The use of power MOSFETs allows a
direct interface between logic and motor power, which permits circuit simplicity as well
as high efficiency. This speed control circuit can be packaged on a 22-pin, double-sided,
3.5 x 4-in. pc board.
A 12 V control supply and a TRW BL11, 30 V motor are used; with minor changes other motor
and control voltages can be accommodated. For example, a single 24 V rail could supply
both control and motor voltages. Motor and control voltages are kept separate here because
CMOS logic is used to start, stop, reverse and oscillate the motor with a variable delay
between motor reversals.
Motor speed is established by potentiometer R2, which applies a corresponding dc voltage
to the + input of comparator U1, whose output is then applied to TMOS device MTP8P08 (Q1).
Zener diode D1 limits the drive to Q1. The output of Q1 drives the permanent magnet motor.
Back emf is obtained from the motor via the network consisting of R8, R9, R10, C2, C3 and
D3; it is applied to-input of comparator U1. |