Multifunctional automatic charger production

Rechargeable batteries such as storage batteries and nickel-metal hydride batteries are widely used in life, but they often die prematurely due to improper charging. In order to save resources and protect the environment, the author designed and produced this multifunctional automatic charging circuit as a production training project for students majoring in electronics in our school. Because it uses discrete components, the circuit is simple and typical, which is conducive to improving students' skills and is a widely used product. Now I would like to use the "Electronic Vocational School Edition" as a communication platform to communicate and discuss with colleagues.

Performance Introduction:

1. The charger has multiple functions such as pulsating current limiting charging, trickle charging, automatic charging stop, etc., thus realizing intelligent charging without human supervision.

2. The charger is triggered by the residual power of the battery. There is basically no voltage output when the battery is not connected. Only when the battery is correctly connected, there will be charging current output. It has short circuit protection or reverse connection protection function.

3. The circuit has strong applicability, as shown in: ⑴ wide input voltage range; ⑵ it can be suitable for charging other types of rechargeable batteries by adjusting the potentiometer; ⑶ by adding a filter capacitor at the output end of the circuit, the circuit can become a PWM adjustable DC regulated power supply.

Circuit principle:

This circuit is designed for single-cell NiMH batteries. As shown in the figure: the mains electricity is transformed by a transformer, rectified by a full bridge, and filtered by capacitor C1 to become DC. LED1 is the power indicator, LED2 is the charging indicator, T1 is the charging control transistor, working in the switch state; T2, T3 and capacitor C2 form a monostable trigger. R6 and RP form a voltage-limited sampling circuit, and R7 is a current-limited sampling resistor.

Standby state : When the power is on, if the battery is not connected, the transistor T2 is cut off due to the lack of base voltage, and the transistor T1 is also cut off, with no voltage output. At this time, only the power indicator LED1 is on.

Charging process : When the rechargeable battery is correctly connected, the transistor T2 is slightly turned on due to the residual power of the battery, its collector potential drops, T1 is quickly turned on, and the output voltage increases; because C2 is a positive feedback, the circuit state quickly reaches a steady state. At this time, T1 T2 is turned on, T3 is turned off, the battery is charged, and the charging indicator LED2 lights up.

Current-limited charging: If the charging current is greater than the limit value, the voltage across the current sampling resistor R7 increases, the BE voltage of transistor T3 is higher than the dead zone voltage, and the monostable trigger state is triggered. T3 is turned on, T1 and T2 are turned off, and charging stops; then the monostable trigger automatically resets and enters the charging state again, and the pulsating charging is repeated in this way. The charging indicator LED2 flashes.

Automatic charging stop: As charging progresses, the voltage at both ends of the battery rises slowly, the pulse width becomes narrower, the charging current becomes smaller, and the flashing of the charging indicator LED2 gradually becomes faster and darker. When the battery is nearly full, the diode D1 is turned on, T3 is also turned on, T1 and T2 are turned off, the charging circuit is turned off, and the charging is ended. In the actual charging process, since the battery voltage drops slightly after being charged for a while, intermittent charging may occur, but the flashing of LED2 is not seen. This silk current charging method is beneficial to prolonging the battery life.

Installation and commissioning :

After the installation is correct, debug according to the following steps: disconnect capacitors C2 and C3, and connect an electrolytic capacitor of about 220uF at the output end. At this time, the circuit is equivalent to an adjustable voltage-stabilized power supply. Do not connect the battery first, turn on the power supply, LED1 lights up, short-circuit the a, b, and e poles of T3, and the charging indicator LED2 should light up. Use a multimeter to measure the output voltage, adjust the potentiometer RP until the output voltage is equal to the final voltage of the charging battery, and then reconnect the capacitors C2 and C3. (The final voltage of the battery charging can be checked from the data or measured; for example: the final voltage of a single nickel-hydrogen battery is about 1.4V, and the single-cell battery is about 2.45V.)

Attached pictures: