Testing Unknown Transformers with Function Generator

Last week, I posted a YouTube video explaining how to use a function generator to test an unknown transformer. I have received quite a few questions since so I thought I would explain this topic a bit more in detail here.

First, let me recap the basic approach I used for testing an unknown transformer.

  • Set the function generator output to around 1 kHz (sine) and set the output level to roughly 1V (RMS)
  • Connect a winding from the transformer to the signal generator output, measure the voltage across the winding and voltages across other windings. Obtain the turns ratio.
  • Adjust the operating frequency, repeat the previous step to obtain the optimal operating frequency of the given transformer.

Depending on the magnetic core material (e.g. laminated iron/steel core, ferrite core, etc.) used, the optimal operating frequency varies. For most laminated core power transformers and signal transformers their operating frequencies range from tens of Hertz to several kHz. For ferrite core transformers, their operating frequencies can range from several kHz all the way into MHz range.

Why using a function generator?

Using a function generator to test a transformer is convenient as we can adjust the output waveform to match the waveform that will be used in a particular circuit (e.g. sine or square) and can also adjust the frequency over a wide range.

It is also a safe way to test. Since both the output voltage and current are limited, we can safely connect our transformer under test without having to worry about the possibility of damaging the signal source or the transformer due to incorrect operating frequencies and voltages.

Of course, you should always avoid touching the output of the transformer during testing as for a step up transformer, the output could reach hundreds of volts depending on the turns ratio. While it probably won’t kill you, it will certainly give you a nasty shock.

Step up or step down, primary versus secondary

Most of the time, the designation of primary and secondary is interchangeable. This means that a transformer can be used as either a step down transformer or a step up transformer depending on which side of the windings the input voltage is applied to.

Sometimes, especially for high power transformers, the designation of primary and secondary is chosen to maximize efficiency and heat dissipation given the topology of the windings. But again, for most use we can safely switch the primary and secondary to make a transformer either a step up or a step down one when heat dissipation is not a primary concern.

Multiple windings, phases

Most of the time the phase between the input and the output windings of the transformer does matter much. But when multiple windings are present or a particular phase designation is required, we can easily test the phases among all the windings with respect to the primary by connecting one of the winding terminal with one side of the primary (i.e. the winding that the signal generator is connected to) and measure the combined voltage across the primary terminal and the other side of the secondary winding terminal.

If the measured voltage across the two windings are higher then the voltage on either of the windings, then the connection point between the two windings are of the opposite phase. And if the measured voltage is smaller than at least one of the windings when measured individually, then the connection point between the two windings are of the same phase.

Or course, you can always use an oscilloscope to observe the phases among different windings. But one needs to be careful as most scope inputs are limited to a few hundreds volts and if the step up voltage is significantly higher the input circuitry of the oscilloscope could be damaged.

Working voltage range

Determine the “correct” working voltage is a bit more complicated. A transformer rated for 110V 60Hz for instance, can operate under the same quiescent current when both the voltage and frequency are doubled (e.g. 220V 120Hz) since the reactance is also doubled as the result of doubling the frequency.

For typical applications, the choice of the input voltage is determined by the desired output voltage, the operating frequency, the turns ratio and the targeted quiescent current and power consumption due to the input winding resistance.

Since each transformer has its optimal operating frequency (determined by the magnetic core in use), the input voltage can be determined if the quiescent power consumption is given (usually less than 1% of the rated power). But for transformers with a wide operating frequency range, we could potentially supply a higher than rated input voltage when operating at a higher than specified frequency. And in a step up transformer configuration, the excess secondary voltage may cause the insulation to breakdown and destroy the transformer in the process. Usually, this is only a concern when the output voltage is above a few hundreds of volts. For this reason, always design the circuit to operate at the lower end of the frequency range with the predetermined quiescent power (e.g. using lower input voltage).

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