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4.5
Capacitance measurement
The coulombs function of the Model 6512 provides a quick
and easy method of measuring capacitance values of capaci-
tors, cables, and connectors. It is especially useful in cases of
cables and connectors because of the very small values of
charge that can be measured.
The basic method involves using a Model 230 voltage source
to apply a step voltage across the capacitor, as shown in Fig-
ure 4-5. Since charge is to be measured, the Model 6512
should be in the coulombs function to make the measure-
ment. Just prior to turning on the voltage source, zero check
should be disabled and the charge suppressed. Then, turn on
the voltage source and note the final charge value.
SHIELD
R
HI
230
V 2
VOLTAGE
V 1
SOURCE
STEP
LO
VOLTAGE
RECOMMENDED R VALUES: < 100pF-10nF 1MΩ
Figure 4-4
Capacitor measurement
The capacitance can then be computed as follows:
C
where:
∆ Q = Q
(final charge) - Q
2
∆ V = V
(step voltage) - V
2
C
HI
6512
ELECTROMETER
IN COULOMBS
Q M
LO
10nF-1µF 100kΩ
∆Q
=
-------- -
∆V
(initial charge, assumed to be 0)
1
(initial voltage, assumed to be 0)
1
As an example of the above procedure, assume that an un-
known capacitor is to be measured. If the step voltage is 100V,
and a Q value of 2nC is obtained, the capacitance value is:
2nC
C
=
-------------- -
100V
4.6
Insulation resistance measurements
At the moderate impedance levels of many circuits, insula-
tion resistance is seldom a consideration, as it is generally
many orders of magnitude above the highest impedance en-
countered in the remainder of the circuit. At very high im-
pedance levels, however, insulation resistance can be a
consideration, since it can lower effective circuit impedance
considerably. Since typical insulation resistances run in the
Ω , their values lie above the measure-
10
16
range of 10
- 10
ment range of ordinary instruments. The high resistance
measurement range of the Model 6512, however, gives it ca-
pabilities to measure such high resistances.
4.6.1 Unguarded resistance measurements
A typical test configuration for making unguarded insulation
resistance measurements is shown in Figure 4-6. The con-
stant-current method is used, and insulation resistances up to
200G Ω can be measured using the Model 6512 ohms func-
tion. As the term implies, the test current through the un-
known resistance is kept constant, while the voltage
developed across the test resistance will, of course, depend
on the value of the insulation resistance. The Model 6512
measures the generated voltage and calculates the resistance
value accordingly. The low compliance voltage of the Model
6512 (<2V on 2G Ω range and lower, except <300V during
overload) keeps error due to voltage coefficient small.
Applications
20pF
=
4-5
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