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Figure 1 is a block diagram of a typical
temperature detection circuit. This represents a balanced
bridge temperature detection circuit that has been modified
to eliminate the galvanometer.
Figure 1
The block consists of a temperature detector
(RTD) that measures the temperature. The detector is felt as
resistance to the bridge network. The bridge network converts
this resistance to a DC voltage signal. An electronic
instrument has been developed in which the DC voltage of the
potentiometer, or the bridge, is converted to an AC voltage.
The AC voltage is then amplified to a higher (usable) voltage
that is used to drive a bi-directional motor. The
bi-directional motor positions the slider on the slidewire to
balance the circuit resistance.
If the RTD becomes open in either the
unbalanced and balanced bridge circuits, the resistance will
be infinite, and the meter will indicate a very high
temperature. If it becomes shorted, resistance will be zero,
and the meter will indicate a very low temperature.
When calibrating the circuit, a precision
resistor of known value is substituted for the resistance
bulb, as shown in Figure 2.
Figure 2
Battery voltage is then adjusted by varying Rb
until the meter indication is correct for the known
resistance.
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