I need to digitize a number of signals of about 40VAC which are derived from 50Hz mains. I need to step the signals down to around 4VRMS and provide electrical isolation so they are safe and within the range of my ADC and a transformer seems like the obvious choice. The big question is what kind of transformer? In order to acquire an accurate signal, I need low distortion and good linearity so the BH curve could be an issue. Options I have considered are:
1) a standard 240/24V mains transformer which would be running well below saturation
2) a 240/24 toroidal transformer which would also be running well below saturation
3) an audio transformer intended for valve (tube) amplifiers.

I imagine that the audio transformer would be the best as it is designed for low distortion and a wide range of amplitudes but does anyone have any other ideas, comments or advice.

Thanks,

Tony

 

Are you trying to measure sine waves at 50Hz or do they have some important higher frequency information?
In other words, what is your range of frequencies?

 

If the freq involved is 50Hz then a small AC mains transformer, if necessary place a small overwind to obtain the required voltage, alternatively, remove turns.
Hammond have very small transformers that may work.
Max.

 

If the signal is only 40vac and can't find a suitable transformer, you could get creative and use a 48vac relay and wind a small overwind on the coil, presumably the load will be trivial.
Max.

 

Are you trying to measure sine waves at 50Hz or do they have some important higher frequency information?
In other words, what is your range of frequencies?

The signals are basically sinusoidal and we are not interested in components at frequencies much higher than 50Hz.

 

If the signal is only 40vac and can't find a suitable transformer, you could get creative and use a 48vac relay and wind a small overwind on the coil, presumably the load will be trivial.
Max.

Exact transformation ratio is not really important. The digitizer is 16 bit so we have plenty of resolution. All inputs will undergo calibration with gain correction performed in software. My concern was that if we calibrate at (say) 40VAC input, we might find that the transformation ratio might be slightly different with a smaller signal (ay 4VAC) due to the shape of the BH curve and other changes e.g. losses.