Adjustable precision shunt regulators, TL431

[walid]

Hi
In its datasheets attached below, and looking to the atteched figure below, we can see that:
Vout = (1 + [R1/R2]) * Vref
If +V = 12v, R1 =10k, R2=2K
What is Vout
I cannt understand how to get Vref value
please help me to solve this
thanks

431-2.pdf

[audioguru]

Vout = (1 + [R1/R2]) * Vref
If +V = 12v, R1 =10k, R2=2K
What is Vout
I cannt understand how to get Vref value

Hi walid,
For the inexpensive TL431C, the datasheet lists the Vref as minimum 2.44V, typical 2.495V and max 2.55V. So in your example the Vout is typically 14.97V.

[Ldanielrosa]

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).

[walid]

thank you guru fore your direct and good answer, it was very good
also thank for Ldanielrosa

[audioguru]

Hi Walid,
The TL431 is a shunt regulator like a zener diode. You have its two resistors setting its voltage to 14.97V.
I didn't notice that your input voltage is only 12V so it won't work.

[walid]

Thank u guru

No problem, I didn't calculate it, only I put random numbers to know the value of Vref, you are clearly mentioned in Your previous answer I got what I want Thank you once again for your attention.

[walid]

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

[audioguru]

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.

[Ldanielrosa]

For R2 I arbitrarily chose 10k- an easy number to work with, low enough current so as not to load heavily but high enough to suppress drift, readily available in 1% tolerance.