Dang! You're right Franc, that doesn't sound right. What the heck was I
thinking? The FET has to be fed by a smaller amplifier driver, or
directly from the output of a PWM circuit. I remember building a
variable PWM circuit with a 555 timer (1st semester freshman year in
digital design 101).

I'll get the part number for the diode. D1 has only two leads.

What the heck is the relay for? Oh ... duhh. It's for the bridge
rectifier ... that's a DC motor! So the Motor Controller Board has it's
own power supply that is completely separate from the Power Supply
Board. The Power Supply board just powers the treadmill computer and
sensors. Yeah, that makes sense. They must have done that to keep the
noise out of the computer electronic circuits.

All right. I'll go stare at the board again.