VOM Measures Complex Impedances ("Z-Box")

by John Bipes
Wurth Electronics Midcom, Inc., 1984

If you occasionally need to measure the resistive and reactive components of complex impedances, you could do the job with a sophisticated instrument, such as a vector voltmeter or a vector impedance meter. Alternatively, you could use a simple volt ohm meter (VOM) and a two component circuit.

1. Connect the load to be measured as shown. Adjust the generator's frequency and and voltage to yield the desired operating point. Note the placement of the VOM's red and black leads (the VOM is naturally set to an appropriate ac range).

2. Place the VOM's leads across the generator terminals and record Egen.

3. Place the VOM's leads across the standard resistor Rs and record Eser.

4. Move the VOM's red lead to the SET terminal and adjust the potentiometer to yield the same voltage as recorded in step 3.

5. Move the VOM's black lead to the right side of Rs and record Etrans.

6. Plug the recorded voltage values into the following equations and solve for Rl and Xl.

The trigonometric relationships among Eser, Egen and Etrans are shown in figure b. In configuring the circuit, choose a value for Rs that is as low as possible, but high enough to allow a respectable resolution reading on the VOM. Note that the generator must deliver a clean sinusoidal signal. If you are using the ac line as a source, be sure to use an isolation transformer (for safety reasons). Bear in mind that the computed reactive component has either a positive or a negative sign, corresponding to an inductive or capacitive reactance. You'll usually have a good idea of which it is by examination of the circuit. If you're unsure, however, determine the sign by adding a capacitor (whose reactance at the measurement frequency is much higher than the computed figure) across the load. It increases the voltage measured across Rs if the load is inductive and reduces it if it's capacitive.