AC line powers microcontroller-based fan-speed regulator
Capacitor drops voltage, decreases waste heat in line-powered controllers.
Abel Raynus, Armatron International Inc, Malden, MA; Edited by Brad Thompson and Fran Granville -- EDN, November 9, 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 51Ω, 1W resistor provides adequate current limiting. R2, a 5W, 160Ω Yageo type-J resistor, and D2, a 1N4733A zener diode, provide 5V regulated power for the microcontroller, a Freescale C68HC908QT2.
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).
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This is responce to Mr Garcia comments:
1.The main objective to this IFD is to show the ways of transformerless feeding the 5VDC microcontrollers from 120VAC line. This is the reason why no software
is included.
2.The Logic Triac L2004F31 (from Teccor)needs 3 mA DC Gate Trigger Current. There is no problem of controlling it directly from any microcontroller as shown in the IFD. We are doing it for years.
3.The Fan and Lamp are used as examples of DC and AC Loads to illustrate the main Idea. If you experience problem with Fan current, there are some ways to overcome it, but they are beyond this Idea and do not contradict it.
Abel Raynus - 2007-5-2 05:00:00 PST -
This design is completely faulty, it is a paper design lacking basic usage of TRIACs.
The Microprocessor in this case needs zero crossing information for proper operation of the TRIAC.
Also, no information on the software.
Figure 1 cooling Fan connected wrong, unless the current is less than 20 to 30 milliamps, C1 limits the supply current.
Hernando Garcia - 2007-3-2 09:29:00 PST
