TEMPERATURE DETECTION CIRCUITRY
Figure 8 Bridge Circuit
Resistors R1 and R2 are the ratio arms of the bridge. They ratio the two variable resistances for
current flow through the ammeter. R3 is a variable resistor known as the standard arm that is
adjusted to match the unknown resistor. The sensing ammeter visually displays the current that
is flowing through the bridge circuit. Analysis of the circuit shows that when R3 is adjusted so
that the ammeter reads zero current, the resistance of both arms of the bridge circuit is the same.
Equation 1-1 shows the relationship of the resistance between the two arms of the bridge.
Since the values of R1, R2, and R3 are known values, the only unkown is Rx. The value of Rx
can be calulated for the bridge during an ammeter zero current condition. Knowing this
resistance value provides a baseline point for calibration of the instrument attached to the bridge
circuit. The unknown resistance, Rx, is given by Equation 1-2.