Do-it-yourself time relay on two transistors. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Do-it-yourself time relay on two transistors. 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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The principle of operation of this time relay is based on the fact that the charge time of a fully discharged capacitor is determined by the product of the capacitance of this capacitor and the resistance of the charge circuit. By setting the value of this product by choosing the capacitance and resistance, you can get the required charge time.

Schematic diagram of the time relay is shown in the figure.

Do-it-yourself time relay on two transistors

This time relay can be assembled with your own hands, because there are no scarce parts in the circuit. When connected to the power supply circuit, the charge of the capacitor C1 begins through the resistors R2 and R3 and the emitter junction of the transistor VT1. it opens and a voltage drop forms on the resistor R3 from the flow of emitter current through it. This voltage drop unlocks the transistor VT2, and the electromagnetic relay K1 is activated, which, with its contacts K1.1, connects the HL1 LED to the power bus. Resistor R4 limits the LED current.

As the charge increases, the voltage across the capacitor increases and the charge current decreases. Accordingly, the emitter current and the voltage drop across the resistor R3 decrease. Finally, at a certain voltage across the capacitor, the charge current becomes so small. that the transistor VT1 is locked, the transistor VT2 is locked behind it. As a result, the relay releases and the LED goes out. For the next start of the time relay, it is necessary to press the button SB1 for a short time in order to completely discharge the capacitor C1.

The required period of time during which relay K1 is in the triggered state is set by selecting the capacitance of the capacitor and the resistances of resistors R2 and R3. If the relay has another pair of contacts, they can be used to turn other consumers on or off. But then the second pair of contacts should be normally closed.

The choice of the type of relay is made according to the value of its operating voltage, which must be equal to the supply voltage of the device. In the circuit, the relay is designed for a supply voltage of 12 V.

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

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