Power SuppliesVoltage Regulators Circuit Diagrams

Low-drop Series Regulator using a TL431 Schematic Circuit Diagram

Like the author you may keep some 12 V leadacid batteries (such as the sealed gel cell type) in stock until you come to need them. A simple way of charging them is to hook up a small unregulated 15 V ‘wall wart’ power supply. This can easily lead to overcharging, though, because the off-load voltage is really too high. The remedy is a small but precise series regulator using just six components, which is connected directly between the power pack and the battery (see schematic) and doesn’t need any heatsink.

Low-drop Series Regulator using a TL431 Schematic Circuit Diagram

The circuit is adequatele proof against shortcircuits (min. 10 seconds), with a voltage drop of typically no more than 1 V across the collector- emitter path of the transistor. For the voltage source you can use any transformer power supply from around 12 V to 15 V delivering a maximum of 0.5 A. By providing a heatsink for T1 and reducing the value of R1 you can also redesign the circuit for higher currents.

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