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Maxwell Wien Bridge Circuit and Theory

This is the modified version of simple Maxwell Bridge. In Maxwell Bridge, we find the value of unknown inductance in terms of known inductance. Instead of using a variable inductor in Maxwell Wien Bridge, we use a variable capacitor. Here we compensate for the positive phase angle of the inductive impedance by the negative phase angle of the capacitive impedance.

Theory of Maxwell Wien Bridge

Maxwell Wien Bridge is essentially an AC Bridge. It operates on the basis of the principle of an AC Bridge. Let us draw the basic circuit diagram of an AC bridge as shown in the figure below.

AC Bridge
AC Bridge

Also, we know the balanced condition of the above circuit is

Now in the Maxwell Wien Bridge, we replace the opposite arm of the unknown inductive arm with a variable capacitor. We keep the other two remaining arms of the bridge as fully resistive.

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Now compare to a general AC Bridge, we can write the expression of the impedance of individual arms as

At the balanced condition of Maxwell Wien Bridge, means when no potential difference appears across the detector (D) of the bridge circuit, we can write

The above equation has two parts. One is real and the other is imaginary.

From the real part of the equation we get

This is the expression of the resistance associated with the unknown inductor

From the imaginary part of the equation we get

From the above expression of unknown inductance, we clearly see that the inductance is in terms of known capacitance and known resistances of purely resistive arms.

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