1.2V NiMH battery

　　Five 1.2V NiMH battery charger circuits

　　A NiCd battery charger for NiMH batteries can be used electronically as shown below. This NiCad, NiMH battery charger uses traditional electronic components. Since the maximum charging voltage measures 1.45 Volts and the higher trigger threshold is around 1.7V, this voltage can be adjusted using potentiometer P1 to a value of 1.45 Volts. TTL control signal output voltage divider R4/R5, built-in constant current source transistor T1, which provides a current of approximately 48 mA. If D1 lights up, it means the battery voltage is charging.

　　If the maximum load reaches 1.15 volts, the Schmitt trigger fluctuates when D1 goes out (charging is complete). Due to the low triggering threshold of about 0.9 volts, each charging cycle must be started by pressing the S1 switch. For a 1.2v/1500mAh rechargeable battery with a current of 150mA, R1 must be 5.6 ohms, R2=12 ohms, and T1=2N2904.

　　This NiCd charger/weed eater has a built-in charger and weed eater circuit to clear short circuits. This circuit provides high current pulses to avoid internal short circuits.

　　The circuit is as shown in the figure below. The 220V mains power is stepped down by transformer B, and after rectification by D1~D4, the two batteries are charged through R1 and D6. D6 lights up to indicate that it is charging. At this time, TWH8778 (it is a high-speed integrated electronic switch, can be used in various automatic control circuits. It has a simple appearance, simple peripheral circuits, and contains overheating and overvoltage protection circuits internally. It has overvoltage, overcurrent, and overheating protection functions internally, and can Suitable for applications in the field of high-current drive switches. Widely used in some practical circuits such as timers and alarms). The opening voltage of control terminal 5 is too small, the circuit does not conduct, and D3 does not light up. As charging continues, the battery terminal voltage gradually increases, and the voltage of pin 5 also reaches the turn-on voltage, which connects its input and output terminals, that is, D2 lights up, indicating that the battery is fully charged.

　　This 1.2V NiMH battery automatic charger has status indication function. The LED emits green light when charging; when fully charged, the protection circuit operates and the LED emits red light, indicating that the battery is fully charged. When the battery is fully charged, the protection circuit automatically cuts off the charging current to prevent overcharging. Therefore, this charger can charge ordinary zinc-manganese batteries. ,

　　Working principle: The circuit principle is shown in Figure 1 (capacitor C1 and diodes VD1-VD4 form a step-down (current limiting) and rectifier circuit. Since the internal resistance of the capacitor is large, the output is approximately constant current, and the battery is charged through diodes VD5-VD7 , and generates a voltage drop of about 2.1V on VD5-VD7 to make the light-emitting diode light up (green) as a charging indicator. When the battery is full, VT is saturated and conducts, automatically cutting off charging. current. At the same time, the potential of point A drops to about 0.5V. At this time, VB>VA causes the red light-emitting diode to light up, indicating the end of charging.

　　Charges four size D cells in a constant current series with automatic voltage limiting. The BC301 transistor acts as a current source and carries a base voltage, which is stabilized at about 3V with 2 LEDs. LEDs are also used to indicate charging status.

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