An electronic switch protects the battery. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Electronic switch protects the battery

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Encyclopedia of radio electronics and electrical engineering / Chargers, batteries, galvanic cells

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In order for the rechargeable battery that powers the radio, player or flashlight to serve for a long time, it must not be subjected to deep discharge, in which irreversible chemical changes occur in the elements. You can, of course, carefully monitor the discharge of the battery during operation, avoiding excessive, but it is better to entrust this important task to an electronic machine.

The described electronic switch automatically disconnects the load when the battery voltage drops below the specified value. For a nine-volt battery (7D-0.115, Nika, etc.), the lower limit of the allowable voltage is about 7 V. In fig. 1 shows a diagram of an automaton that turns off, for example, a radio receiver at precisely this voltage.

Electronic switch protects the battery

Turn on the device with a button (without fixation) SB1. In this case, the supply voltage is applied to the radio receiver and at the same time there is a reversible avalanche breakdown of the emitter junction of the transistor VT3, which is connected as a zener diode (the collector output is not used).

The zener diode current passes through the emitter junction of the transistor VT2 and opens it. The collector current VT2, in turn, opens the transistor VT1, which blocks the contacts of the button SB1. All the described processes occur almost instantly, and after a short press, the SB1 button can be released - the receiver will remain on.

To turn off the receiver, briefly press the SB2 button. Its contacts are closed by the emitter junction of the transistor VT2. It will close, and after it the transistor VT1 will close, turning off the power to the receiver. In the off state, a negligible current is consumed from the battery, not exceeding a fraction of a microampere, much less than the battery self-discharge current. Resistors R1 and R4 contribute to its reduction. Resistors R2 and R3 are needed to limit the current of the zener diode and the base currents of transistors VT1 and VT2.

When the battery voltage during receiver operation drops below the permissible value, the VT3 zener diode and both transistors close and the battery is disconnected from the receiver.

After manufacturing, additional advantages of the proposed device were revealed. If the receiver has turned off due to low battery, but you need to listen to some important news (for example, a weather report), this can be done by holding down the SB1 button. Moreover, an accidental short circuit of the wires leading to the receiver causes the device to instantly turn off, saving the battery.

Inspired by the result obtained, the author designed a similar switch for a rechargeable flashlight with a battery of three D-0,25 disk batteries (Fig. 2). It fires when the battery voltage drops below 2,5 V.

Electronic switch protects the battery

As a zener diode, series-connected low-power silicon diodes VD1-VD3 are used here, which open at a forward voltage above about 0,6 V on each. Pay attention to another option for turning on the SB2 button. Any of the options is applicable in both devices. Due to the large current consumption and lower voltage, the resistances of the resistors have been changed and a more powerful transistor VT1 has been installed.

Establishing the switch comes down to setting the required shutdown voltage by selecting an instance of the transistor VT1 (Fig. 1) or the number and type of diodes connected in series (Fig. 2). It should be noted that KT315 transistors with any letter index have a rather large variation in opening voltages when used in zener diode mode.

Author: V.Polyakov, Moscow

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