Simple charger

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

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As before, radio amateurs pay great attention to the creation of automatic devices for charging nickel-cadmium batteries and batteries. This article describes a charger that monitors the voltage on the battery being charged and automatically turns it off when the nominal value is reached. The simplicity of the circuit solution, ease of setup and operation make this design available for repetition by a wide range of radio amateurs.

Chargers (chargers) according to their circuit design can be divided into two groups: with constant current charging and voltage control and with constant voltage charging and current control. It is preferable to use the first option, since in this case the recharge of the batteries and their failure are almost completely excluded.

In the described device, the voltage on the battery is measured continuously during charging (see diagram). On transistors VT1, VT2, a Schmitt trigger is assembled, which compares the voltage on the rechargeable battery GB1 with the exemplary one that comes from the divider R1-R3. If a discharged battery is connected to the charger, the transistor VT2 is closed, and the transistors VT1 and VT3 are open. The collector current of the transistor VT3, the value of which is determined by the resistance of the resistor R9, charges the battery. As soon as the voltage on it reaches the specified threshold value, the trigger is activated. Transistors VT1 and VT3 close, and the transistor VT2 opens and turns on the HL1 LED, indicating the end of charging. The SB1 button switch is designed to force the charger to start (for example, if the battery is not completely discharged). Pressing the switch SB1 in this case leads to setting the trigger to the state corresponding to the charging mode.

The charger is designed to charge a battery of two AA nickel-cadmium batteries (usually so much is used to power a player, radio or flash) with a nominal capacity of 750 mAh. Charging current is about 75 mA.

Some discrepancy between the charging current and the nominal value, caused by an inaccurate selection of the resistances of the resistors R4, R5, R6, R9, as well as the selected trigger voltage, will not greatly affect the quality of the charge. To set the current, variable resistors are included - in parallel with R4 and in series with R6 or R9, and an ammeter is inserted into the collector circuit of the transistor VT3.

According to the above diagram, you can assemble a memory for almost any type of battery. When calculating device parameters, the following points should be taken into account:

1. The current of the divider, which forms the exemplary voltage, must be 10 times higher than the base current of the open transistor VT1.

2. It is necessary that the collector current of the transistor, determined by the resistance of the resistor R6, ensure the normal brightness of the HL1 LED.

3. The resistance of resistors R4 and R5 must be sufficient to saturate the transistor VT1.

4. The total resistance of the feedback resistors R7 and R8 must be greater than the resistance of the resistors R4 and R5, so that the currents flowing through the resistors R4, R5, R7, the base-emitter junction of the transistor VT2, the resistor R6 and the resistors R4, R5, R7, R8, the battery GB1 were as small as possible (it is necessary to exclude the opening of the transistor VT3 by a voltage drop across the resistor R4 on the one hand, and partial recharging of the battery GB1 with current through the resistor R8 on the other).

The power supply of the device is assembled on a KR142EN5A chip. For this device, it is essential to use a stabilized power supply. Other options for its construction are possible: the main thing is that the stabilized supply voltage provides a stable charging current.

The device uses MLT resistors, a push-button switch - KM1-1. Instead of transistors KT315B and KT626V, you can use any with similar parameters. A heat sink for the KT626V transistor is not required.

Setting up the device (setting the reference voltage level at the current set as described above) must be carried out as follows. Bring the slider of the variable resistor R2 to the upper position according to the diagram, connect a freshly charged battery and apply the supply voltage. Move the potentiometer slider until the HL1 LED turns on. This method is good because you do not need to know the specific value of the set reference voltage, which depends on the type of battery, and in the described device, also on the charging current and feedback resistances.

The charger has also been successfully used for partial restoration of galvanic cells (in this case it is necessary to set a different trip level).

Author: V.Kosolapov, Orel

See other articles Section Chargers, batteries, galvanic cells.

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