Maxwell bridge

The Maxwell bridge is used to measure lossy inductors, it was named after the physicist James Clerk Maxwell.

Theory

The bridge is similar in structure to the Wheatstone's bridge, but it can contain complex resistors and must be operated with a sinusoidal AC voltage like all AC bridges.

The bridge is balanced when

Applies. Here, the real part and the imaginary part is considered. Are real and imaginary parts in the two bridge branches of the same proportion, the bridge balanced in phase and amplitude (U is zero).

If one of the bridge arms no imaginary components, has the voltage at node (junction of the voltmeter U) of this branch no phase shift with respect to the AC supply voltage. Then the ratio of the real and imaginary parts of the elements Z in the other bridge arm must correspond to zero balance - only then the nodes of this bridge has 0 ° phase shift.

In principle, capacitors and their equivalent series resistance ( equivalent series resistance, ESR ) and dissipation factors can be measured with such a bridge circuit.

Execution

L1, R1 lie in a branch with the measured inductance Lx, consisting of the inductance L2 and the equivalent series resistance ( loss of resistance) R2

R1 and R3 are adjustable. L1 is a Referenzinduktivität.

R1 and R3 are set so that D = 0. This is achieved by the phase position of the node of the left branch of the bridge is brought up to 0 ° with respect to the supplying alternating voltage of the amplitudes of R1 and node points of the two bridge branches to be aligned with R3. Only when these two conditions are met, U = 0

In practice, this must be R1 and R3 are usually repeated alternately adjusted to minimum.

We then have:

Real part ( loss resistance or equivalent series resistance ) of the inductance Lx:

Imaginary part ( inductance ) of the inductance Lx:

• Measuring bridge