What is the disadvantage of Maxwell Bridge?

Question

Examoogle · Accepted Answer

Inductance cannot be measured over a wide range

Explanation: The Maxwell bridge is an AC bridge used to measure an unknown inductance in terms of a known capacitor. A primary disadvantage is its limited range, as it cannot be used to measure low Q-factor inductors effectively nor cover a very wide range of inductances due to the variable components required.

Working Principle: The bridge operates by balancing the impedances of four arms. At balance, the product of opposite arms is equal. Since one arm contains a capacitor and a resistor in parallel to balance an inductor and resistor in series, the balance equations depend on the values of these components. If the inductor is low Q or requires large variations, the variable standard components (capacitor and resistor) become physically impractical or non-linear.

Key Formulas: $L_x = R_2 R_3 C_4$ — Inductance of the unknown coil in terms of bridge components; $Q = \omega R_4 C_4$ — Quality factor of the inductor measured by the bridge

Key Points: Used for measuring medium-Q inductors ($1; Balance equations are independent of supply frequency.; Variable capacitor is generally more expensive/complex than variable resistors.; Limited effectiveness for low Q coils because $R_4$ becomes excessively high.

Advantages: Frequency independent balance equations; Useful for comparing inductances; No frequency error due to series resistance

Disadvantages: Cannot measure over a wide range of inductances; Requires a high-quality variable capacitor; Not suitable for low Q coils; Calibration of variable capacitor is difficult

Applications: Laboratory measurement of unknown inductors; Determination of magnetic properties of materials

Additional Info: The bridge is specifically named after James Clerk Maxwell.. Option B is incorrect because the balance equations for a Maxwell bridge do not contain the frequency term $\omega$ in the final result ($L_x = R_2 R_3 C_4$), making it frequency independent.

Answer: A is correct — The Maxwell Bridge is not suitable for measuring inductances over an extremely wide range, especially low Q-factor inductors, due to the physical limitations of the required variable components.

Exam Tip: Always remember that while Maxwell bridge is frequency independent, Hay's bridge is a modification of Maxwell's bridge specifically designed for high Q-factor inductors.

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