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Have You Seen A Diac Before?

By on September 7, 2010


 smd diac

Above are the sample of Diac photos and you have to take note that certain Diac may come in the form like a normal signal diode. If you could see a center line across the component then you can be certain that it is a Diac. Otherwise you need to refer the part number with the help of semiconductor databook to confirm the component. Set your meter to X10 K ohm using analog meter and it should have no reading either way. It can be found in blender and in light dimming circuit.



  1. Sam C

    September 7, 2010 at 2:14 pm

    Thanks for sharing. Learned something new today.

    Does a Diac have a part number or are they all the same ?

  2. Waleed

    September 7, 2010 at 5:06 pm

    sure, i have seen this in an emergency lights that switchs between AC charging and DC to light up the light when there is no electricity. it took me a while to figure it out..

  3. admin

    September 7, 2010 at 9:16 pm

    Hi Waleed,

    Thanks for the info.


  4. admin

    September 8, 2010 at 10:43 am

    HI Sam,

    Some Diac have part number printed on it but some just center stripe.


  5. Jim

    February 11, 2011 at 6:25 pm

    Hi, im currently working on a Philips projection Tv, when turned on the DIAC is flashing and making a cracking sound, when i put the analog meter on it i get a full reading, one way and nothing when i reverse the leads, would it be safe to think that this componant is bad?, if so, do you know where i can get a replacement

  6. admin

    February 13, 2011 at 1:24 am

    HI Jim,

    If you test the Diac offboard and got a reading most probably the DIAC have problem. May i know what is the board marking of the Diac?


  7. Amir Mukhtar

    March 10, 2011 at 1:50 am


    Great topic and marvelous work of DIAC in every electronic circuit.
    In short, diac is a commonly used as switching device. diac devices are use the fact, that a diac only conducts current only after A CERTAIN BREAK DOWN VOLATAGES has been exceeded. The actual breakdown voltage will DEPEND UPON THE SPECIFICATION for the particular component type.
    Most diacs have a breakdown voltage of around 30 volts, although the exact specifications will depend upon the particular type of diac device.
    The circuit symbol showed below for learners.

    MT1 -|

  8. Amir Mukhtar

    March 10, 2011 at 1:52 am

    DIAC Symbol

    MT1 -|>|- MT2

  9. TomMcNally

    January 8, 2016 at 12:41 pm

    my first view of a photo of a DIAC
    Ive been looking for what I thought was a zener diode
    I have (3)different speed control TRIAC assemblys,each one has a
    copper colored ,clear covered axial component with a single silver band dead center. When I cut them out, They all read between .2 to .7 nf ,approx.
    .......Anyway,this is the second time an in depth search during my self education I have come across your site. I will immediately purchase a set of your books and stop the surfing for an education MADNESS!!

    • Jestine Yong

      January 8, 2016 at 3:16 pm

      Hi TomMcNally,

      You are welcome.


  10. james

    July 23, 2016 at 7:42 pm

    hello, if i remove a diac from pcb , how would i know what value it is to obuy one, if as you say, it must give no reading with analog meter

  11. kriyen

    July 23, 2016 at 7:44 pm

    great bit of information. i have a diac fitted inside a transmission and runs in hot oil. Can the heat affect the working conditions of the diac. if it is faulty, how do i order one if there are no numbers written on it, thanks and regards

    • Jestine Yong

      July 26, 2016 at 7:50 am

      Hi James Kriyen,

      Good question. Common Diac part number are DB3/DB4/DB6 and the spec are the volt and ampere. This means you need to find out roughly what will be the voltage and ampere usage in your electronics circuit then only you choose the right Diac. You may also use the higher spec of Diac to test it out first.


  12. Bilal Ahmad

    May 27, 2017 at 9:30 am

    Yes i have seen it in heat gun which is used in mobile repairing. It is connectes to gate of BT134 triac which controls the air in this heat gun

  13. Ulises Aguilar Pazzani

    August 22, 2017 at 1:21 pm

    Yes good infor Mr Yong Diac DB3-1 it was alot of help

  14. sagie

    May 25, 2019 at 3:28 pm

    i have seen many of these wire harness replaced and only fault is diac. This part is fitted inside a car transmission and runs in hot transmission oil, The car is obviously 12 volt but the connections are run inside a computer box of the car so how do i test. Can i connect voltmeter on the circuit of the diac to see what voltage it runs. and if i see what voltage runs on that circuit, how do i determine what diac to buy. Also i noticed when connecting meter to both sides of diac, i get reading of about 5,9 Kohms, then when applying heat gun to it, it chages dratiscally

    • Casey

      February 16, 2020 at 11:10 am

      That sounds more like a heat sensor in the transmission oil

  15. Richard Print

    August 5, 2021 at 3:16 pm

    In your top picture with a blue band around it. What is the rating of it? How does it compare with an ST2 Diac? What ratings are DB3, DB4, DB6 Diac? Thank you.

  16. Cristiano

    December 26, 2022 at 6:01 am

    Hi, can I use a bidirecional TVS diode as a diac?

    • Jestine Yong

      December 27, 2022 at 9:11 pm

      Hi Cristiano,

      Both are different kind of components serving different function with different breakdown voltage. Use the exact replacement to avoid unnecessary issue.



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