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# What is a wheat stone bridge ?

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## 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 values. 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.Physics

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