Instrumentation

# Wheatstone Bridge for strain gauge – Working – Operational characteristics

A Wheatstone bridge is used to detect electrical resistance changes. This circuit is used in signal conditioning. The circuit works like this: Four resistors are wired in the manner shown, with a galvanometer as a very sensitive current detector.

A voltage source, E, produces two opposing current flows in the legs of the resistance bridge as shown. If the resistance values are adjusted properly so that I1 equals I2, it is possible to cause a net current of zero to occur through the galvanometer. This happens only if the following resistor ratios are maintained.

There are virtually an infinite number of resistor combinations that will satisfy the conditions above. In reality, there are certain desirable combinations of resistors that work better than others for strain gauge applications, but that is beyond the scope of our discussion.

### Working:

If one of the resistors is made variable, say R1, and one of the other resistors, say R4, is replaced by the dc resistance of our strain gauge, it is possible to adjust R1 so that the ratio formed by R3/R4 is precisely equal to the ratio formed by the fraction R1/R2.

When this occurs, there will be no current flow through the galvanometer; the galvanometer’s indicator needle will read zero in a straight-upward position. A mismatch of these two ratios will produce an imbalance of currents in each of the two current paths of the bridge, causing the galvanometer’s needle to deflect to one side or the other of centre zero. Once the balanced or nulled condition has been reached, any movement of the galvanometer’s needle to the right or left of centre zero will be a certain indication of a change in the strain gauge’s resistance brought on by a stressed condition to the gauge.

### Operational Characteristics:

In reality, the temperature will affect the internal resistance of the strain gauge. This is one of the more notable and undesirable characteristics of a strain gage. So how can we nullify the effects of temperature? Which is shown below:

R1 was the variable balancing resistor for the Wheatstone bridge and R4 was the strain gauge, we could insert a duplicate gauge into the bridge circuit that was not subject to the measured strain. This duplicate gauge would be subjected to the same temperature as that experienced by the stressed gauge. The duplicate gauge, or dummy gauge as it is often called, can be inserted in place of R2 to offset the temperature-created resistance in the stressed strain gauge, R3.

### Sivaranjith

Instrumentation Engineer