I got a call from a company who was having problems getting an AC current regulator working. They just needed some basic ideas. I often get such calls and I try to help as long as it doesn't take up too much of my time.
As near as I can tell, they are using the current to test and calibrate current transformers. They needed to route 10 amps of pure 60Hz sine wave AC current through a single heavy wire, as part of a calibration circuit for one of their products. They said the total resistance of the loop was about 0.025 ohms. Their first approach used a small variac and a toroid transformer, to produce the needed current. However, it was not well regulated. The variac approach worked fine, as long as the line voltage remained at 120vac. But since they wanted to keep the 10 amps to an accuracy of +-0.5%, any line voltage change would certainly change the current. They considered attaching the Variac to the output of a ferro-resonant constant 120vac supply or use a motorized Variac but they really wanted something much more compact. They were also concerned about distortion of the 60Hz waveform coming from wall outlet, especially at the waveform peaks. I suggested that perhaps a cleaner 60Hz signal could be synthetically generated and used as a reference. The signal would be fed to an audio amp, which would drive the winding of a low voltage transformer, perhaps 24vac. A custom wound secondary of the transformer could then generate the needed high current. By using another current transformer in line with the test loop, in conjunction with an op amp circuit, the AC current in the loop of wire could be kept constant. Can anyone think of another way this could be done?
David A. Johnson, P.E. --- Consulting Engineer