Sometimes it’s necessary to drive a resistive load such as an incandescent bulb or heating element from a DC voltage that’s higher than the nominal load voltage. An example would be driving a 6.3-V, Type 47 bulb1 from an 18-V DC power supply. A ballast resistor would waste power and cause heating on the printed circuit board (PCB), and a switching power supply would add cost and complexity.
A simple and effective solution is to use the pulse-width-modulation (PWM) output from a microprocessor to drive the load through a switched MOSFET. This article shows how to compute the PWM duty cycle so that the RMS, or effective voltage of the PWM signal, is correct when the load is connected directly to the PWM output.