Battery discharge limiter. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Battery discharge limiter. Encyclopedia of radio electronics and electrical engineering

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

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During the operation of batteries, it is very important to prevent their deep discharge. For this, special discharge limiting devices are usually used, which disconnect the battery from the load when the voltage on it falls below a certain level. The main requirements for such devices are simplicity and efficiency, and in the case of use in wearable equipment, also small dimensions. All these conditions can be met if powerful field switching transistors are used [1].

The scheme of the device is shown in fig. 1. The limiter is included in the open circuit between the power switch and the consumer (load). The field effect transistor VT1 performs the function of an electronic key, and the DA1. (parallel voltage stabilizer [2]) - voltage comparator. Capacitor C1 is used to start. When the contacts of the SA1 switch are closed, the voltage from the battery is supplied to the device. The field-effect transistor opens, and if the voltage on the battery is greater than a certain set value (the battery is charged), it remains open.

(click to enlarge)

The voltage drop across an open transistor depends on its parameters, the values of the supply voltage and the current consumed by the load. The main parameters of the IRFR5505 transistor indicated in the diagram: open channel resistance - 0,11 Ohm, maximum drain current - 18 A, drain-source voltage limit - 55 V. At a battery voltage of 12 V and a load current of 2 A, the measured voltage drop across the transistor was 0,17 V, and at 7 V and 1 A - 0,7 V.

As the battery discharges, the voltage of the battery decreases, and when it becomes less than the set value (the battery is discharged), the voltage on the microcircuit will begin to increase, and in the gate-source section of the field-effect transistor VT1, it will decrease. Therefore, the transistor closes, the voltage at the load decreases, which, in turn, will lead to a further increase in the voltage on the microcircuit and, accordingly, the closing of the transistor. Thus, the limiter abruptly disconnects the load from the battery. The current consumed by the device in this mode is fractions of a milliamp. The cut-off voltage is set by the tuning resistor R3.

After opening the switch SA1, the capacitor C1 is discharged, and after a while the device is ready to start. To start, you can use a button that is connected in parallel to the microcircuit (between pins 1 and 2, 3, 6, 7) or to a field effect transistor (between drain and source). In this case, the capacitor C1 and the resistor R2 can be eliminated by installing a jumper wire instead of the resistor. If the connecting wires are of considerable length, it is advisable to install 0,1 microfarad capacitors at the input and output of the device.

The gate-source voltage at which the field-effect transistor opens exceeds 4 ... 4,5 V, the minimum voltage on the microcircuit is about 2 5 V. Therefore, the input voltage of the device must be at least 7 V and not more than 20 V field effect transistor). Capacitor C1 must be rated for the appropriate voltage.

Any microcircuit of the TL431 series or the domestic analogue KR142EN19 is applicable in the device, the field effect transistor is any with a p-channel from the list specified in the article [1], while you can increase the output current of the device and reduce the voltage drop (for example, IRF4905 with an open channel resistance of 0,02 .74 Ohm and allowable drain current up to 19 A). Trimmer resistor - SPZ-28, SPZ-2 or similar imported, constant - MLT, S33-1, R4-1, R12-50, capacitor - series K53, KXNUMX.

Since the number of parts is small, the device can be assembled by surface mounting without using a printed circuit board. If parts for surface mounting are used, they are placed on a printed circuit board made of one-sided foil fiberglass with dimensions of 15x15 mm, a sketch of which is shown in fig. 2.

Literature

Powerful field-effect switching transistors from International Rectifier. - Radio, 2001, No. 5, p. 45.
Nechaev I. Unusual application of the KR142EN19A microcircuit. - Radio, 2003, No. 5, p. 53, 54.

Author: I. Nechaev, Kursk

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