Low-Drop Series Regulator Using a TL431 (EE Tip #134)

You likely have a stash of 12-V lead acid batteries (such as the sealed gel cell type) in your lab or circuit cellar. Below is a handy tip from Germany-based Lars Krüger for a simple way to charge 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.EL2009-Kruger-SeriesReg

The circuit is adequatele proof against short circuits (minimum 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.

Resource: http://focus.ti.com/lit/ds/symlink/tl431.pdf. This tip first appeared in Elektor July/August 2009.

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