The TL431 integrated circuit with adjustable shunt regulator can form a very simple charger.
working principle:
The circuit is shown in the drawing. The mains capacitors are stepped down, bridge rectified, and capacitively filtered. The DC voltage is output and the two nickel-tin batteries are charged through D5. The magnitude of the charging current and the level of the voltage are determined by the adjustment potentiometer W.
TL431 has high stability, good switching characteristics, and can output large current. The fixed voltage of the reference terminal REF and the anode terminal A is 2.5V. When the voltage at both ends reaches 2.5V (the Battery voltage reaches 2.5V through the voltage dividing circuit), the TM31 is turned on, and the charging current is shunted. At this time, K, A The voltage between them is maintained at about 2V. If the battery voltage is lower than 2V, the TL431 is turned off again, and the circuit begins to enter the charging state again. The device skillfully utilizes this integrated circuit with switching characteristics, and the charger can ensure that the battery is not charged, thereby prolonging the service life of the battery.
Component selection:
The TL431 in the circuit is a precision adjustable integrated voltage regulator circuit, and other manufacturers can also choose LM431, Lin A431 and so on. D5 uses IN4148 switching diode, the resistor must use 1/2W carbon film resistor, W selects IOk potentiometer. The other components are selected by the icon. After the circuit is installed, it must be fixed in a plastic box. Because the circuit is not isolated from the mains, it must be safe to prevent electric shock.
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