Study of LCR or Maxwell Bridge.
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•LCR bridge meter
A Maxwell Bridge, also known as the Maxwell-Wien Bridge, is an AC bridge circuit
used for measuring an unknown inductance in terms of a known capacitance by
balancing the loads of its four arms, one of which contains the unknown inductance.
For AC bridge to balance, the impedance ratios of each branch must be equal:
The impedance quantities in the above equation are complex, accounting for both
magnitude and phase angle. It is insufficient that the impedance magnitudes alone be
balanced; without phase angles in balance as well, there will still be voltage across the
terminals of the null detector and the bridge will not be balanced.
As shown in circuit diagram, one arm of the Maxwell bridge consists of a capacitor in
parallel with a resistor (C1 and R1) and another arm consists of an inductor Lx in
series with a resistor (Lx and Rx). The other two arms just consist of a resistor each
(R2 and R3). The values of R2 and R3 are known, and R1 and C1 are both adjustable.
The unknown values are those of Lx and Rx.
Like other bridge circuits, the measuring ability of a Maxwell Bridge depends on
'balancing' the circuit. Balancing the circuit in Figure means adjusting C1 and R1 until
the current through the bridge between points A and B becomes zero. This happens
when the voltages at points A and B are equal. When the Maxwell Bridge is balanced,
it follows that
Rx = R2R3/R1
And Lx = R2R3C1
Note that the balancing of a Maxwell Bridge is independent of the source frequency.
One type of inductance measuring device is a multifunction meter, such as an
inductance/capacitance/resistance (LCR) meter. The LCR meter uses a principle of a
balancing bridge or an auto balancing bridge. The measurement device excites the
inductor using built-in signal generator and bridge balancing technique to measure the
value of impedance of the inductor. Specifically, in an LCR meter, the signal source is a
sinusoidal wave, whereas in a real operating environment, the signal may be a square
wave with a high voltage and a high current component.
1. Connect the unknown inductance to the bridge meter.
2. Turn on the meter.
3. Set the parameter, frequency, display and range.
4. Set the indirect functions. Indirect functions are Clear “0”, Sorting, Auto-LCZ,
Buzzer, Change cursor, Advanced set, and State save & exit. Press setup key to
enter menu in measurement state. Pressing setup key again, the instrument returns
back to the measurement state.
5. Select the appropriate settings. The value of unknown inductance will be
displayed on the LCD screen.