This provides bidirectional speed regulation for small motors and requires no tachometer. The voltage th
at summing amplifier IC1A applies to the motor"s windings equals: where Vc is the command voltage and IM is the motor current. If you set the motor"s winding resistance and brush resistance (RM) equal to: the command voltage will be proportional to the motor winding"s counter emf. CI provides compensation. Set Rl"s value so that it equals 5 to 10% of RM"s value. You can generally find RM"s value in a motor"s spec sheet.