A Wheatstone bridge is an electrical circuit used to measure an unknown electrical resistance by balancing two legs of a bridge circuit, one leg of which includes the unknown component.
Wheatstone Bridge Circuit
By replacing R4
above with a resistance of known or unknown value in the sensing arm of the Wheatstone bridge corresponding to RX
and adjusting the opposing resistor, R3
to “balance” the bridge network, will result in a zero voltage output. Then we can see that balance occurs when: R1R2=R3RX=1(Balanced)
The Wheatstone Bridge equation required to give the value of the unknown resistance, RX
at balance is given as: VOUT=(VC−VD)=(VR2−VR4)=0RC=R2R1+R2andRD=R4R3+R4Atbalance:RC=RD So,R2R1+R2=R4R3+R4∴R2(R3+R4)=R4(R1+R2)R2R3+/R2R4=R1+R4/R1R4∴R4=R2R3R1=RX
Where resistors, R1 and R2
are known or preset
Wheatstone Bridge Example No1
The following unbalanced Wheatstone Bridge is constructed. Calculate the output voltage across points C and D and the value of resistor \(R_4\)
required to balance the bridge circuit.
For the first series arm, ACB VC=R2(R1+R2)×VSVC=120Ω80Ω+120Ω×=60volts
For the second series arm, ADB VD=R4(R3+R4)×VSVD=160Ω480Ω+160Ω×=100=25volts
The voltage across points C-D is given as: VOUT=VC−VD∴VOUT=60−25=35volts
The value of resistor, R4 required to balance the bridge is given as: R4=R2R3R1=120Ω×480Ω80Ω=720Ω
We have seen above that the Wheatstone Bridge has two input terminals (A-B) and two output terminals (C-D). When the bridge is balanced, the voltage across the output terminals is 0 volts. When the bridge is unbalanced, however, the output voltage may be either positive or negative depending upon the direction of unbalance.