This DC power supply controller is controlled by pulse width modulation PWM, generated by the circuit around timer IC2 7555 according to the formula:
Duty cycle = ((R1 + (VR1/2)/(R1+R2+VR1)) x 100%
Diode D1 is used to bypass resistor R2 and half the effective resistance of the “lower” section of VR1, allowing the duty cycle to be varied from 0.01% to 99.9%.
The output at IC2 pin 3 is fed directly to power MOSFET TR1 to control the load. For loads greater than 3A, a heatsink must be used with TR1.
The DC power controller circuit is powered at 12V, regulated down to 8V by IC1 78L08. Capacitors C1, C2 and inductor L1 prevent noise being fed back into the power line when the load is being switched. Diode D2 prevents the generation of back-e.m.f. when switching inductive loads. It should be changed to a high power version if the load is greater than 3A.