Electronic switch. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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At present, electronic switches are often used in electronic equipment, in which one button can be used to turn it on and off. It is possible to make such a switch powerful, economical and small-sized if a field switching transistor and a digital CMOS microcircuit are used.

A diagram of a simple switch is shown in fig. 1. Transistor VT1 performs the functions of an electronic key, and trigger DD1 controls it. The device is constantly connected to the power source and consumes a small current - units or tens of microamperes.

If the direct output of the trigger has a high logic level, then the transistor is closed, the load is de-energized. When the contacts of the SB1 button are closed, the trigger will switch to the opposite state, a low logic level will appear at its output. Transistor VT1 will open, and the voltage will go to the load. The device will remain in this state until the button contacts are closed again. Then the transistor will close, the load will be de-energized.

The transistor indicated in the diagram has a channel resistance of 0,11 Ohm, and the maximum drain current can reach 18 A. It should be borne in mind that the gate-drain voltage at which the transistor opens is 4 ... 4,5 V. At a supply voltage of 5. ..7 V, the load current must not exceed 5 A, otherwise the voltage drop across the transistor may exceed 1 V. If the supply voltage is higher, the load current can reach 10 ... 12 A.

When the load current does not exceed 4A, the transistor can be used without a heat sink. If the current is greater, a heat sink is required, or a transistor with a lower channel resistance should be used. It is not difficult to select it according to the reference table given in the article "Powerful switching transistors from International Rektifier" in Radio, 2001, No. 5, p. 45.

Other functions can be assigned to such a switch, for example, automatic shutdown of the load when the supply voltage drops or exceeds a predetermined value. In the first case, this may be necessary when powering the equipment from a rechargeable battery in order to prevent its excessive discharge, in the second case, to protect the equipment from overvoltage.

The diagram of an electronic switch with a shutdown function when the voltage drops is shown in fig. 2. The transistor VT2, a zener diode, a capacitor and resistors are additionally introduced into it, one of which is rigged (R4).

When you press the SB 1 button, the field-effect transistor VT1 opens, the voltage is supplied to the load. Due to the charging of the capacitor C1, the voltage on the collector of the transistor at the initial moment will not exceed 0,7 V, i.e. will be logic low. If the voltage at the load becomes greater than the value set by the trimmer resistor, a voltage sufficient to open it will be supplied to the base of the transistor. In this case, the input "S" of the trigger will remain a low logic level, and the button can turn the load power on and off.

As soon as the voltage drops below the set value, the voltage on the trimming resistor engine will become insufficient to open the transistor VT2 - it will close. In this case, the voltage on the collector of the transistor will increase to a high logic level, which will go to the input "S" of the trigger. A high level will also appear at the trigger output, which will lead to the closing of the field effect transistor. The load will be de-energized. Pressing the button in this case will only lead to a short-term connection of the load and its subsequent disconnection.

To introduce protection against overvoltage, the machine should be supplemented with a transistor VT3, a zener diode VD2 and resistors R5, R6. In this case, the device works in the same way as described above, but when the voltage increases above a certain value, the transistor VT3 opens, which will lead to the closing of VT2, the appearance of a high level at the input "S" of the trigger and the closing of the field-effect transistor VT1.

In addition to those indicated in the diagram, the device can use the K561TM2 chip, bipolar transistors KT342A-KT342V, KT3102A-KT3102E, and a KS156G zener diode. Fixed resistors - MLT, S2-33, R1-4, tuned - SPZ-3, SPZ-19, capacitor - K10 17, button - any small-sized with self-return.

When using parts for surface mounting (CD4013 microcircuit, bipolar transistors KT3130A-9 - KT3130G-9, zener diode BZX84C4V7, fixed resistors P1-I2, capacitor K10-17v), they can be placed on a printed circuit board (Fig. 3) made of one-sided foil fiberglass with dimensions 20x20 mm. The appearance of the mounted board is shown in fig. 4.

The establishment of the machine is reduced to setting the voltages of the device operation with tuned resistors. The capacitance of C1 may need to be increased - it should be such that the transients in the powered device are completed before the capacitor is charged to a high logic level.

Author: I. Nechaev, Kursk

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