1.5V rechargeable battery

　　Low-cost multi-cell series 1.5V rechargeable battery charging circuit

　　The integrated circuit KA7500B described in this article is a dedicated pulse-width modulation switching power supply integrated controller produced by Samsung. It is fully compatible and interchangeable with TL494. The circuit scheme is shown in the figure below.

　　The following is an explanation of the working principle of this circuit:

　　The circuit depicted in the figure is essentially a power supply circuit with a constant current/constant voltage output function. It has two feedback circuits, which are current feedback and voltage feedback. The positive and negative poles of the current feedback correspond to pins 1 and 2 of KA7500B. The output current produces a voltage drop on the resistors R12 and R20, which is passed through R9 and R10, R14, and R15 resistors feed back. When the voltage of pin 1 of KA7500B is greater than the voltage of pin 2, KA7500B will reduce the output pulse width (pins 8 and 11) to reduce the current. Otherwise, the pulse width will be increased to make the output The current is constant at the preset value, and its constant current value conforms to the following formula:

　　In the formula, R is the resistance value of R12 and R20 in parallel, so the theoretical calculated constant current value is 735mA. The positive and negative poles of the voltage feedback in the circuit correspond to pins 16 and 15 of KA7500B. After powering on, pin 14 of KA7500B outputs a stable 5V voltage. This voltage makes the LED light up as a power indicator, and the 5V voltage is used as a reference. Voltage, pin 15 is provided to KA7500B as the voltage reference. After the output voltage is divided by R19, R10, VR1 and R17, it is compared with the voltage reference. When the voltage is too large, the pulse width is reduced, and if it is too small, the pulse width is increased. To maintain a constant output voltage value, the output voltage value conforms to the following formula:

　　Since the two-channel feedback of KA7500B is controlled internally after being "ANDed", when the output voltage is lower than the constant voltage value, the current feedback plays a controlling role. When the output voltage reaches 8V4, the voltage feedback plays a controlling role. , in this way, the circuit completes the constant current/constant voltage control function. Its principle is exactly the same as the working principle of a regulated power supply, except that this circuit is a switching power supply control method, so it has high efficiency and low temperature rise.

　　D1 in the figure is the input diode to prevent polarity reversal, D2 is the freewheeling diode for switching power supply operation, and T1 works in the switching state.

　　After understanding its working principle, it is much simpler to extend its application, which is described below:

　　1. During normal use, adjust the VR1 potentiometer to obtain the corresponding output voltage.

　　2. Charge lithium-ion batteries of different capacities: the resistance values of R12 and R20 can be changed, and the required current value can be calculated and set according to the formula mentioned above. If the current is too large, a heat sink needs to be installed on T1, and the freewheeling diode should be changed to 1N5822 to withstand greater current.

　　3. Charging high-voltage lithium batteries: Just change R10 or R17. The specific required output voltage value can be calculated and set according to the formula.

　　4. Make a high-precision regulated power supply: Its output voltage principle is consistent with the above, but you need to pay attention to the ripple. If possible, you should increase the inductance and filter capacitance of the inductor in the circuit to reduce the ripple.

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