# What is Wheatstone Bridge and How it is used in Automation field?

What is Wheatstone Bridge?

A Wheatstone Bridge is essentially an electrical circuit designed to compare resistances or measure the unknown value of a resistor’s resistance by generating a balance between the bridge circuit’s two legs. It is often referred to as a “resistance bridge”. To calculate the value of unknown resistance, Wheatstone bridge employs the comparison approach.

The Wheatstone Bridge is made up of four resistors connected in a diamond-shaped circuit. Although it was designed to measure the value of unknown resistance, it may now be used to calibrate measuring instruments such as voltmeters and ammeters using changing resistance and a simple mathematical calculation. The Wheatstone bridge circuit provides an extremely accurate value for the measured resistance.

How does Wheatstone BridgeWorks?

Whetstone bridges are simple series-parallel resistor arrangements that are connected between a supply voltage and ground. Wheatstone bridge circuit has two input terminals and two output terminals with four resistors each. The bridge is used to precisely determine the unknown resistance by comparing it to a known value of resistance. The null or balanced condition is utilized to find the resistance in this circuit. To achieve a balanced condition, the voltages at B and D must be equal, as illustrated in the above figure. In order for no current to flow through the meter and a balanced condition to be achieved, one of the resistors must be variable.

Its operation is as follows: A balanced voltage between the sites allows the measurement by using the ratio of three known resistors to measure the fourth unknown resistance. To balance the device, the resistors are changed to adjustable variable resistors, and the mathematical ratio is employed to determine the fourth resistance.

Construction of Wheatstone Bridge

A Wheatstone bridge circuit has four arms, two of which have known resistances and the other two of which have an unknown resistance and a variable resistance. A galvanometer and a source of electromotive force are also indulged in the Wheatstone bridge’s circuit. The galvanometer is connected between points c and d, whereas the emf source is connected between points a and b. The potential difference across the galvanometer determines the current that runs through it.

Errors occur in Wheatstone Bridge

The inaccuracies that can arise when measuring a value with the Wheatstone bridge are listed below.

• The true value and the mentioned value of the resistance may differ, resulting in a measurement error.
• Due to the galvanometer’s lower sensitivity, measurement errors may occur.
• The bridge’s self-heating could change its resistance and cause a mathematical error.
• The creation of thermal causes can result in mistakes in low-value resistance measuring.
• When the individual taking the reading is not cautious, personal errors can occur.

How can we avoid errors in Wheatstone Bridge?

We can avoid the inaccuracies described above by utilising the best resistor and galvanometer available. To reduce the inaccuracy caused by resistance to self-heating, we should measure the resistance as soon as possible. Thermal mistakes can be reduced by putting a reversing switch between the battery and the bridge.

Application of Wheatstone Bridge

In many electronic applications, balanced bridge circuits are used to sense changes in light intensity, strain, or pressure. Potentiometers, LDRs, strain gauges, and thermistors are examples of resistive sensors that can be utilised in a Wheatstone bridge circuit.

Wheatstone bridge applications detect electrical and mechanical parameters. However, the most basic Wheatstone bridge use is the light measurement with a photo resistive device. A light-dependent resistor is used in place of one of the resistors in the Wheatstone bridge circuit.

An LDR is a passive resistive sensor that converts visible light levels into a resistance change and, eventually, a voltage. LDRs can be used to measure and monitor the intensity of light. LDR has several Mega ohms resistance in dim or dark light, over 900 at a light intensity of 100 Lux, and down to around 30ohms in bright light. We may measure and monitor changes in light levels by connecting the light-dependent resistor in the Wheatstone bridge circuit.

• The Wheatstone bridge is mainly used to measure the low resistance.
• To measure physical characteristics like temperature, light, and strain, a Wheatstone bridge and an operational amplifier are utilised.

Advantages of Wheatstone Bridge

• The Wheatstone Bridge’s key advantage is that it can be readily interfaced into multiple combinations.
• We can measure variations in the bridge down to m ohms’.
• The unknown resistance is relatively straightforward to find because the other three are well known.

Disadvantages of Wheatstone Bridge

• If the Wheatstone Bridge is not balanced, it will fail.
• It can produce results ranging from a few ohms to a few mega-ohms.
• The Wheatstone may fail if the quality of the galvanometer is poor
• The results of testing large resistances are often non-linear.
• The resistance value can decline depending on the temperature and EMF applied.