TEMPERATURE DETECTION CIRCUITRY
When calibrating the circuit, a precision resistor of known value is substituted for the resistance
bulb, as shown in Figure 12.
Figure 12 Resistance Thermometer Circuit with Precision
Resistor in Place of Resistance Bulb
Battery voltage is then adjusted by varying Rb until the meter indication is correct for the known
Because of changes in ambient temperature, the resistance thermometer circuitry must be
compensated. The resistors that are used in the measuring circuitry are selected so that their
resistance will remain constant over the range of temperature expected.
compensation is also accomplished through the design of the electronic circuitry to compensate
for ambient changes in the equipment cabinet. It is also possible for the resistance of the
detector leads to change due to a change in ambient temperature. To compensate for this change,
three and four wire RTD circuits are used. In this way, the same amount of lead wire is used
in both branches of the bridge circuit, and the change in resistance will be felt on both branches,
negating the effects of the change in temperature.