Testing2-Terminaldevices - Keithley 486 Instruction Manual

SECTlON 3
Front Panel Operation
3.15.2 Testing 2-Terminal Devices
The Model 487 is ideal for sourcing voltage and measur-
ing current to test voltage coefficients of resistors, leak-
age current of capacitors, and reverse current of diodes.
Generally, these tests are performed on high impedance
devices allowing either source V measure I configura-
tion to be used. (Figure 3-10).
NOTE
For the following test examples, use
Figure3-10 as the circuit configuration. The
device under test (DUT) is shown installed in
the Keithley Model 8002A test fixture.
Voltage Coefficient Tests of Resistors
High megohm resistors often exhibit a change in resis-
tance with applied voltage. This resistance change is
characterized as the voltage coefficient.
To determine voltage coefficient of a resistor, two V/I
ohms measurements at two different voltage values will
be required. The voltage coefficient in s/V can then be
calculated as follows:
Voltage Coefficient (s/V) =
lOO(R, - R1)
R v
1
where; R, is the measured resistance with the first
applied voltage.
R, is the measured resistance with the second
applied voltage.
V is the difference between the two applied
voltages.
Example: Assume that the following values are
obtained.
R, = 1.01 x 10%
R2 = 1 x 10%
v=5v
The resulting voltage coefficient is:
Voltage coefficient (s/V) = loo(* x IO')
1 x 10'"(5)
= 0.2%/V
Note that the voltage coefficient of a particular device
may apply only across the selected voltage range and
may vary with different voltage increments in the same
approximate range.
Capacitor Leakage Tests
An important parameter associated with capacitors is
leakage current. The amount of leakage current in a
capacitor depends both on the type of dielectric as well
as the applied voltage. With a test voltage of lOOV, for
example, ceramic dielectric capacitors have typical leak-
age currents in the n4 to pA range. By using the V/I
ohms function, the leakage current measurement can be
automatically converted into the insulation resistance
ValUe.
When measuring leakage currents on capacitors, stabil-
ity and noise performance can be maintained by adding
a resistor in series with the capacitor under test. The
value of this resistor should be around 1MQ. For larger
capacitor values (>lpF), the value of the series limiting
resistor can be made lower in order to improve settling
times; however, values below 1OkQ are not generally
recommended.
After the voltage is applied to the capacitor, the device
must be allowed to charge fully before the measurement
can be made. Otherwise, an erroneous current, with a
much higher value will be measured. The time period
during which the capacitor charges is often termed the
"soak" time. A typical soak time is 7 time constants, or
7RC, which would allow settling to less than 0.1% of
final value.
WARNING
Hazardous voltage may be present on the
capacitor leads after performing this test.
Discharge the capacitor before removing it
from the test fixture.
Diode Reverse Current Test
The reverse current of a diode can be checked using the
Model 487 to source voltage and measure current.
To perform this test, the diode must be reverse-biased
by programming the V-Source to apply a positive (+)
voltage to the cathode of the diode (anode to V-Source
common). Forward biasing the diode will, in most cases,
cause the Model 487 to go into I-limit.
To check the reverse current of a lN3952 germanium
diode, set the V-source of the Model 487 to apply +4.5V
to the cathode of the diode. The measured reverse cur-
rent displayed on the Model 487 should be approxi-
mately 4fiA.
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