The Wheatstone bridge is an important electrical circuit that can be used to accurately measure unknown electrical resistances. It is commonly used in electrical and electronic systems to measure resistances due to its accuracy when the circuit is balanced.
This principle is also used in many measuring devices such as resistance based sensors and strain gauges. This is useful knowledge for engineering students who may be studying electrical measurements.
A Wheatstone bridge is a circuit consisting of four resistors connected together to form a diamond shape. A DC power supply is used to supply power to the circuit between two diagonally opposite points. A Galvanometer or suitable voltage measuring device will be connected between the other two points.
Three of the resistances may be known or variable. The fourth resistance will be the unknown resistance. The Wheatstone Bridge compares the ratio of two resistances. Once the circuit is balanced there will be no potential difference between each point of the galvanometer therefore no current can flow.
The main components of a Wheatstone bridge are:
The arrangement of these components allows the unknown resistance to be calculated from the known resistance values when the bridge is balanced.
The Wheatstone bridge compares the ratio of two branches. We can explain how it works using the balanced/unbalanced conditions.
When the circuit is balanced the potential difference between the two points that connect to the galvanometer will be equal and therefore no current will flow.
Say our four resistors are P, Q, R and S. At balance;
P/Q = R/S
We can rearrange this to get;
S= (Q x R)/P
Where S is our unknown resistance. As long as we know what P, Q and R are, we can calculate the unknown resistance.
If the ratio of the resistances aren’t equal then there will be a potential difference between the points connecting to the galvanometer. Current will flow through the galvanometer.
The direction of the current will tell us which point has the higher potential. We can then alter the resistances until there is no current flowing through the galvanometer showing the circuit is balanced.
Example
Say our known resistances are;
P = 2 Ω
Q = 4 Ω
R = 6 Ω
At balance;
S= (Q x R)/P
S= (4 x 6)/2
S=12 Ω
We can see that our unknown resistance is 12Ω.
The Wheatstone bridge can be used for a variety of applications.
The Wheatstone bridge is an important electrical circuit that can be used to accurately measure resistances. This works by comparing the ratio of two resistances. Once the circuit is balanced there will be no current flowing through the galvanometer. The Wheatstone bridge can be used in many applications such as strain gauges, temperature sensors and industrial. Knowing how this circuit works will give engineers a good base of knowledge on how to measure electrical resistances.
To accurately measure unknown resistances.
The potential difference between the galvanometer points will be equal and no current will flow.
Traditionally a galvanometer is used to determine if current is flowing through the detection branch.
Yes, if resistance based temperature sensors are used such as RTDs.
The balance equation for the resistor configuration mentioned in this article is P/Q=R/S.
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