Capacitor Leakage Measurements - Keithley 6512 Instruction Manual

Applications
100µA
10µA
1µA
1N914
DIODE
CURRENT
1N4148
(I)
100nA
1N4006
10nA
1N765
1nA
0.1
Figure 4-2
Diode curves
4.4

Capacitor leakage measurements

An important parameter associated with capacitors is their
leakage currents. Once the leakage current is known, the in-
sulation resistance can be calculated. Ideally, a capacitor
should have no leakage current, and thus infinite leakage re-
sistance. However, capacitors, like all practical devices, are
not ideal, so these parameters can become important, espe-
cially to circuit design and component engineers. The
amount of leakage current in a given capacitor depends on its
dielectric as well as the applied voltage. Ceramic dielectric
capacitors typically have leakage currents in the nA to pA
range, while polystyrene and polyester dielectric devices ex-
hibit much lower leakage currents–generally in the fA range.
(These values are for test voltages in the 100V range).
The basic configuration for this test is shown in Figure 4-4.
A Model 230 Voltage Source is used to impress a voltage
across the capacitor, C. The resulting leakage current is then
measured by the Model 6512.
4-4
0.2 0.3 0.4 0.5
V F
HI
230
VOLTAGE
SOURCE
LO
RECOMMENDED R VALUES: < 100pF-10nF 1MΩ
Figure 4-3
Capacitor leakage tests
The resistor R is necessary to limit current to a safe value in case
the capacitor is shorted, and it also helps to reduce noise. Typi-
cally a value of about 1M Ω should be used, although that value
can be decreased for larger capacitor values. However, values
under 10k Ω are not recommended. (Refer to paragraph 2.13.8.)
0.6
At the start of the test, the Model 6512 should be placed in
the amps mode and on the 20mA range. The Model 230 Volt-
age Source is then programmed to the desired voltage, and its
output is turned on. Once the required soak time has passed,
the Model 6512 can be placed on the proper current range to
make the current measurement. (The soak time is the period
necessary to fully charge the capacitor; typically 10RC.)
Once the test is completed, the voltage source should be
turned off to allow the capacitor to discharge.
The leakage current can be directly read from the Model
6512 display during the test procedure. If the leakage resis-
tance value is required instead, the value can be calculated
from the source voltage and the measured current.
This basic procedure could be used to test a number of capac-
itors on an automated basis. A test fixture that holds a num-
ber of capacitors could be constructed, and a Keithley Model
7001 or Model 7002 Switch System, equipped with appro-
priate scanner cards, could be used to select among the vari-
ous devices to be tested. For a higher degree of automation,
the switch system, voltage source, and Model 6512 could be
controlled from a computer via the IEEE-488 bus. In this
way, measurements that would otherwise be tedious and time
consuming could be conducted on a more routine basis.
SHIELD
I L
R C
HI
ELECTROMETER
IN AMPS
I M
LO
10nA-1µF 100kΩ
6512