Back emf 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 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.
Back emf motor speed control  - schematic

Motor speed is established by potentiometer R2, which applies a corresponding dc voltage to the + input of comparator Ul, whose output is then applied to TMOS device MTP8P08 (Ql). Zener diode Dl limits the drive to Ql. The output of Ql drives the permanent magnet motor.Back emf is obtained from the motor via the network consisting of R8, R9, RIO, C2, C3 and D3; it is applied to—input of comparator Ul.




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