Electronic switch on field-effect transistors. Scheme, description

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Electronic switch on field-effect transistors. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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In the August 2002 issue of Radio magazine, p. 60 has been published article by V. Polyakov on the electronic switch, which is able to turn off the power to the load when the battery voltage drops below the permissible level.

I became interested in this idea and built my own version of such a switch. It differs from the prototype in that shortly before the automatic power off of the load, a flashing LED starts flashing, indicating that the load will soon be de-energized. The device is made on cheap field-effect transistors and microcircuits, which improved its load and ergonomic performance. A load with a direct current consumption of up to 0,4 A can be connected to an electronic switch, but using a more powerful n-channel MOS transistor instead of microcircuit current switches, the current of the connected load can be increased to several amperes, which will be discussed below.

The scheme of the device is shown in fig. one.

FET electronic switch

When the contacts of the SB2 button are closed, the full supply voltage is applied to the load. Through resistor R3, an opening voltage is supplied to the gate of the p-channel MOSFET VT1. The transistor opens, therefore, a high-level voltage is supplied to the gates of the transistors of the microcircuits (pins 1,8 DA1-DA3). The keys on the DA1 - DA3 microcircuits, each of which is a high-voltage n-channel MOSFET with a protective two-anode zener diode in the gate-source circuit, open. To increase the load capacity and reduce power and voltage losses, all three switches are connected in parallel.

The resistance of the resistor R1 is chosen so that when the battery voltage drops below 7 V (a battery of seven nickel-cadmium cells), the transistor VT1 starts to close. Since the voltage at the gates DA1 - DA3 is still quite high, these keys are still fully open. As soon as the source-drain voltage VT1 exceeds the threshold opening voltage VT2, this transistor will start to open, the bipolar transistor VT3 will also open, the blinking HL1 LED will flash brightly.

With an even greater decrease in the battery voltage, the transistor VT1 closes so much that the voltage across the resistor R4 becomes insufficient to keep the keys DA1-DA3 in the state of minimum resistance of the open channel, which leads to an avalanche closing of both VT1 and DA1-DA3. The load is de-energized, the LED stops flashing. You can forcefully turn off the load power by briefly pressing the SB1 button.

The resistance of the resistor R2 is chosen so that if the blinking LED flashes start at a battery voltage of 7 V, then the load is completely turned off when the battery voltage drops to 6,9 V. But the values \u2b\u47bof these voltages may be slightly different - it all depends on the parameters of field-effect transistors. If the resistor R7,5 is taken with a resistance of 7 kOhm, the LED will start flashing at 1 V, and the load power will turn off at 2 V. Ceramic capacitors CXNUMX and CXNUMX increase the noise immunity of the device.

Resistors can be taken with any small-sized power of 0,05-0,25 W, for example, C1 -4, MLT VS, C2-23. Non-polar capacitors are suitable for types K10-7, K10-17, KM-6; oxide - K50-35, K50-24. The blinking LED can be taken from any of the L36B, L56B, L796B, L816B series. In order not to increase the average discharge current of the battery too much when the LED is turned on, it is advisable to increase the resistance of the resistor R5 to 3 kOhm, and take the LED with increased brightness. Instead of a blinking LED, you can install, observing the polarity, a piezoceramic sound emitter with a built-in generator (HPA17AX, HPA24AX); such a replacement would be appropriate if the electronic switch is equipped with a "silent" device: multimeter, frequency meter, electronic thermometer, etc.

Field-effect transistors can be replaced by any of the KP301, KP304 series, preferably with the lowest possible threshold opening voltage. The bipolar transistor can be replaced by any of the KT3102, KT342, KT645 series. If you want to significantly increase the load capacity of the device, for example, for its use with an electrified self-propelled model, a radio station, a portable radio, then it is advisable to replace the DA1 - DA3 microcircuits with one powerful n-channel field-effect transistor, for example, types KP723G, KP727V, KP736G, IRLZ44. With one of these transistors, it is permissible to connect a device with a current consumption of 3 ... 5 A to an electronic switch. It is not necessary to install a field-effect transistor on a heat sink.

By selecting a resistor R1, the switch can be configured both for a nominal operating voltage of 9 V - a battery of seven nickel-cadmium batteries, and for 12 V - 10 batteries. When the device is powered by a battery of ten such batteries, the load power should be turned off when the battery voltage drops to 9,7 ... 10 V. If p-channel field-effect transistors with a relatively low gate-source threshold voltage, less than -3 V, are used, then by selecting the resistor R1, the device will be able to be configured to work with a lower nominal voltage, but not less than 4,5 V.

The electronic switch can be mounted on a printed circuit board made of one-sided foil-coated fiberglass with dimensions of 80x35 mm (Fig. 2).

FET electronic switch

The pinout of transistors and microcircuits is given in fig. 3.

FET electronic switch

Author: A.Butov, village of Kurba, Yaroslavl region

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