An Owen Bridge is an AC bridge circuit used for measuring an unknown inductance by balancing the loads of its four arms, one of which contains the unknown inductance. Figure 1 below shows a diagram of the Owen Bridge.  

Figure 1.  The Owen Bridge

As shown in Figure 1, one arm of the Owen bridge consists of a capacitor in series with a resistor (C1 and R1) and another arm consists of an inductor L1 in series with a resistor (L1 and R4).  One arm contains just a capacitor (C2) while the fourth arm just contains a resistor (R3).  The values of C2 and R3 are known, and R1 and C1 are both adjustable. The unknown values are those of L1 and R4.

Like other bridge circuits, the measuring ability of an Owen Bridge depends on 'balancing' the circuit. Balancing the circuit in Figure 1 means adjusting R1 and C1 until the current through the bridge between points A and B becomes zero.  This happens when the voltages at points A and B are equal.  When the Owen Bridge is balanced, it follows that Z2/Z1 = R3/Z4 wherein Z2 is the impedance of C2, Z1 is the impedance of the arm containing C1 and R1, and Z4 is the impedance of the arm containing L1 and R4.  Mathematically, Z2 = 1/(2πfC2);  Z1 = R1 + 1/(2πfC1) while Z4 = R4 + 2πfL1.

Thus, when the bridge is balanced,

1/(2πfC2)/[R1 + 1/(2πfC1)] = R3 / [R4 + 2πfL1]; or

[R4 + 2πfL1]= (2πfC2R3) [R1 + 1/(2πfC1)]; or

R4 + 2πfL1 = 2πfC2R3R1 + C2R3/C1

When the bridge is balanced, the negative and positive reactive components are equal and cancel out, so

2πfL1 = 2πfC2R3R1 or

L1 = C2R3R1.

Similarly, when the bridge is balanced, the purely resistive components are equal, so

R4 = C2R3/C1.

Note that the balancing of an Owen Bridge is independent of frequency.

See Also:   Bridge Circuits;  More Articles