The microcontroller requires 2 to 5.5V DC power supply, which can be easily provided by batteries or auxiliary power sources. However, in some cases, microcontroller-based products must operate directly from 120 or 220V AC power outlets without using a step-down transformer or resistive voltage drop. Additionally, polyester/ polypropylene film capacitors rated for AC power can be used as non-dissipative reactance (see Figure 1). The capacitor C1 of 2 µF AVX FFB16C0205K rated at 150V rms can provide significant AC voltage drop, thereby reducing the voltage applied to the diode bridge rectifier D1. The fireproof metal film resistor R1 can limit the current spikes and transient voltages induced by lightning and sudden load changes in the AC power line. In this application, the AC current does not exceed 100 mA rms and a 51 Ω, 1W resistor provides sufficient current limiting. R2 is a 5W, 160 Ω Yageo J type resistor, D2 is a 1N4733A zener diode that provides a 5V regulated power supply for the microcontroller Freescale C68HC908QT2.
Figure 1 C1 provides capacitive reactance, which limits the AC input current without dissipating excessive heat in this DC fan speed controller.
This schematic shows a representative circuit of a microcontroller-based fan speed controller, where a thermistor senses the air temperature and the microcontroller drives the fan motor. Figure 2 shows a light dimmer based on inexpensive two-pole rectifiers and a bidirectional thyristor–light controller that share a common ground. IC2 is a Fairchild MOC3021-M bidirectional thyristor driver opto-isolator, which separates the return path of the light from the ground loop of the microcontroller (see Figure 3). In each of these three circuits, Kingbright W934GD5V0 LED indicators contain a built-in current limiting resistor (not shown).
Figure 2 shows a two-diode rectifier and light control bidirectional thyristor sharing a common return path to the AC line.
Figure 3 shows the opto-isolator separating the high current AC return path of the thyristor from the power supply of the microcontroller.
Hubei Zhu Shaohua compiled