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into the probe tip and comprises one leg of a bridge
circuit as shown in simplified circuit diagram section of
Figure 4. Two 1.5V batteries are used to power both the
bridge circuit and operational amplifier U2. Since the
bridge must be balanced to provide 0°C and 0° Е indica-
tions, separate range or temperature scale resistors RA
and RB are included in the bridge circuit. When RB is
shorted, the °C scale is selected and the bridge is calibrat-
ed by R2 to null at 0°C. Conversely, when КВ is open,
the 02 Е scale is selected and the bridge is calibrated by
R2 to null at 0" Е. Deviations above and below 0° provide
a bridge output of approximately 2.45 mV/*C.
Figure 4. Simplified Circuit Diagram
Operational amplifier U2 is used to measure the bridge
output and scale itto a | mV/degree signal. Since the °C
and the °F scale are sloped differently, the scale for U2
must be matched with the scale selected for the bridge
circuit. Shorting resistor RD selects the °C scale. Con-
versely, when КО is open the ° F scale is selected. Resistor
К4 calibrates both scales.
The output voltage used to drive the external voltmeter
is taken from the output of U2(P2) and the reference side
of the bridge (P1). Since U2 is operating as an inverting
amplifier, its output is used as the low input to the volt-
meter. This enables the voltmeter to display an increase in
temperature as an increase in voltage.
GENERAL
MAINTENANCE
Access Information
The
battery,
the temperature
scale jumpers
and
the
calibration pots are located on the interior of the temp-
80T-150
erature-to-voltage converter assembly.
Access to these
locations is accomplished by removing the three screws
from the bottom side of the assembly and removing the
plastic box.
Battery Replacement
The 807-150 uses two 1.5 volt, size N, alkaline batteries.
They provide a useful operating life of =1000 hours. To
replace the batteries pull the old ones out and press the
new ones into the battery holders. Match polarity mark-
ed on the pcb. Mercury or equivalent N-size batteries
may be installed in the 80T-150.
Temperature Scale Jumpers, °C or °F
The jumper locations for selecting either the °C or °F
temperature scale are shown in Figure 5. The absence of
jumpers at both points indicates selection of the °F scale.
The presence of solder bridges at both points indicates
selection of the °C scale. Under no circumstances should
one point be open while the other is bridged.
TEMPERATURE
SCALE JUMPERS
80T-3001
Figure 5. Temperature Scale Jumpers
PERFORMANCE
TEST
Complete
the calibration
procedure
without
opening,
the temperature-to-voltage converter assembly and with-
out making any calibration adjustments. Observe the
readings given in [brackets]. Other readings are for cal-
ibration only.
CALIBRATION
A calibration cycle of | year is recommended to maintain
the unit within the specifications given earlier. The equip-
ment required for calibration is listed in Table 2.
NOTE
Values given in brackets apply to the Per-
formance Test.
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