Wheatstone Bridge

In 1843 Sir Charles Wheatstone, a British scientist, developed this circuit in order to determine unknown resistances. When this circuit is used in strain gage based load cells, it is used to determine a change in resistance.



The images above are representations of the Wheatstone bridge. The ratio of the output voltage, measured by the voltmeter, and the supplied voltage from the voltage source is a function of resistors 1, 2, 3 and 4 and may be seen below.



V 0 = Output Voltage (V)
V S = Supplied Voltage (V)
R 1 = Resistance 1 (Ω)
R 2 = Resistance 2 (Ω)
R 3 = Resistance 3 (Ω)
R 4 = Resistance 4 (Ω)

When no load is applied to a load cell all gages are assumed to hold the same resistance. When loaded two of the arms of the bridge will experience tension (lets assume R1 and R3) and two will experience compression (lets assume R2 and R4). The arms with gages in tension will have an increased resistance and increased voltage drop. The arms with gages in compression will have a decreased resistance and decreased voltage drop. This will result with a voltage differential across the output nodes.