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It is dificult task to use unknown transformer. If it is a 50Hz traf from power supply you can measure resistance of windings and connect the most resistant to the mains through small fuse or beter output of another transformer connected to the mains. If fuse does not burn, you can measure output voltages and calculated transformer ratio. If measure carefully diameter of winding wire you can estimate max current through it. But it is a very dangerous approach! You must be very carefull!!!
If it is a transformer from switching power supply you need a L meter at least, and some detail for material of the core.
how detailed do you need your transformer parameters??
If you only need primary to secondary turn ratio, this is quite easy. You only need to supply any ac voltage on the primary (be careful not to saturate the core) and measure the secondary voltage.
If you need the elements of the equivalent circuit diagram you have to measure more than that. Then you have at last to measure voltage and current on the primary for the two cases: open- and short- circuit secondary.
For dividing into inductive and resestive parts measurements must be done with phase information of voltage and current.
In open circuit case you can measure the main inductance and core losses, modeled by an inductance in parallel to a resistor.
When short circuiting the measurement will show the parasitic inductance of primary and secondary as well as ohmic values of all windings.
If you have to divide the stray inductance and the resistive parts to each winding (in case you have more than one secondary winding) you must exchange measurement probes from primary and secondary and redo the above measurement.
It is important to notice that if the windings of primary and secondary are not absolutely symmetric (this almost never happens) you have to orient all measuring results to one side. That means you gain as a result an equivalent cuircuit where the values of the transformer are all oriented to the primary voltage. The transformer itself can be seen as an ideal component that transforms voltage with M:N and current with N:M.
some info on transformers can be found here:
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Sorry, this document is german only :-(. **broken link removed**
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