How do you control an Silicon controlled rectifier with a potentiometer in a variable high amperage circuit?

Hi! I'm trying to build a constant voltage variable high amperage circuit. I know that you can use a silicon controlled rectifier in inverse parallel to conduct the full waveform in AC. How can you use a potentiometer send the signal to the SCR without blowing up the small potentiometers available (circuit will be up to 200 amps.) Thanks to anybody who can shed some light on this!