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3-14
Volts and Ohms Measurements
Application
Capacitor dielectric absorption
Dielectric absorption occurs when randomly oriented permanent dipoles of molecules with a
capacitor dielectric are aligned by an applied electric field. After a capacitor has been discon-
nected from a discharge circuit, a residual charge remains on the capacitor, and a voltage will be
re-established across the capacitor terminals.
For timing and integrating applications, dielectric absorption (or a residual capacitor voltage)
can seriously degrade the accuracy of the circuit. Thus, a capacitor's dielectric absorption must
be known and compensated for in circuits where capacitance tolerance is a significant factor in
circuit accuracy.
Dielectric absorption is not normally specified by a manufacturer since its importance is
application dependent. The parameter can be defined as the capacitor's discharge current at a
designated time following the initiation of a discharge cycle. The capacitor is typically charged
up to the maximum voltage that will be applied. The measurement of the discharge current is
usually made at a discharge time interval that will be used in the application of the device, or no
longer than one minute. Acceptable capacitors have current levels below a required maximum
limit.
Dielectric absorption can also be expressed as a percentage of residual voltage with respect
to a charging voltage. This ratio is determined by charging the capacitor to the rated voltage. The
capacitor is then discharged for a second time interval. Finally, the capacitor is open-circuited,
and the residual voltage across the capacitor is measured after a third time constant.
The Model 6514 is particularly useful in measuring dielectric absorption because it draws vir-
tually no charge from the capacitor during the measurement, nor does it induce charge on the
capacitor being measured.
The test circuit in Figure 3-9A uses Keithley Model 230 as a voltage source and Model 6514
to perform the voltage measurements. Figure 3-9B shows the voltage waveform across the
capacitor during the three phases of the test.
Initially, capacitor C is charged through R
Soak time is typically one or two minutes, depending on the capacitor value. Next, the voltage
source is turned off, and the capacitor is discharged through R
sit for a few minutes with S
6514. Dielectric absorption is then calculated as follows:
% Dielectric Absorption = (Residual Voltage / Soak Voltage) × 100%
for the required soak time (t
1
and S
open (t
), and the residual voltage is then measured by Model
2
1
3
in Figure 3-9B).
1
(t
). The capacitor is allowed to
2
2
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