SECTION 4
APPLICATIONS
4.1 INTRODUCTION
Applications for the Model 617 are many and varied and will
depend on the user’s needs. Basically, the Model 617 can be
used to make many of the same measurements in the range of
ordinary DMMs; however, special characteristics such as
high input resistance and high sensitivity give the instrument
much better capabilities than those of the ordinary DMM.
For example, the typical input resistance for an ordinary
DMM is on the order of lOM0. In contrast, the Model 617
has an input resistance of greater than ZOOTQ (2 x 10143).
The Model 617 can detect currents as low as O.lfA (lo-16A),
while a typical DMM might be limited to current measure-
ments in the pA range.
In this section, then, we will discuss some possible applica-
tions for the Model 617 Electrometer. Keep in mind that these
examples are only representative of what is possible with this
highly sophisticated instrument, and by no means exhaust the
possible uses for the unit.
4.2 INSULATION RESISTANCE
MEASUREMENTS
At the moderate impedance levels of many circuits, insulation
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 con-
sideration, since it can lower effective circuit impedance con-
siderably. Since typical insulation resistances run in the range
of lOlo-lOM2, their values lie above the measurement range
of ordinary instruments. The high resistance measurement
range of the Model 617, however, gives it capabilities to
measure such high resistances.
A typical test configuration for making insulation resistance
measurements is shown in Figure 4-l. In this case, the con-
stant current method is used. Using this method, insulation
resistances up to 2WGfI can be measured. As the term irn-
plies, the tet current through the unknown resistance is kept
constant. The voltage developed across the test resistance
will, of course, depend on the value of the insulation resis-
tance. The Model 617 measures the generated voltage and
calculates the resistance value accordingly. The low com-
pliance voltage of the Model 617 ( < 2V on 2G0 range and
lower, except <3OOV during overload) keeps error due to
voltage coefficient small.
For resistance measurements above 1080, or for cables longer
than three feet, guarded measurements are recommended, as
shown in Figure 4-2. In this case, the rear panel V, R GUARD
switch is used to internally apply a guard signal to the inner
shield on the connecting cable. The guard is carried through
to the inner shield of the test fixture. The inner shield must be
insulated from the outer shield, which is a safety shield. In-
cidentally, a shielded fixture is recommended for both
unguarded and guarded configurations for measurements
above 107n if stable readings are to be expected (in the
unguarded mode, the shield should be connected to input
IOW).
With the constant current method just discussed, the Model
617 can make rneasurenients as high as 200GR. However, the
insulation resistance of such materials as polyethylene may lie
above this range. By using the Model 617 to make resistance
measurements in the constant-voltage mode, measurement
range can be extended up to 10160. Also, for a given
resistance range, the V/I mode will be faster.
A typical configuration for using the Model 617 in this man-
ner is shown in Figure 4-3. Here, the built in voltage source of
the instrument is used to force a current, I, through the
unknown resistance, R. The insulation resistance is then
automatically calculated by the Model 617 as follows:
V
R=-
I
where I is the current through the resistance as measured by
the instrument, and V is the programmed voltage.
Note that COM is connected to input LO thru 1OGQ and ap-
pears in series with the resistor under test. This resistance is
below the resolution of the instrument on ranges above
2MSl.
4-l
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