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How To Test Adjustable Precision Shunt Regulator TL431

By on June 10, 2008

tl 431 ic 

If you repair the SMPS that have the optoisolator IC connected between the primary and the secondary section, you would usually see a transistor like component. It is actually the TL431 IC adjustable precision shunt regulator and it just like the shape of a C945 transistor.  How am i going to test it?

Simple-Set your analog meter to X 10 K ohm range. Black probe to pin1 and red probe to pin 2 and 3 (must have readings) then black probe to pin 2 and red probe to pin 1 and 3 (must have readings). If black probe to pin 3 and red probe to pin 1 and 2-it should have no reading. If there is a reading then it is consider shorted. How acurate is this test? It is 90 over percent and this case is quite similar to my previous post about testing the PWM UC3842 IC. That means, if you still suspicious about the reading then the only way is to direct replace the component and retest the smps again. This component is quite cheap and easily get from those used electronic board especially the SMPS that have the optoisolator in it. 

What happen to the power supply if this component is shorted or leaked? It would cause power blink/cycling, low power or even intermittent no power problem.



  1. Ricardo Berbis

    January 16, 2009 at 3:59 am

    Muchas gracias Jestine por tu colaboración, tus comentarios son muy útiles para los que nos dedicamos al servicio electrónico.

  2. admin

    January 17, 2009 at 1:58 pm

    Usted es bienvenido Ricardo.


  3. larry

    July 19, 2009 at 2:34 am

    this is gross neg of the worst kind. in the last two instruction of testing the tl431 you list in your test,
    black probe to pin 2 and red probe to pin 1 and 3 (must have readings)

    If black probe to pin 3 and red probe to pin 1 and 2-it should have no reading. If there is a reading then it is consider shorted.

    If red probe to pin 3

    do this test again students tend to believe anything

  4. admin

    July 29, 2009 at 8:40 pm

    Hi Larry,

    I'm testing the IC using Sunwa meter and this meter have a reverse polarity output as compare to other brands. May i know what brand of analogue meter you are using?


  5. Roberto Ramirez

    December 12, 2009 at 7:52 pm

    Hi Jestine very good info about this componet so. how much must be the measure ? or only have any measue in the x10k scale?

  6. admin

    December 13, 2009 at 4:04 pm

    HI Roberto,

    Follow the test using X10 K ohm and you have to get the same reading as mine.


  7. Tal

    April 30, 2010 at 7:51 pm

    Thanks a lot Jestine,
    You just save me long hours to understand that TL 431 thingy.

  8. admin

    May 1, 2010 at 10:27 am

    HI Tal,

    You are welcome!


    • Benson

      April 13, 2017 at 10:25 am

      hi Jestine im using a fluke 101 digital multimeter how to test that tl431a

      • Jestine Yong

        April 17, 2017 at 9:01 am

        Hi Benson,

        From experience, the best is still direct replace the IC and retest. This is an IC thus it is not 100% accurate to check it with analogue or digital multimeters.


  9. job

    June 1, 2010 at 11:19 am

    Black probe to pin1 and red probe to pin 2 and 3 (must have readings) then black probe to pin 2 and red probe to pin 1 and 3 (must have readings). If black probe to pin 3 and red probe to pin 1 and 2-it should have no reading. If there is a reading then it is consider shorte. how do i start numbering the legs? is the logic be the same to all three legs components? aha i was told that only transistors have three legs but i can see you disapproved it

  10. admin

    June 2, 2010 at 8:05 pm

    HI Job,

    The numbering of the leg can be seen from the photo above. Usually you count start from the left when the component is facing you. Not all the same for the leg numbering but i make it into standard for easy explanation for my blog readers. You need to see the part number first before you determine if it is a transistor or other type of components.


  11. Sam C

    July 10, 2010 at 1:57 am

    Try cooling the TL431. If this allows the PS to run on, replace he shunt regulator. Does not work 100% but much easier and faster than pulling the TL431 out of circuit.

  12. admin

    July 10, 2010 at 10:24 am

    HI SAM,

    Thanks for sharing.


  13. jose luis saleg

    July 11, 2010 at 5:17 pm

    ecxelente informacion muy agradecido por la ayuda que presto a mis aprendizage felicitaciones

  14. admin

    July 13, 2010 at 9:53 am

    Usted es bienvenido


  15. Daniel F

    September 22, 2010 at 6:31 pm


    Can the TL431 be tested with a Fluke 179 Digital Meter?
    It has a bar graph on the display which is supposed to act a bit like a needle on an analog meter.

    From Fluke - Fluke 179:
    "The bar graph is like the needle on an analog Meter.
    overload indicator (>) to the right and a polarity indic
    the left.

    Because the bar graph updates about 40 times per second
    it is 10 times faster than the digital display, the bar graph
    is useful for making peak and null adjustments and for observing
    changing inputs.

    The number of lit segments indicates the measured value and is
    relative to the full-scale value of the selected range."

    So I wonder if it is possible to use my meter for testing TL431.
    Furthermore, as your Sunwa analog meter has reverse polarity outputs, does this mean I have to reverse my red/black leads and test as above?

