Introduction to the Wheatstone Bridge
The Wheatstone bridge is an electrical circuit for the precise
comparison of resistances. Sir Charles Wheatstone is most famous for this
device but never claimed to have invented it - however, he did more than anyone
else to invent uses for it, when he came across a description of the device in
1843.
The Wheatstone bridge is an electrical bridge circuit used to measure
resistance. It consists of a common source of electrical current (such as a battery)
and a galvanometer that connects two parallel branches, containing four
resistors, three of which are known. One parallel branch contains one known
resistance and an unknown (R4 in the example below); the other parallel branch
contains resistors of known resistances. In order to determine the resistance
of the unknown resistor, the resistances of the other three are adjusted and
balanced until the current passing through the galvanometer decreases to
zero. This is one method of using it and
is known as the null balance bridge. In
this case, Vo is always zero.
It can also be used as a deflection bridge in which case three resistors are fixed and not adjusted and the output voltage varies as the fourth resistor (e.g. RTD) changes.
The Wheatstone bridge is well suited also for the measurement of small changes of a resistance and, therefore, is also suitable to measure the resistance change in an RTD. It is commonly known that the RTD transforms temperature applied to it into a proportional change of resistance.
The Wheatstone bridge is a DC device that is typically used to modify a change in resistance to a change in voltage which can be used more easily for display purposes. The basic form of the Wheatstone bridge is shown as follows: (both diagrams are equivalent)
The equation for the output voltage is easily determined by dividing the circuit into two parts as shown below. The output voltage is the potential difference between B and D, or VBD. This is easily determined by calculating the potential at points B (VAB) and D (VAD). VBD is then the difference between these
VBD = VAB - VAD
VAB =? VAD = ?
