Wheatstone Bridge

In the world of electrical engineering, circuit analysis plays a crucial role in understanding the performance and operation of various electronic components. One impedance measuring technique that has stood the test of time – and is still extensively employed today – is known as the Wheatstone Bridge. This simple yet powerful tool is used to measure unknown electrical resistance by balancing two legs of a bridge circuit, which then allows for accurate determination of resistance.

Wheatstone Bridge Configuration

The Wheatstone Bridge was invented in 1843 by British scientist Sir Charles Wheatstone, who initially used it to perform precise measurements for telegraph calibrations. It consists of a four-armed arrangement connected to a sensitive galvanometer (a device used to detect and measure electric current) and a power source. The four arms constitute four resistors – three are known resistances (typically labeled R1, R2, and R3), whereas one’s value is unknown (labeled Rx). These resistance values form a loop connecting the voltage source on one side and a galvanometer on the opposite side.

Working Principle

In its balanced state, the ratio of known resistances – i.e., R1/R2 – equals the ratio of unknown resistance and another known resistance in the bridge network – i.e., Rx/R3. As there is no current flowing through the galvanometer during this state, calculations can be made based on simple laws of parallel and series circuits.

Using Ohm’s Law (V = IR) and Kirchhoff’s Voltage Law (the algebraic sum of potential difference in any closed loop must be zero), one can solve for the unknown resistor value:

Rx = R3(R1/R2)

Special Cases

The Wheatstone Bridge is not limited to one universal configuration; it also allows for the inclusion of capacitive or inductive components. When capacitors or inductors replace any or all resistive elements, it results in capacitive or inductive bridges commonly employed to evaluate capacitance or inductance.

Applications

1. Measurement of Electrical Resistance: Its primary application lies in measuring unknown electrical resistance with high accuracy.

2. Strain Gauges: A popular application in mechanical engineering involves attaching strain gauges as resistive elements in a Wheatstone Bridge circuit to measure minuscule dimensional changes due to stress imposed on materials.

3. Differential Pressure Sensing: In pressure transducers, diaphragm-based sensors serve as variable resistors calibrated to generate minute changes corresponding to varying fluid pressure.

4. Temperature Sensing: The measurement of temperature variations by means of temperature-dependent resistive elements such as thermistors and resistance temperature detectors (RTDs) incorporates Wheatstone Bridge circuits for precise monitoring.