Automatic load switching. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Automatic load switching. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Clocks, timers, relays, load switches

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In household use, one often has to deal with a situation where household appliances must operate in a periodic mode. Without this, for example, an electric heater can overheat the serviced object, and a fan can create an unpleasant feeling of draft. Modern elements of radio electronics make it easy to solve the above problem.

A diagram of an automaton for this purpose is shown in the figure. It includes the timer KR1006VI1 - DD1 [1] operating in the multivibrator mode, the triac optocoupler AOU160A - U1 [2] and the power switch on the triac - VS1. The functions of the controlled load are performed by the motor M1 of the electric fan. Capacitor C1 with resistors connected to it forms a timing circuit that determines the duration of the on and off state of the load.

Automatic load switching

This device works as follows. When power is applied to the DD1 chip, the capacitor C1 begins to charge and, as a result, a voltage close to the supply voltage appears at pin 3 of DD1. At the end of charging C1, a transistor opens inside the DD1 microcircuit, connecting its seventh and first conclusions, as a result of which the capacitor C1 is discharged through the resistor R2. After that, the cycle of operation of the device is repeated. A voltage close to the supply voltage, which periodically occurs at the output of the DD1 microcircuit, is fed through the current-limiting resistor R3 to the LED located in the control circuit of the optocoupler U1. Under the influence of the light energy emitted by it, the triac, which is part of the optocoupler, passes into a conducting state and the power triac VS1, which opens as a result, turns on the M1 motor. The most important function of the optocoupler, designed for a voltage between the input and output circuits of about 1500 V, is reliable electrical isolation of the input and output circuits.

Before the advent of such electronic components, the problem of separating circuits was solved using bulky electromagnetic relays. Trinistor VS1 with bilateral conductivity opens with the beginning of each half-cycle of the mains voltage, while there is a signal at the output of the DD1 microcircuit and the optocoupler LED is on. The power of the controlled load is determined by the allowable current of the triac VS1. The DD1 chip itself and the optocoupler LED at a supply voltage of 6 V consume a current of the order of 8 ... 12 mA, so even LR6 galvanic cells (foreign AA standard) can be used to power them.

The machine uses resistors MLT-0,125 (R1 - R3) and MLT-0,5 (R4), a capacitor - K52-1 B. The MT1 microtumbler is used as the SA1 switch. With the ratings of the elements of the time-setting circuit indicated in the diagram, the period of switching on and off the load is 0,3 and 0,2 minutes, respectively. By choosing other ratios of ratings, you can change the cycle of operation of the device. The resistance of the resistor R3 should be chosen so that with a fresh battery, the current through the optocoupler LED is 10 ... 12 mA (recall that the maximum allowable current is 40 mA). When mounting the device, it is important to ensure that the output circuit of the optocoupler and the power triac are reliably isolated from the circuits associated with the DD1 chip and from the walls of the case (if it is made of metal).

Depending on the power connected to the load device, the power triac may require a heat sink. In this case, the case should be provided with ventilation holes. To connect an electrical appliance-load (in our case, an engine) to the machine, a standard socket X2 is attached to its case, which is connected to the network with a flexible cord with an X1 plug.

See other articles Section Clocks, timers, relays, load switches.

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