Approx circuit of DC-51 motor speed controller (D6 and D8 are the wrong way around, and there may be more errors). And the more I looked at the circuit, the more I liked the speed controller. Update ...

Maxon Precision Motor's ESCON 50/5 OEM module is a 4 quadrant Pulse Width Modulated (PWM) servo motor controller that can efficiently control permanent magnet brushed DC ...

Update: see the circuit and a stab at explaning it here. Safety note for folks not used to working with the mains: All electronic parts of this DC-51 motor controller are connected to live mains and ...