How to measure I max of a transformer

[polashd]

I bought a transformer from a local shop.

I can measure the V out of the transformer easily using Multimeter.

The shopkeeper told current output (I max) would be 600mA.

But I'm not sure about the 'Imax'. Without connecting to proper Load I can't measure it.
The efficiency also not known.

Is there any way to find out max current of the transformer?

note: It will be connected to 220-230V main.

 

[Reloadron]

But I'm not sure about the 'Imax'. Without connecting to proper Load I can't measure it.
The efficiency also not known.

You said it right there. You also do not mention the secondary voltage?

If the transformer is (for example) rated with a secondary output of 12 volts @ 600 mA (0.6 amp) then you would need to apply a load. 12 Volts / .6 Amp = 20Ω. So placing a 20 ohm load across the secondary it should draw 600 mA. The power dissapated by the load would be the current times the voltage or 12 X .6 = 7.2 watts. So to properly load the secondary for .6 amp you want at least a 10 watt 20Ω resistor.

You place your ampmeter in series with the load. That is important!

In my 12 volt example the voltage should remain a constant 12 volts when loaded with the 20 ohm 10 watt resistor.

Ron

 

[MikeMl]

However, I'll bet that the application for the transformer involves a full-wave bridge and filter capacitor. When you load the tranny with just a resistor, the current waveform in the winding is sinosoidal; when you load the transformer into a rectifier/filter capacitor, the current flows in short spikes, but the peak current is several times higher than the sinosoidal current. The losses (that heat the transformer) are proportional to the square of the current, so the losses are much higher with the pulsing current.

The only meaningful test is to load the transformer in the way you intend to use it, and then measure the temperature rise over 30min or so. If your transformer starts to smoke, you have overloaded it. If it gets hot enough that you cannot keep your hand on it, it may still be within its maximum allowed temperature rise. I use an IR thermometer to measure the temp. rise. 50 to 60degC is ok. 80+degC is not.