Applications
4-7
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:
INPUT
ON
TRIAX CABLE
SAFETY
SHIELD
SIGNAL
GUARD
RX
6512 SET TO OHMS, V
Ω
, GUARD ON
A. CONNECTIONS
A/D CONVERTER
6512 PREAMP
R
I
I
RX
B. EQUIVALENT CIRCUIT
V
Ω
, GUARD
OFF
HI
GUARD
GND
COM
UNKNOWN
RESISTANCE
WARNING: SAFETY SHIELD RECOMMENDED FOR GUARDED
RESISTANCE MEASUREMENTS ABOVE 30G
Ω
. UP
TO 300V MAY BE PRESENT ON GUARD.
V
SIGNAL
GUARD
SAFETY SHIELD
LO
COM
R
V
I
----
=
R
100V
1 pA
---------------
10
14
Ω
=
=
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.
Summary of Contents for 6512
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