This circuit is a very compact switching regulator for small DC motors. I use it for my small printed circuit board drill
(18 Volt, 1.5 Amp), but it is suitable for many other applications (e.g. 12V DC halogen dimmer).
R1, R2, C1, D1, D2 and schmitt trigger IC1B build a 1kHz oscillator with adjustable duty cycle.
The output of the oscillator passes through buffer IC1B/C/D which drives power mosfet T1.
When T1 is switched on, a current will flow from the +Vmotor supply via the motor and T1.
When T1 is switched off, the motor current is interrupted; because the motor acts as an inductor,
it will try to maintain its current, which can flow through freewheel diode D3.
The avarage current through the motor depends on the duty cycle of the oscillator.
The circuit losses are very low, for my 1.5 Amp drill, no heatsink is required for T1.
Source : unknow …….