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WHAT IS WHEATSTONE BRIDGE?
WHAT IS WHEATSTONE BRIDGE?
The Wheatstone bridge is the most straightforward type of bridge for measuring resistance (> 1).
It is the most precise method for measuring resistance known and is widely used in laboratories.
Wheatstone Bridge device
Fundamental of Wheatstone Bridge
Fundamental of Wheatstone Bridge
It is being used to compute an unknown electrical resistance by comparing the two legs of a bridge circuit, one of which consists the unidentified component.
The basic idea is that two voltage dividers were being supplied by the same source.
The circuit outcome is obtained from the outputs of both voltage dividers. A galvanometer (a very sensitive dc current gauge) is linked between the output terminals to analyze the current flowing through one voltage divider with the other.
If the ratios of the two voltage dividers are accurately the same, the bridge seems to be balanced, and no flow of current thru the galvanometer in any direction.
Wheatstone Bridge at balanced condition
Wheatstone Bridge at balanced condition
(voltage perspective)
(voltage perspective)
As a result, for the Wheatstone bridge: “At balance, the product of the resistances of opposed arms is always equal.”
It is important to note that the “unknown” resistor might be ANY of the four resistors!
Wheatstone Bridge alternative explanation
Wheatstone Bridge alternative explanation
(current perspective)
(current perspective)
The final equation states the conditions for a Wheatstone bridge's balance and is relevant for measuring the value of an unknown resistor once balance is already acquired.
If the value from one of the resistors changes even slightly, the bridge becomes imbalanced and current flows through the galvanometer.
Do not memorize the notation of 𝑹𝟑 𝐨𝐫 𝑹𝟐, 𝐞𝐭𝐜. Always refer to the circuit.
Sensitivity of the Wheatstone Bridge
Sensitivity of the Wheatstone Bridge
When the bridge is imbalanced, current flows thru a galvanometer, leading its indicator to deflect.
The magnitude of deflection is determined by the galvanometer's sensitivity.
Sensitivity is described in terms of deflection per unit current.
For the same current, a more sensitive galvanometer deflects by a higher amount.
Total Deflection can be evaluated in either linear or angular units (such as mm, degrees or radians).
Thus, the sensitivity is defined as deflection per unit current :
So, it follows that, the total deflection is:
What happened if unbalance condition?
What happened if unbalance condition?
To determine the amount of deflection that would result for a particular degree of unbalance, the following steps are taken:
Find the equivalent voltage (i.e. Thevenin voltage, VTh)
Find the equivalent resistance (Thevenin resistance, RTh) by removing the galvanometer and short circuiting the voltage sources
Find the deflection current of the galvanometer due to the unbalanced condition.
Wheatstone Bridge
Wheatstone Bridge
Thevenin Equivalent Circuit
Thevenin Equivalent Circuit
How do I calculate the 'open circuit' voltage (Vth) between terminals a-b?
Or
Amount of current flow through the meter can be obtained by Thevenin Equivalent Circuit
To find Rth:
Setting all sources to zero (voltage source are replaced by short circuits and current sources by open circuits)
Finding the resultant resistance between the two marked terminals.
Thevenin’s equivalent resistance circuit is :
Current flows through the meter:
Wheatstone Bridge
Wheatstone Bridge
Application
Application
Slight changes in resistance are to be monitored, as in sensor applications.
Wheatstone Bridge in a transducer converts a resistance change to a voltage change.
The combination of a Wheatstone bridge and an operational amplifier is widely utilised in industry for different transducers and sensors.
Strain gauges, whose electrical resistance fluctuates proportionately with strain in the instrument, are most often employed to measure strain.
- Strain Gauges
In practice, strain gauge resistance ranges from 30 ohms to 3000 ohms.
A Wheatstone Bridge circuit is linked to the strain gauge.
2. Loop Tests to find Cable faults
2. Loop Tests to find Cable faults
Locate the problem in an underground cable.
There are two types of loops test which is Murray Loop Test and Varley Loop Test.
Murray Loop Test
Murray Loop Test
To locate a defect in an underground wire by installing a Wheatstone Bridge in it
The Wheatstone bridge's balance is maintained by changing the resistance of the ratio arms R1 and R2 until the galvanometer deflection is zero.
The position of the fault may be determined by comparing the resistance.
In this experiment, wires with a known length should be used.
LOCATING A GROUND FAULT
LOCATING A GROUND FAULT
The cable terminals of the problematic conductor of length Lb are linked to the cable terminals of a healthy conductor of length La.
The test set is linked to these two conductors to formed the loop.
R2 is an adjustable resistor that balances the bridge.
The ratio of R2 to R1 is commonly referred to as the rms ratio.
At balance:
The resistance value is proportional to the length of the cable. As a result, the value of Rx is proportional to Lx's length.
If both conductors consist of the same material and cross sectional area, then
In a multicore cable the healthy has the same length and same cross sectional as the faulty cable, so that La=Lb. Therefore,
Varley Loop Test
Varley Loop Test
The Varley Loop Test is the same as the Murray Loop Test, but with a little change in configuration.
Instead of calculating it from the known lengths of the cable, determine the fault site in an underground cable by constructing one Wheatstone Bridge and comparing the resistance.
R1 and R2 are fixed ratio arms, and the balance position is achieved by altering the known variable resistance.
If the switch S is in position 1, the variable resistance R need to be adjusted to balance the circuit.
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