Engineering Principles V11

Clive W. Humphris

MEASUREMENT: Wheatstone Bridge.

The Wheatstone bridge is a simple circuit which forms the basis of many test instruments. The circuit should be viewed as a pair of voltage dividers connected in parallel across a DC supply and joined by R5.

Providing the resistor values in each half of the circuit are equal, the junction voltages will be the same and zero voltage (potential difference) will be developed across R5, therefore no sense current flows. A circuit imbalance causes current to flow in one or other direction through R5.

Suppose the value of R3 is unknown, depending upon whether it is larger or smaller than R4 a current will flow through R5 in a direction relative to their difference in voltage potential.

This is a measure of the unknown value against R4. Adjustment of R4 for zero sense current against a calibrated scale could provide the unknown value of R3. Also shown is how the bridge sense current can be calculated. Simple voltage dividers for each leg of the bridge are used. In theory R5 sense current needs to be taken into consideration when calculating Va and Vb, but ensuring R5 has a high value means it can be disregarded. A much more accurate method of calculation would be to apply one of the Circuit Theorems.

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