# How do you find the power output of an unknown transformer?? (SOLVED)

I got this one transformer. It’s pretty small in size.The transformer’s secondary output is center tapped. It will take in 240 volts and spit out 9volts on two different (yellow) cables when measured from the third cable(ground) and 18 volts when the PD is measured between the two (yellow) cables .

My question is: Is there a way to find out the maximum safe current of the secondary winding of this transformer and therefore its power output?

Void

Ideally you need to know the power dissipation limit - measure the resistance of each winding (primary and secondary), then calculate the dissipation for a given load current (I-squared-R for both primary and secondary combined), if its too high calculate for a lower current till you home in on roughly the right value. This is then an upper limit for the rms current output.

Current in the primary depends on the secondary current via transformer action, but also on the inductance of the primary, which complicates things. You can measure the current taken by the primary with secondary unconnected - however this involves measuring current on the mains side which has safety implications of course.

Find a transformer of the same size in a catalogue / online site to get an idea of power dissipation.

Typically a small transformer would be around 80% efficient, so the VA rating and power dissipation are related - but these things do vary.

If in doubt de-rate the device from your calculated values, and monitor for overheating.

And one last thing a picture is worth a 1000 words - if you post a photo here someone may recognise it....

Thanks MarkT
The transformer is labelled 3-246
I attached some images

If you knew a lot about transformer design and you could get all that information (wire gauges, coupling efficiency, etc.) it'd amount to an educated guess.

Once the output voltage at load is less than 3/4 of the unloaded, things start heating quickly.

Mr pancake could you please provide me with such information? (a link maybe? ) Thanx

Some similar looking transformers I was looking at were 150mA current from either 9V or 18V (but not both). It is definitely in the 75 - 300mA range. (unless that's some sort of novelty coin)

I would scan the main component distributor web sites (Farnell/Newark, RS, Digikey etc.) for transformers of similar construction and similar size, and see what VA rating those have.

Load the transformer until the output voltage measures 15V and record the current, then try the same thing at 12V. That would give you a good idea of what the transformer can supply. running it at a lower voltage would probably be overloading.

Thank you very much people :)