The useful TL431 chip

I haven’t made any electronics projects for a while now but I was working on some circuits at work that used a TL431 shunt regulator. This chip can be used to provide a 2.5V voltage reference and also 5V with the addition of two 10K resistors between the Ref pin and supply voltage and another to ground. A 470 ohm resistor on the input to the circuit limits current so not to damage anything. I looked at using one to improve the accuracy of one of my previous projects, an AA rechargeable battery tester.

I had made two testers based on completely different designs, the second uses the 1.1V internal reference to calculate the supply voltage which is used as a reference. The former simply uses a 10K resistor attached to the 3.3V out to the vref pin so it is nowhere near as accurate as the second tester I built. The second tester can also test up to 3 batteries at once.

 

Going back to the first tester I will use one of the TL431 chips to provide a 2.5V reference and hopefully that will improve accuracy as the 3.3V supply is provided by the CH340 on this clone and is far from ‘3.3V’. I don’t expect the accuracy to improve by much but it’s gonna be better than using a supply voltage as a reference as these can vary especially with cheap voltage regulators found on the Arduino clones. As the tester will only be measuring 1.5V max, 2.5V reference is enough. Unless of course you wanted to modify it to test lithium cells in which case you would need to configure the TL431 to provide a 5V reference instead.