My Google is failing me.
I want to make an isolated AC voltmeter capable of working at 120 or 240 VAC.
I've found a suitable 240 volt to 5 volt transformer to use (current capacity doesn't matter – the measurement will be at very high impedance).
What would it do if it were fed 120 volts? Ideally the transformer would simply output 2.5 volts, but I have a memory of transformers getting very cranky when fed the wrong voltage.
Best Answer
Ignacio Vazquez-Abrams is generally right in his comment. But what you seem to have here is a 240V 50Hz transformed that you want to use at 120V 60Hz and at a current much below the rated one. In general 60Hz transformers may overheat when used at 50 Hz. But not viceversa, and not at low load loads, so this isn't a great concern here.
But you need to beware that any transformer will output voltage above the rated one when running open circuit or at low load. So don't expect exactly 2.5V. In fact if the transformer is low power one (<10VA) you can easily have 50% more voltage at the output than what it's rated for at max current.