Measuring the input impedance, bandwidth and efficiency of transformers.

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Test setup 1

To determine the input impedance of a transformer, we must load the output of the transformer with a load resistor RL.
The input impedance depends on the load resistor. Test setup for measuring the input impedance (Z.in) of a transformer Z.in must be at least 10 times higher then the output impedance of the signalgenerator, otherwise this test setup gives gives too much error.

V= voltmeter, this must also give a reliable reading at low voltages and frequencies upto 10 kHz .
Instead of a voltmeter, we can better use a oscilloscope with a 10:1 probe.
RL= load resistor
Tr= transformer
G= signal genarator with output resistance Rg=50 Ohm.
R1= potentiometer with a value between 40x and 100x the resistance of the transformer primary winding.

The voltmeter measures the voltage across the load resistor.

- Set the generator to 1 kHz and about 10 Volts amplitude.
- Close switch S, and measure the voltage across RL, this is voltage U1.
- Open switch S1 and adjust the potentiometer, so you measure a voltage of 0.5xU1.
- Disconnect the potentiometer from the circuit and measure it's value (R1).
- The input impedance of the transformer (Z.in) is equal to R1+Rg.

Test setup 2 Test setup for measuring the input impedance of transformers. If the input impedance of the transformer has a low value (between 50 and 1000 Ohm), we can better use this circuit. This circuit is less suitable for high input impedances (>100 k.Ohm.), in that case use test setup 1.

- Open switch S, and turn the potentiometer to 0 Ohm.
- Measure the voltage from the generator (U1).
- Close the switch, and adjust the potentiometer so you measure halve the voltage of U1.
- Disconnect the potentiometer from the circuit, and measure it's value (R1).
- The input impedance of the transformer is:  Z.in=R1+Rg.

Rg is the output resistance of the generator, most times this is 50 Ohm. Test setup to determine the bandwidth of transformers.

First we must measure the input impedance of the transformer, as described above.

- Adjust the potentiometer R1, so R1+Rg is equal to the input impedance of the transformer.
- Vary the frequency of signal generator, and search for the frequency which gives the highest reading on the voltmeter, often this is about 1 kHz, the voltage on this point is U1.
- Vary the frequency of the generator, and determine the two frequencies where the voltage across the voltmeter is reduced to 0.707xU1.
These two frequencies are the -3dB point of ther transformers bandwidth.

Note: the bandwidth is depending on the load resistor RL. Test setup for measuring the efficiency of a transformer. Two voltmeters are shown, but we can also use one voltmeter, and measure the input voltage and output voltage separately.

For this measurement, we must know the input impedance of the transformer (Z.in).
A test method for this is described above.

- Vary the frequency of the generator, and search for the point where the voltage across RL is the highest, this must be somewhere in the middle of the bandwidth as described above.
- Measure the voltage across RL, this is voltage U.out.
- Calculate the output power of the transformer: P.out= U.out x U.out / RL

- Measure the voltage at the input of the transformer, this is voltage U.in.
- Calculate the input power of the transformer: P.in= U.in x U.in / Z.in

- Calculate the efficiency:  Efficiency= P.out / P.in