Isolating transformer disparity on reading volts out

paddy ryan said:

my DMM reads the output as 265 volts.

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What is the measured transformer input voltage?

Transformers have a rated output voltage at their rated load, so an unloaded transformer will likely read somewhat higher to allow for voltage reduction under load due to primary and secondary winding resistances.

As Crutschow says.

The variation from no load to full load is called the Regulation; for a tiny transformer it could be eg. 30% high off load, for larger ones the difference gets steadily smaller.

The one you have has a regulation of 8% according to the data sheet? (1.08 x nominal voltage, off load).

If you are in the UK, the mains input could easily be anything up to 250V, which will increase the output proportionally.

With the no load output of 1.08 times nominal, 265 V mean that the input is 245 V, which is well in the normal range for the UK.

paddy ryan said:

I have a BLOCK TIM 100 isolating transformer, designated at 230 Volts 1 amp primary

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https://www.block.eu/en_EN/productversion/tim-100/
That transformer is rated at 100 W, so on 230 V primary that is a bit under half an amp.

Thanks for all that, it eases my mind. There was no load on it when I tested it for the output, so it all makes sense. Thanks again.