Automatic dimmer. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Lighting

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Regulators (Fig. 1,2) allow you to perform two functions: automatically maintain a given level of illumination, regardless of changes in the level of external illumination, and smoothly adjust the desired level of illumination. The noted properties of the regulators make it possible to use them to maintain constant illumination of corridor areas, for photo printing, setting the thermal (light) mode in industrial and domestic installations (incubators, aquariums, greenhouses, thermo- and photostats, etc. devices).

A light-emitting element (incandescent lamp) with a power of up to 200 W can be included in the thyristor load circuit in direct current (Fig. 1, 2) or in alternating current - in a break in the network wire.

The operation of the thyristor is controlled by a relaxation RC generator made on the avalanche transistor VT2 (K101KT1). At the initial moment of time, the capacitor C1 is charged from a positive half-cycle of voltage taken from the anode of the thyristor VS1 through a resistor R2 and a transistor VT1 (Fig. 1) or resistors R2 and R4 and a diode VD1 (Fig. 2). In parallel with capacitor C1, a potassium sulfide photoresistor of the FSK-2 type is connected, the resistance of which in the dark exceeds 3 MΩ. Thus, if the photoresistor is located in a darkened zone (in the absence of an optical connection between the light emitter EL1 and the photoresistor R3), the latter almost does not bypass capacitor C1.

When the voltage on the capacitor plates exceeds 8 V, an avalanche breakdown of the transistor VT2 occurs and the capacitor is discharged to the control electrode of the thyristor VS1. The thyristor opens for the current half-cycle of the mains voltage and the mains voltage is applied to the incandescent lamp. For each subsequent half-cycle of the mains voltage, the process is repeated. Up to 95% of the input power is released on the lamp, which is typical for all types of thyristor and triac controllers. If the illumination of the photoresistor is increased, its resistance drops to 200 kOhm or less. Since the photoresistance is connected in parallel with the storage capacitor C1 of the generator, its shunting leads to a decrease in the charge rate of the capacitor and a delay in the moment of turning on the thyristor. As a result, the incandescent lamp in each half-cycle begins to turn on with a delay proportional to the level of illumination at the location of the photoresistor. Accordingly, the total illumination is stabilized at a certain (specified) level. The potentiometer R1, included in the emitter circuit of the transistor VT1 (Fig. 1) or R2, connected in parallel with the collector-emitter section of the transistor VT1 (Fig. 2), are designed to set the maximum illumination level and allow you to smoothly adjust the specified level.

If necessary, the device can be converted into a thermostat operating on a similar principle. When mounting the device, the photoresistor should be positioned in such a way that the light from the incandescent lamp does not directly fall on the working area of ​​the photoresistor, because otherwise, the generation of flashes of light is possible, the frequency of which is a phenomenon (optical feedback) that can be used to generate light pulses, determine the distance between the reflective coating and the light emitter / receiver, in various electronic devices.

Author: M. Shustov, Tomsk; Publication: N. Bolshakov, rf.atnn.ru

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