Making a simple charger circuit with TL431

The TL431 integrated circuit, known for its adjustable shunt regulator functionality, can be used to create a very simple and cost-effective battery charger.

Working Principle:

The circuit is illustrated in the diagram. It begins with a step-down transformer that reduces the AC voltage from the mains. This low-voltage AC is then rectified using a bridge rectifier and filtered by a capacitor to produce a DC output. This DC voltage is used to charge two nickel-tin batteries via diode D5. The charging current and the voltage level are controlled by the variable resistor W, which allows for fine-tuning of the charging process.

The TL431 is a highly stable and reliable component, offering excellent switching performance and the ability to handle relatively high currents. Its reference terminal (REF) and anode (A) have a fixed voltage of 2.5V. When the voltage across the battery reaches 2.5V—through a voltage divider network—the TL431 turns on, allowing the excess current to be shunted away. At this point, the voltage between K and A is kept around 2V. If the battery voltage drops below 2V, the TL431 turns off again, restarting the charging cycle. This smart use of the TL431’s switching characteristics ensures that the battery is never overcharged, helping to extend its lifespan and improve safety.

Component Selection:

In this circuit, the TL431 is a precision adjustable voltage regulator. Other compatible alternatives include the LM431 or Lin A431 from different manufacturers. Diode D5 is a fast-switching IN4148 type, ensuring efficient operation during the charging process. Resistors must be 1/2 watt carbon film resistors to handle the current safely. The potentiometer W should be a 10k ohm variable resistor, allowing for easy adjustment of the charging parameters. All other components are selected based on standard values and manufacturer recommendations. Once assembled, the circuit should be housed in a plastic enclosure to ensure electrical safety. Since the circuit is not isolated from the mains supply, it's crucial to take precautions against electric shock. Always follow proper safety procedures when working with AC circuits.

This simple charger design demonstrates how a single IC like the TL431 can be used effectively in practical electronics projects. Whether you're building a basic battery charger or exploring more advanced power management systems, understanding the role of components like the TL431 is essential for any electronics enthusiast or hobbyist.

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