 2,00 to 2,50.