PH337 Electronics
Experiment #3

AC BRIDGE LAB

OBJECTIVES

  1. Assemble a Maxwell ac bridge circuit using a two channel oscilloscope as a null detector.
  2. Measure the inductance of an unkonwn inductor in terms of a known inductor
  3. Reconfigure an alternate form of an AC bridge to measure an unknown capacitor.

DISCUSSION

Generalized AC Bridge

The general ac bridge is shown with complex impedances in each leg of the bridge. At balance, the unknown impedance, Zx, is
Zx = Z2 [Z3 / Z1]

Maxwell Bridge is a particular form of the generalized ac bridge. The components are arranged such that
Lx = C1 R2 R3 and
Rx = [R2 R3] / R1
  1. the unknown impedance is an inductor and it is in position Zx. (Inductors inevitably have a series resistance Rx in the wires), and
  2. a known capacitor is in position Z1. (an adjustable resistance, R1, is put in for phase balancing)

      3. PROCEDURE

      Part 1. Measure Inductance with Maxwell bridge.
      1. Set up components of the MAxwell ac bridge. You will use both channels of the oscilloscope to measure Va and Vb. Connect one input channel to point Va on the bridge; the other scope input channel to point Vb. [Explain why this is necessary. What would happen if you connected the two points to a single channel of the scope?]
      2. Balance the bridge voltage by adjusting the values of the resistors R2 and R3 to get both the amplitude and phase of Va and Vb equal. Make these measurements at 1 kHz. Readjust the frequency to 10 kHz. Is the bridge still balanced? Explain. Rebalance as necessary.
      3. Note and record which technique of using the oscilloscope gives you the best sensitivity in detecting the bridge balance;
        • Viewing the dual channels
        • Channel 1 minus Channel 2 (1 added to inverted 2)
        • X-Y display
      4. Calculate the value of Lx from the Maxwell bridge equations at both 1kHz and 10 kHz. Your instructor will give you a value to use for C1. Then compare these results on your bridge with measurements you make on a commercial ac bridge that is set up in the lab.


        5. Part 2. An alternate AC bridge for Capacitance Measurement
      5. Reconnect the components as shown in figure 2.
      6. Derive the formula:
        Cx = C1 [R3/R4]
      7. Balance the bridge by adjusting the resistances R3 and R4
      8. Calculate the value of the unknown capacitor and compare to an independent measurement on a commercial bridge.