    Daniel F

  16. admin

    September 23, 2010 at 2:48 am

    Hi Daniel,

    Since i do not have this meter thus i can't give an exact answer to you-sorry. As for the probes, yes you have to reverse your probes and test the above.


  17. euclides

    November 19, 2010 at 7:11 pm

    how do I test an TL431AC with an digital meter?

  18. admin

    November 20, 2010 at 5:11 am

    HI Euclides,

    Since this is an IC and cheap thus the best is to directly replace and retest. Even if i use analog meter to test it , it still not 100% accurate. Due to this i don't test this with digital meter.


  19. marvin appleby

    February 25, 2011 at 3:57 pm

    Hi: Read your article on testing shunt regulator tl431a.I have a fluke 113, so i have to reverse, and use red to 1 and black to pin 2 and 3.No reading on pin 2.Tested red to pin 2, black to pin 1 and 3. No reading on pin 3. Red to pin 3, black to pin 1 and 3 , no reading.
    Tried this on other 431a's and got the same results. Went to hatachi i am fixing and took out 1431a, shunt regulators, and got the same results as above.My question is, am i doing it right or are they all bad? Hope you can help.


  20. admin

    February 26, 2011 at 11:01 am

    HI Marvin,

    I suggest that you use an analog meter to test it because digital meter is not accurate in testing the IC due to low output from the test probes.


  21. sallehuddin z

    May 11, 2011 at 9:04 pm

    I'am in Penang, I'am interested in LCD repair, how do i contact u online, do u hav ym or other medium of online interative...?

    I hav 2 set of LCD (hd & samsung) which is have a common fault. that is it's can display the moment the switch in ON, but after few second the display is off. If we ON again after 1/2 an hours or more, it's can display and again only for a few second before it's completely no display.
    thank you

    saz png.

  22. admin

    May 12, 2011 at 8:05 am

    HI Saz,

    You can contact me using this email jestineyong(at)

    In the monitor case, the problem could be bulged caps, dry joints in high voltage transformer or one of the backlight have problem.


  23. marius

    February 17, 2012 at 7:23 pm

    hello everyone dose anybody knows how to test wt7510

  24. admin

    February 18, 2012 at 10:06 pm

    Hi Marius,

    I have not check this ic before but you can check if there is supply to the vcc pin and if possible try check pin 8 with your analog meter to see if there is any flick or not.


  25. marius

    February 18, 2012 at 11:34 pm

    Hi jestine thank you for your fast response.i will give it a try.But on my power supply there are two smds, a transistor and a diode which are marked W2F(TRANSISTOR) AND A2(DIODE) and i can`t find the datasheet for them ca you help me please.

    thank you very much Jestine

  26. admin

    February 22, 2012 at 11:18 pm

    HI Marius,

    Usually i use the below website to search for smd spec:


  27. satish

    June 30, 2012 at 6:46 am

    Hi jestine,
    I tried your instructions.I got readings when black was on pin 1 and red on pin2; blackon pin2 and red on pin1. I got no readings when black probe was on third pin. But problem is, I got no readings when black on pin1 and red on pin3; black on pin2 and red on pin3.Now, whether device is ok ? plz reply.Thanks. 

    • Jestine Yong

      July 3, 2012 at 8:45 am

      HI Satish,

      Are you using the Sunwa 360tre analog meter and have set to x10k ohm? If yes then probably the TL431 have problem. Try replace with a new one and retest the equipment.


  28. satish

    July 7, 2012 at 4:03 am

    Hi jestine,
    I am actually using a digital multimeter but the device itself seems to be defective.Thank you again for the great help.

  29. loon

    July 8, 2012 at 5:30 am

    Hi Jestine
    Suspected AP431 in acer power board shorted. There is reading when black probe at pin 3 and red probe at pin 1 (intermittent). Any suggestion for equivalent?

    • Jestine Yong

      July 9, 2012 at 5:26 am

      HI LOON,

      This is an IC and you need to get back the same part number. If you can't get AP431 then try TL431 or S431. I can't guarantee if both numbers can work 100% or not because in some power supply design, it has to use back the same part number which is the AP431.


  30. Dan Metzger

    September 12, 2016 at 11:59 pm

    Worked in my case! I had two identical TVs that were lightning damaged. This test showed me which of the two shunt regulators would possibly work. I used his method of testing the MOSFETs as well. I repaired one TV with parts from the other. Free TV! I can fix the other one for just a few bucks worth of parts.
    Thanks Mr. Yong!

    • Jestine Yong

      September 13, 2016 at 9:57 am

      Hi Dan,

      You are welcome.


    • Endale

      January 6, 2018 at 11:37 pm

      I have a supermax mini receiver but it´s power light is continue sly or about 5-6 hours blinking, after that turning on and when the power line disconnect for a few miniute it blinks, there is Tl431A transistor inside the power supply , i think the problem is this transister. BC547 ior BC1815 is preferable to replace it, if i didn´find Tl431A help me!

  31. Ulises Aguilar

    July 1, 2020 at 11:25 am

    good after noon Mr yong, hope every one around You are OK , Take care

    • Jestine Yong

      July 1, 2020 at 4:21 pm

      Hi Ulises,

      I'm fine thanks.


  32. Ulises Aguilar

    July 3, 2020 at 12:11 pm

    Ok good Sir will ordering a Book take care


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