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OFF
VΩ, GUARD
SAFETY SHIELD
SIGNAL
GUARD
Figure 4-6
Insulation resistance measurement (guarded)
A typical configuration for using the Model 6512 to make
V/I resistance measurements is shown in Figure 4-8. Here,
an external Model 230 Voltage Source is used to force a cur-
rent, I, through the unknown resistance, R. The insulation re-
sistance can then be calculated as follows:
I is the current through the resistance as measured by the
Model 6512, and V is the programmed Model 230 voltage.
For example, assume that the applied voltage is 100V, and
the measured current is 1pA. The resistance is calculated as
follows:
100V
R
=
-------------- -
1 pA
TRIAX CABLE
COM
INPUT
ON
6512 SET TO OHMS, VΩ, GUARD ON
I
R X
V
R
=
----
I
14
Ω
10
=
WARNING: SAFETY SHIELD RECOMMENDED FOR GUARDED
RESISTANCE MEASUREMENTS ABOVE 30GΩ. UP
TO 300V MAY BE PRESENT ON GUARD.
A. CONNECTIONS
COM
LO
6512 PREAMP
R
I
V
HI
GUARD
GND
B. EQUIVALENT CIRCUIT
Since the Model 230 voltage source can be adjusted over a
wide range of values (up to ±101V), this configuration can be
used for voltage coefficient studies, as described later in this
section.
Note that COM is connected to input LO through 100 Ω and
appears in series with the resistor under test. This resistance
is below the resolution of the instrument for measurements
above 2M Ω .
In addition to the measurement of insulation resistances, this
basic method can be used to measure unwanted leakage re-
sistances. For example, leakage resistance between PC board
traces and connectors can be made with either of the two
methods described above, depending on the resistance values
involved.
R X
UNKNOWN
RESISTANCE
SAFETY
SIGNAL
SHIELD
GUARD
A/D CONVERTER
Applications
4-7
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