Typical circuit of a microcontroller-based fan speed regulator

    　Microcontrollers require a DC operating power supply in the 2V to 5.5V range, which can be easily supplied by batteries or secondary power supplies. In certain cases, however, microcontroller-based products must operate directly from a 120V or 220V AC outlet without a step-down transformer or resistors that generate heat to reduce the voltage. As an alternative, polyester/polypropylene film capacitors specified for AC line service can act as non-dissipative reactance (Figure 1). Capacitor C1 is a 2mF AVX FFB16C0205K rated at 150V rms, providing a significant AC voltage drop that reduces the voltage applied to diode bridge rectifier D1. Flame-resistant metal film resistor R1 limits current spikes and transient voltages in the AC power line caused by lightning and sudden load changes. In this application, the AC current does not exceed 100 mA rms, and a 51Ω, 1W resistor provides sufficient current limiting capability. R2 is a 5W, 160Ω Yageo J-type resistor and D2 is a 1N4733A Zener diode, which provide a regulated 5V supply to the Freescale C68HC908QT2 microcontroller.

　　The schematic shows a representative circuit for a microcontroller-based fan speed regulator, where a thermistor senses air temperature and the microcontroller drives the fan motor. Figure 2 shows a light intensity regulator based on an inexpensive dual-diode rectifier and a triac lamp controller that share a common ground. IC2, a Fairchild MOC3021-M triac driver optoisolator, isolates the lamp return path from the microcontroller's ground return (Figure 3). In each of the three circuits, the Kingbright W934GD5V0 LED indicator includes a built-in current-limiting resistor (not shown).