Voltage in and out of an isolation transformer?

[riotubes]

Why am I not measuring the same voltage on the primary and secondary of an isolation transformer? I'm using a triad n68x isolation transformer for a power transformer for the analog stage of my dac. It measures 120V on the primaries and 139 VAC on the secondaries and 178 VDC on the output of the Schottky bridge and 1uF film cap.

I thought an isolation transformer is supposed to generate the same output voltage as the input voltage? What would be the reason I am measuring 138 VAC on the output?

[dmills]

I am guessing you are very lightly loading the transformer?

All transformers have losses, so to make up for this the windings will not be quite 1:1 so that when under full load the extra voltage cancels the winding losses to provide the correct voltage output.

Small transformers have worse regulation then large ones, so a small transformer lightly loaded will have a higher secondary voltage then a large one lightly loaded.

16 percent is poor however, and I am wondering if the waveform is a bit off and your meter is assuming a sine or something.

Regards,Dan.

[riotubes]

Thanks Dan! That explains it.

[sharpi31]

I recently purchased a 1KVA isolation transformer and couldn't understand why all of the transformers in attached components buzzed when connected to it.

I measured Vin and Vout and found that the isolation transformer was raising the supply from 240V to >250V (hence the buzzing).

I reversed primary and secondary to give me Vin = 240V and Vout = 230V. This brought about a correspondingly significant drop in component transformer buzz compared to direct connection to the mains - bonus!