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The bridge operates by placing Rx
in the circuit, as shown in Figure 1, and then adjusting R3
so that all current flows through the arms of the bridge
circuit. When this condition exists, there is no current flow
through the ammeter, and the bridge is said to be balanced.
When the bridge is balanced, the currents through each of the
arms are exactly proportional. They are equal if R1
= R2. Most of the time the bridge is constructed
so that R1 = R2. When this is the case,
and the bridge is balanced, then the resistance of Rx
is the same as R3, or Rx = R3.
When balance exists, R3 will be equal to the
unknown resistance, even if the voltage source is unstable or
is not accurately known. A typical Wheatstone bridge has
several dials used to vary the resistance. Once the bridge is
balanced, the dials can be read to find the value of R3.
Bridge circuits can be used to measure resistance to tenths
or even hundredths of a percent accuracy.
When used to measure temperature, some
Wheatstone bridges with precision resistors are accurate to
about + 0.1°F.
Two types of bridge circuits (unbalanced and balanced) are
utilized in resistance thermometer temperature detection
circuits. The unbalanced bridge circuit (Figure 2) uses a
millivoltmeter that is calibrated in units of temperature
that correspond to the RTD resistance.
Figure 1
Figure 2
The battery is connected to two opposite
points of the bridge circuit. The millivoltmeter is connected
to the two remaining points. The rheostat regulates bridge
current. The regulated current is divided between the branch
with the fixed resistor and range resistor R1, and
the branch with the RTD and range resistor R2. As
the electrical resistance of the RTD changes, the voltage at
points X and Y changes. The millivoltmeter detects the change
in voltage caused by unequal division of current in the two
branches. The meter can be calibrated in units of temperature
because the only changing resistance value is that of the
RTD.
The balanced bridge circuit (Figure 3) uses a
galvanometer to compare the RTD resistance with that of a
fixed resistor. The galvanometer uses a pointer that deflects
on either side of zero when the resistance of the arms is not
equal. The resistance of the slide wire is adjusted until the
galvanometer indicates zero. The value of the slide
resistance is then used to determine the temperature of the
system being monitored.
Figure 3
A slidewire resistor is used to balance the
arms of the bridge. The circuit will be in balance whenever
the value of the slidewire resistance is such that no current
flows through the galvanometer. For each temperature change,
there is a new value; therefore, the slider must be moved to
a new position to balance the circuit.
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