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Bridge Circuits

A bridge circuit consists of two voltage dividers connected in parallel as illustrated below.

Voltage Dividers !

If the upper voltage divider (comprising resistors R1 and R2) divides the supply voltage in the same ratio as the lower voltage divider (comprising resistors R3 and R4), the potential between points C and D is zero (UD = 0). In this state, the bridge is said to be balanced. The balancing condition is

Voltage Dividers !

If resistors R3 and R4 are replaced by an adjustable resistor, the bridge circuit can be used to measure resistances; this kind of circuit is termed Wheatstone bridge after the English physicist of the same name (see the diagram below).

Voltage Dividers !

Rx is the unknown resistance, RN a (usually switchable) standard resistance for comparison. To perform the measurement, the bridge is balanced (UD = 0) and Rx determined using the following relationship:

Voltage Dividers !

 

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