Transistor power switch. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Transistor power switch. 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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Most small, low-power devices powered by 220 V AC (so-called power adapters, DVD players, chargers, etc.) do not have a power switch that completely disconnects them from the mains. This not only leads to useless, albeit small, consumption of electricity, but also increases the likelihood of device failure.

If it is not possible to install a mechanical power switch in such a device (for example, due to lack of free space or unwillingness to significantly change the design of the device being finalized), then it can be equipped with a simple electronic switch-attachment controlled by two buttons.

Transistor power switch
Fig. 1

A schematic diagram of a possible variant of such a switch is shown in fig. 1. It is made on three high-voltage transistors, two of which (VT2, VT3) form a composite transistor, and all three are an analogue of a lockable trinistor with low control and hold currents. After applying 220 V to the device, the load connected to the XS1 socket remains de-energized, as the capacitor C1 is discharged and all transistors are closed. When the contacts of the SB1 button are closed, this capacitor is charged to a voltage of about 2,5 V and transistors VT2, VT3, and after them VT1 open. As a result, the diagonal of the rectifier bridge on diodes VD1-VD4, where the composite transistor and diodes VD5-VD7 are connected, closes and the supply voltage is supplied to the load. Due to the voltage drop across the diodes and the open transistor VT3, it is several volts less than the mains, but this does not affect the performance of the load. Diodes VD5-VD7 limit the voltage across the resistor R6 and thereby protect the emitter junction of the transistor VT1 from overload.

To turn off the power to the load, it is enough to briefly press the button SB2. In this case, the capacitor C1 is instantly discharged, the transistors are closed and the load is disconnected from the network.

The device can work with any load up to 40 watts. The effective voltage value at a load of 16 W (incandescent lamp) is approximately 4 V less than the mains voltage, at a load of 40 W - by 8 V. In the first case, the heating of the VT3 transistor case is practically absent, and in the second its temperature rises to 50 ... 60 °C (at an ambient temperature of 22 °C).

In the event of a short power failure, the load is disconnected and remains de-energized when it is restored. To reconnect it to the network, you must press the SB1 button.

Transistor power switch
Fig. 2

The device is assembled on a printed circuit board, the drawing of which is shown in Fig. 2. It contains all the details, except for the buttons. Resistors and diodes are mounted perpendicular to the board. Resistors - MLT, C1-4, C1-14, C2-23, capacitor - oxide of any type, domestic or imported. Buttons SB1, SB2 are small-sized membrane buttons with a plastic pusher at least 10 mm long, for example, SDTG-644/648, SDTX644/648, SDTA644. (When installing the switch in a device with a metal case, their metal mounting parts must be electrically isolated from it). Fusible insert FU1 - any small-sized. If the device being modified has its own fusible link in the 220 V circuit, the one shown in the diagram can be omitted.

Diodes 1N4007 are replaceable by any others with a forward current of at least 1 A and a permissible reverse voltage of at least 400 V (1 N4005,1 N4006, UF4005-UF4007, 1 N4936, 1N4937, KD243G, KD243D, KD247G).

Possible replacement of the transistor 2SB1011 - 2SB1074, 2N6520,

2SA1625K, and transistor MJE13003 (VT2) - MJE13001, 2N6517. As VT3, instead of MJE13003 (the maximum collector-emitter voltage is 400 V, the maximum collector current is 1,5 A, the maximum power dissipation on the collector is 40 W), you can use more powerful ones, for example, MJE13005 (respectively 400 V, 4 A, 75 W), MJE13007 (400V, 8A, 80W), MJF13007 (400V, 8A, 40W). It is advisable to use these transistors when operating a device with a load equipped with a switching power supply. When the transistor case is heated above 50 ° C, it must be provided with a small heat sink. It should also be done if the structure where the described switch is built in, itself noticeably heats up during operation. When replacing transistors, it should be noted that their pinout may differ from the pinout of the transistors used by the author.

Instead of two transistors (VT2, VT3), you can use one composite, for example, 2SD1141 (400 V, 6 A, 40 W). Resistor R7 is excluded.

If the switch is made as a prefix, the mounted board is placed in a plastic case of suitable dimensions. The buttons SB1, SB2 are installed on its upper wall, and the XS1 socket is installed on one of the side ones. Assembled from serviceable parts and without installation errors, the device starts working immediately after being connected to the network and does not require adjustment. Since all parts of the switch are under mains voltage, when checking its performance and in all cases when access to the installation is open, it is necessary to observe electrical safety precautions - avoid touching uninsulated metal structural elements with bare hands.

If the electronic switch is to be operated in conjunction with a device that uses a network switching power supply, then it is necessary to connect a constant resistor (preferably wire) with a resistance of 10 ... 1000 Ohm with a power dissipation of 1 ... 3 W in series with it. Its resistance is chosen in such a way that when the device is running, a voltage of 1 V drops across the resistor. It is not recommended to use non-wire resistors, as they can quickly fail.

Author: A. Butov

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

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