Maxwell’s Inductance-Capacitance Bridge and Measurement of Inductance

Maxwell’s inductance-capacitance (LC) bridge is a widely used method for the measurement of inductance. It is primarily used for measuring the inductance of medium Q coils. Let us understand why this is the case.

Working Principle of Maxwell’s LC Bridge

The Maxwell’s LC bridge is based on the principle of comparing the unknown inductance with a known capacitance and resistance in a balanced bridge circuit. The bridge consists of four arms - two arms with known resistances and capacitances, and two arms with the unknown inductance and resistance.

The bridge is balanced when the ratio of the unknown inductance to the known capacitance is equal to the ratio of the unknown resistance to the known resistance. In this balanced condition, the bridge output voltage becomes zero.

Measurement of Inductance

When the Maxwell’s LC bridge is used for the measurement of inductance, the bridge is initially balanced with the unknown inductance disconnected. The bridge is then unbalanced by connecting the unknown inductance into one arm of the bridge.

The bridge is rebalanced by adjusting the known capacitance until the bridge output voltage becomes zero again. At this point, the ratio of the unknown inductance to the known capacitance is equal to the ratio of the unknown resistance to the known resistance.

Selection of Q Coils

The quality factor (Q) of a coil determines its ability to store energy. It is defined as the ratio of the energy stored in the coil to the energy dissipated in the coil per cycle. Q is directly related to the resistance and inductance of the coil.

Low Q Coils: Low Q coils have a high resistance compared to their inductance. These coils dissipate a significant amount of energy as heat and have a low ability to store energy. When such coils are used in the LC bridge, the energy dissipated in the coil affects the accuracy of the measurement. Therefore, low Q coils are not suitable for measurement using Maxwell’s LC bridge.

Medium Q Coils: Medium Q coils have a balance between resistance and inductance. They have a moderate ability to store energy and dissipate less energy as heat compared to low Q coils. Maxwell’s LC bridge is primarily designed for measuring the inductance of medium Q coils. The bridge circuit is optimized for this range of Q values, ensuring accurate measurement results.

High Q Coils: High Q coils have a low resistance and a high inductance, resulting in very low energy dissipation. These coils are used in applications where the energy loss needs to be minimized. While the Maxwell’s LC bridge can measure the inductance of high Q coils, the accuracy may be compromised due to the small energy dissipation in the coil.

Conclusion

In conclusion, Maxwell’s inductance-capacitance bridge is used for the measurement of inductance of medium Q coils. The bridge circuit is optimized for this range of Q values, ensuring accurate measurement results. Low Q coils dissipate a significant amount of energy, affecting the measurement accuracy, while high Q coils have very low energy dissipation, compromising the accuracy of the measurement. Hence, medium Q coils are the most suitable for measurement