I'm trying to design a high voltage AC signal generator for some lab experiments.
This schematic is as far as I've got and I wanted to sanity check the basic approach before I continue as I'm used to working on low voltage DC stuff.
EDITED because I completly mischaracterised the nature of the wave forms:
I need to generate various bipolar pulses with a peak no load voltage of around 120V, but at very low power – around 3 watts maximum. The pulses are of a fixed length, but are sent at a variable frequency (So there are gaps between pulses for anything below max frequency.
For Example: A sawtooth wave where the wave length is 2.5 milliseconds, and one complete wave may be sent anywhere from once every second to 400 times a second.
I've based it around a 3 watt fully differential audio amplifier driven by two 12 bit DACs (an MCU will control the two DACs.) The amplifier output drives the secondary coil of a transformer – currently 6V secondary and 240V primary 50/60Hz. The primary coils go through a relay so they can be disconnected from the equipment's output terminal.
The equipment needs to be portable so it will run from a battery.
Is this approach going to work, or am I heading in completly the wrong direction?
As a bare minumum I need to reproduce the following waveform. It is 5ms long and would, at the maximum rate, equal 200Hz. Anything beyond this is a bonus.
I did some bench tests with a DAC, an audio amp (NOT the one in the schematic) and a 6V 2.5VA to 240V 50/60Hz transformer. Unfortunatly the output from the audio amp is a bit distorted.
Best Answer
It may work between 50 Hz and 400 Hz. Over that range your transformer will avoid saturation. If you want it to work down to 1 Hz, you will need to apply a maximum of only 1/50th of your transformer rated voltage to each winding, and can expect to get only 1/50th of the rated voltage out of any winding. A minimum frequency of 10 Hz will mean you must apply no more than 1/5th of the rated voltage, and so on.
A low cost mains transformer may struggle with loss at 400 Hz. You can get transformers rated for 400 Hz, but they cost more due to the thinner laminations.
You can get power audio transformers, which will have the right sort of voltage ratings if they're designed to be driven by valves and drive a low impedance loudspeaker. Again the low frequency response will be limited.