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Adjustable precision shunt regulators, TL431


walid

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This si a cool little device!  A few years ago I had a job that included scrapping power supply modules, and I noticed a few reusable parts on them.  The TL431 was one such.  I have a film jar full of them (very short leads though).

Then I became aware of a PCB promotional offer, so I included a daughterboard in my submission.  It works great!  Two SMT resistors, one TO-92, and two header pins (it also workd for LM334 and LM335).

That's what I used on my PIC programmer for both of my zeners.  I also used one to make a 2.55V reference (should have been 2.56) for a projet that monitors it's own supply voltage (still not done- the human interface was a bad design, and right now everything is packed up).

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Hi
I have another question about the shunt regulator 431:
We Know that Vout = (1 + [R1/R2]) * Vref
If I want Vout 12.5V and assume Vref = 2.5V then 12.5/2.5 = 5
that is R1/R2 =4
400K/100K = 4
4ohm/1ohm =4 ???!!!!!
I read three different 431 datasheets from three different companies trying to find something about the values of that resistos, but nothing.
can u please help
thanks

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Don't use 1 ohm and 4 ohms as a voltage divider for a little TL431. At 12.5V then their current will be 2.5A and the little TL431 will melt if the source voltage rises.

The reference input current is only typically only 2uA and doesn't change much when the temperature changes, so the current in the divider could be 200uA. Then the divider's values are 12.5k and 50k.

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