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Back EMF PM Motor Speed Control


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.

Parts List:
Component: Value: Datasheet: Qty:


Not Available 01

1K Pot

Not Available 01


Not Available 01
Resistor 46.4K Not Available 01
Resistor 2M Not Available 01
Resistor 15K Not Available 01
Resistor 10K Not Available 01
Resistor 100K Not Available 01
Resistor 6.6K Not Available 01
Resistor 499K Not Available 01
Capacitor 0.01uF Not Available 02
Capacitor 0.1uF Not Available 01
Diode 1N4004 Available 02
Zener 1N4740 Available 01
Darlington MTP8P08 Not Available 01
IC LM311 Available 01