5.5 Maxwell’s Bridge

Definition: The Maxwell bridge is the bridge that is used to measure the circuit's self-inductance. It's a more sophisticated version of the Wheatstone bridge. The Maxwell bridge operates on the comparison principle, which means that the value of an unknown inductance is determined by comparing it to a known or standard value. It is also known as “Maxwell Inductance Capacitance Bridge.”

Function: It is used to measure the unknown inductance of a circuit in terms of a known capacitor.

The component of each branch are:

• Branch a-c= known standard resistor

• Branch b-d=a known a variable resistor

• Branch a-b= known capacitor in parallel with a known resistor

• Branch c-d=unknown resistor in series with unknown inductor

• Branch B-C=an AC null detector to detect balanced condition

Like other bridge circuits, its measuring ability depends on “balancing” the circuit that is :

• Adjusting R3 until the current/null detector between points b and c shows null/zero. That is, until the bridge current = 0 and Vbc= 0 (i.e. Vb=Vc)

The impendances in each branch in mathematical forms are as follow

Therefore, from the above condition for bridge balance the unknown inductance value can be determined by comparison with variable standard capacitor.

However, this bridge is limited to the measurement of only a low “Q-factor” (less than 10) and can be calculated as follows.

The advantages of Maxwell bridge

· The balance equation is independence of frequency

· It is useful for measurement of wide range of inductance at power and audio frequency

· The scale of resistance can be calibrated to read inductance directly

The disadvantages of Maxwell bridge

· It cannot be used for the measurement of high Q values (Q≥10)

· It cannot be used in the measurement of very low Q values, because of balance converge problem.