30
c
HAPTER
2:
Cooling System Design and Temperature Control
Model 372 AC Resistance Bridge and Temperature Controller
2.5.4.1 Voltage Excitation Mode—Measurement Input Only
The user specifies an input voltage range that is a “not to exceed” setting for the
instrument. The instrument selects the appropriate current setting that keeps the
voltage across the resistor as high as possible without exceeding the voltage range
setting. Current is changed when the resistance range changes to maintain the volt-
age limit. This mode is preferred when measuring negative temperature coefficient
materials common in cryogenic temperature sensors. With voltage limited, exci-
tation power in the resistor reduces with lowering temperature (P = V
2
/R) limiting the
effect of self-heating.
2.5.4.2 Current Excitation Mode
The user specifies a fixed excitation current. The instrument selects the appropriate
voltage gain for the selected resistance range. The instrument changes gain when the
resistance range changes. In this mode it is easier to calculate the actual power in the
measured resistor (P = I
2
× R) because the true current is displayed. In voltage exci-
tation mode the voltage range displayed is the maximum not the actual voltage.
2.5.5 Resistance
Ranges
2.5.5.1 Measurement Input
The Model 372 measurement input has full-scale resistance ranges from 2 m
to
2 M
. Many ranges are available, but with several different excitation levels, it makes
resistance range selection somewhat more involved than it initially appears. The goal
is to use enough excitation to maintain reasonable accuracy, resolution and settling
time without putting too much heat into the resistor. Higher resistance ranges from
6.32 M
to 63.2 M
, are available but not specified because the effects of lead capac-
itance cannot be eliminated at such high resistances. These ranges should be used for
relative measurements only.
The selected resistance range and excitation are always shown on the instrument dis-
play. Refer to FIGURE 1-13 through FIGURE 1-15 for resistance and excitation ranges.
As shown in the figures, not every combination of excitation and resistance range is
possible. Unusable combinations of range and excitation are automatically locked
out. The Model 372 also continuously calculates and displays the excitation power
being dissipated in the resistor as additional feedback to the operator. The excitation
current source is shorted inside the instrument to minimize transients during range
changes.
Manual operation allows the user to choose both the excitation and resistance range.
The instrument does not change ranges automatically. If the input resistance is too
large for the range an overload condition will be indicated. This mode is used when
the system is very sensitive to excitation changes or if changes in input resistance are
too fast for the auto range algorithms.
The Model 372 has an autorange feature that will increase the resistance range any
time the measured resistance is greater than full scale for the range and decrease the
range any time the measured resistance is below 20% of full scale. The ranges have
an approximate 20% over range capability so in most cases the instrument will
increase range before an overload appears. Autorange works with either voltage or
current excitation. It will not change the excitation setting in order to reach a resis-
tance range that is not available for the present selection.
2.5.5.2 Control Input
The Model 372 control input has full-scale resistance ranges from 2 k
up to 632 k
.
Unlike the measurement input, the excitation current is a function of the selected
resistance range. The resistance range is selectable from the Control Input (Input A)
menu. Also, when in the Control Input display mode, the selected resistance range
and current excitation are displayed.