Stepwise switching on of a powerful load. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Clocks, timers, relays, load switches
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The device is designed to gradually supply mains voltage to an active load.
From experience it is known that most often powerful lamps and heaters fail at the moment of switching on. This is due to the fact that the heating filament of the lamp in a cold state has a resistance of more than 10 times less than when warmed up. Because of what, a surge of current is possible when voltage is applied. If the inclusion accidentally hit at the time of the maximum voltage amplitude in the network, an impulse overload occurs.
Shown in fig. 1 scheme facilitates the load operation mode, reducing current surges due to a gradual (within 4 s) increase in the amplitude of the applied voltage. This allows you to significantly extend the life of the lamps, in addition, the level of mains interference at the time of switching on is reduced. The electrical circuit works as follows. The electronic triac switch VS1 is controlled by a unijunction transistor generator VT1.
Rice. 1 (click to enlarge)
To open the triac at any ambient temperature, a current of at least 80 ... 100 mA must pass through the LED. The use of a unijunction transistor makes it possible to have a power supply for the control circuit of low power, since the energy necessary to open the triac is accumulated on the capacitor C1 and is given off during a short pulse.
The voltage diagram is shown in Fig. 2
Fig. 2
When turned on, at the initial moment, the transistor VT2 is locked (for about 4 s), just like VT3. The value of the resistor R3 determines what minimum initial voltage will be applied to the load A1. As soon as C2 is charged, a current will appear through VT3, which will lead to the opening of VT2, - the resistor R3 will be shorted by the emitter-collector junction of the transistor. This will reduce the charge time for C1, i.e., transistor VT1 will generate an impulse to open VS1 earlier. We select the value of the resistor R4 so that at the same time there is a maximum voltage in the load.
Since the circuit facilitates the operation of the triac VS1, the device allows you to switch the total load power up to 10000 watts.
The circuit uses MLT resistors, and capacitors C1 - K73-9, C2, C- - K52-1B for 63 V. The optocoupler is installed on the radiator (when using a circuit with a load of up to 500 ... 1000 W, it is not necessary).
The topology of the printed circuit board is shown in fig. 3.
Ris.3
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