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AC line powers microcontroller-based fan-speed regulator

Abel Raynus, Armatron International Inc, Malden, MA; Edited by Brad Thompson and Fran Granville -November 09, 2006

A microcontroller requires dc operating power in the 2 to 5.5V range, an amount that a battery or a secondary power source can easily supply. However, in certain situations, a microcontroller-based product must operate directly from a 120 or 220V-ac power outlet without a step-down transformer or a heat-producing, voltage-decreasing resistor. As an alternative, a polyester/polypropylene film capacitor rated for ac-line service can serve as a nondissipative reactance (Figure 1). Capacitor C1, a 2-µF AVX FFB16C0205K rated for 150V rms, provides a significant ac-voltage drop that reduces the voltage you apply to a diode-bridge rectifier, D1. A flameproof metal-film resistor, R1, limits current spikes and transient voltages induced in the ac-power line by lightning strikes and abrupt load changes. In this application, the ac current does not exceed 100 mA rms, and a 51O, 1W resistor provides adequate current limiting. R2, a 5W, 160O Yageo type-J resistor, and D2, a 1N4733A zener diode, provide 5V regulated power for the microcontroller, a Freescale C68HC908QT2.

Figure 1 C1 provides capacitive reactance, which limits ac-input current without dissipating excessive heat in this dc fan-speed controller.

The schematic shows a representative circuit for a microcontroller-based fan-speed regulator in which a thermistor senses air temperature and the microcontroller drives a fan's motor. Figure 2 illustrates a light-intensity regulator based on an inexpensive two-diode rectifier and a bidirectional-thyristor-lamp controller that share a common ground. IC2, a Fairchild MOC3021-M bidirectional-thyristor-driver optoisolator, separates 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).

Figure 2 A two-diode rectifier and lamp-control bidirectional thyristor share a common return path to the ac line.

Figure 3 An optoisolator separates the bidirectional thyristor’s high-current ac-line return path from the microcontroller’s power supply.

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