RLC Measurement

The circuits shown are types of bridge circuits, which are very useful in measuring unknown values of R, L, and C based on known values of the other components in the same bridge circuit. 

The potentiometers and other variable components in the bridge are adjusted until the reading of the meter at the center of the bridge becomes zero, in which case the bridge is said to be balanced.

Figure :  Circuit Diagrams for a Resistance Bridge (left) and a Capacitance Bridge (right)

In Figure , the circuit on the left is a simple Wheatstone bridge. When this bridge is balanced, R1/R2=R3/R4.  The circuit on the right is a capacitance bridge which is balanced when:  C2 = C1(R1/R2) and R4 = R3(R2/R1). 

 

Figure :  Diagram for an Inductance Bridge Circuit

The circuit in Figure is used for measuring unknown inductances. When this bridge is balanced, the following equations apply:

L1 = R1R4C1 / [1 + (2πfR3C1)²]; and

R2 = (2πfC1)²R3R2R4 / [1 + (2πfR3C1)²]

where f is the frequency of the applied input voltage.