Check transformer input output voltage how to

In this tutorial you will learn how do you check input and output voltage of a transformer with multimeter and oscilloscope. Transformers are major electrical part in electronics and electrical field of engineering. Sometimes you have a transformer and you might not know the rating of the transformer. You might not know which is primary and secondary side, the input and output voltage of the transformer. Here multiple solution is provided and demonstrated on how you can check the input and output voltage of a unknown transformer.

Check Transformer with Multimeter

First we should know which is the primary and secondary winding of the  transformer. This can be done with a multimeter. The multimeter is set to continuity mode and is connected to one side of the winding. If it is primary winding and is a step down transformer then usually there is no continuity because of the high resistance of the transformer primary winding in step down transformer. Then we can check the secondary winding in continuity mode. If this is secondary winding then there will be beep sound indicating there is continuity. 

Another way to test the transformer sides is to measure the resistance of the two winding sides. If the transformer is step down transformer then the primary winding is one which has higher resistance than the resistance of the secondary winding.

Now to check the input and output voltage, we can apply known ac signal to the primary and measure the output voltage. This measurement can be done using a multimeter which should be set in AC measurement mode. One can connect the primary side to house hold AC power supply which has voltage of 220V and frequency of 50Hz or 60Hz. This can be measured using multimeter and most multimeter do have ac measurement mode. But one should be careful with the power supply from house electricity outlet. If you body part touches both side of the electricity wire, then high current at high voltage will flow from the positive to negative and you will get not only shock but can also have fatal consequence.

Similarly to measuring the ac voltage on the primary as explained above, one can use the multimeter in ac mode to measure the ac voltage on the secondary side when primary side is connected to the ac power supply.

The following video shows how to check a transformer input and output voltages with multimeter.


Check with Oscilloscope

Another method to measure and check the input and output signal of a transformer is to use oscilloscope. AC signal from house electricity outlet can be applied to the primary side and the signal at the primary and secondary can be measured using oscilloscope.

The following video shows how to check a transformer input and output voltage with oscilloscope.

Check with PC soundcard

Instead of using ac electricity from house electricity outlet, one can use a function generator to apply ac signal at the primary and then measure the voltage output at the secondary. Instead of function generator we can also use ac signal from other sources such as generating sine wave from Arduino boards or using audio signal generator software such as Soundcard Scope. Soundcard Scope and other PC based function generator can apply ac signal upto 1V and frequencies upto many KHz. Such signal can be applied at the primary and the signal voltage at the secondary can be measured. An example of using such soundcard signal was shown in the tutorial  How to build base biased BJT amplifier on breadboard and test with PC soundcard based oscilloscope.

 Other solutions

Other methods can be used to know the transformer input and output voltage. The other methods include using signal from a function generator or even using from Arduino. The input signal voltage is known and the output voltage can be measured using either multimeter or oscilloscope. It was shown in the tutorial Generate Waveform using Arduino Due how to generate sine wave. If the input signal voltage is low enough, the Arduino can also be used to measure the voltage.

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