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Hello,This component has very little information on the internet.

  1. Jun 21, 2011 #1
    Hello,

    This component has very little information on the internet. I learned the classic exploitation of it with 2 transistors(attached) and I get that. But i feel that is just an analogy.

    Does anybody here at PF EE know the details behind this component like, explanation with junctions, which are forward biased, inverse biased etc. and all this put together, why there is a specific voltage when it starts to conduct. I mean it makes sense to me when you are talking about 2 transistors.
    But in my opinion there are no 2 transistors there,just the effect is the same. Or is it in fact, literally 2 transistors in couple to produce this effect.

    Thanks
     

    Attached Files:

  2. jcsd
  3. Jun 21, 2011 #2

    Averagesupernova

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    Re: Thyristor(SCR)

    It is actually 4 layers. The two transistors are pretty accurate but just eliminate the leads between the two transistors and you have the 4 layers I am referring to.
     
  4. Jun 21, 2011 #3
    Re: Thyristor(SCR)

    I do not understand what are u trying to say. In picture there are in deed 4 layers.
     
  5. Jun 21, 2011 #4

    Averagesupernova

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    Re: Thyristor(SCR)

    So what's the problem? That's what I am saying. 4 layers....
    -
    Just take an NPN transistor and stick a P layer on top of the collector and you have it. Of course there is more to it in manufacture such as layer thickness, but that should be clear enough.
    -
    The junctions are biased just as they would be if there were two separate transistors. Try building one out of 2 discreet transistors and you may have a better understanding of it. Also, look up SCS.
     
    Last edited: Jun 21, 2011
  6. Jun 21, 2011 #5

    uart

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    Re: Thyristor(SCR)

    No, in the picture is two three layer devices with interconnecting wires between two of the layer. Not the same thing.

    Having said that however the two transistor model of the SCR is reasonably good at explaining the most significant features of the device.
     
  7. Jun 21, 2011 #6
    Re: Thyristor(SCR)

    Well how do they work? From the characteristic curves of thyristor, they aren't trivial, and I couldn't interpret them just from looking at the layers. Generally they are explained with 2 transistors.

    I've read somewhere an article that describes this effect but not with transistors. But this article was shallow too and they weren't very qualitative.

    How does this effects occurs just by looking at the layers? What is happening?
     
  8. Jun 21, 2011 #7
    Re: Thyristor(SCR)

    Well when you are explaining them with transistors, you have to break them into to parts.

    if it satisfies you:

    [PLAIN]http://pokit.org/get/81c0c19a36b82454b42f3eae01522201.jpg [Broken]

    How does this work, without using transistor analogy.

    There must be an explanation somewhere that describes this in detail.
     
    Last edited by a moderator: May 5, 2017
  9. Jun 21, 2011 #8

    Averagesupernova

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    Re: Thyristor(SCR)

    You don't really need to break them into two transistors to explain it. They are in fact two transistors but they are sharing layers with each other. Maybe if you look at it that way it will make sense.
     
  10. Jun 21, 2011 #9
  11. Jun 21, 2011 #10
    Re: Thyristor(SCR)

    Keeping up with you is quite an undertaking. I am still preparing some pages of explanation about other forms of diode so I will be brief here.

    Instead of thinking about four layers, think about three junctions.

    Counting from the anode calling them J1, J2, J3.

    The device is forward biased by making the anode positive relative to the cathode.
    This forward biases junctions J1 and J3 and reverse biases J2.

    Thus J2 contols the device (blocks) until the forward voltage reaches the breakdown of this junction.
    When this junction breaks down the total forward drop reduces suddenly to around 1 volt and a large current passes making the device a voltage operated switch.
    Current injection at either cathode gate or anode gate will precipate this process.

    Mathematical modelling is best caried out with the connected two transistor model - the second picture in Bob's Wikipedia reference, although they do not show the maths (There is about a page of it).

    If you want to go into things things deeply you really should get a textbook, the one I believe I recommended before (Pierce) has about eight pages on these devices.
     
  12. Jun 21, 2011 #11
    Re: Thyristor(SCR)

    Yes that is what I was talking about. Thank you very much!!!
     
  13. Jun 21, 2011 #12
    Re: Thyristor(SCR)

    The properties, physics and design of semiconductor devices

    Shive (Bell Telephone Labs)

    Physics of Solid state Electronics

    Shive

    Semiconductor Junction Devices

    Pierce (Oak Ridge National Laboratory)
     
  14. Jun 21, 2011 #13
    Re: Thyristor(SCR)

    Thank you. I am asking these kinds of questions because, I am having my final exam in 2 days. And my professor asks for a understanding of a component. I am not given that much material of all components.

    I must know this because I got high exam marks on written test and now on the final exam, where you are being questioned by professor live, I will be probably asked some serious questions, because I did well on the written exam. I cannot be caught off guard.

    Thank you all, especially you Studiot. Lifesaver.
     
  15. Jun 21, 2011 #14
    Re: Thyristor(SCR)

    I got everything. Just one thing bothers me. When i reach the the reverse breakdown, by applying high voltage, shouldn't that voltage drop still be on that reversed junction even after the breakdown occurs? I mean I am concluding that directly from the characteristic curves of a normal diode. Yes a large current suddenly surges, but in the graph, voltage drop still stays the same...
     
  16. Jun 21, 2011 #15
    Re: Thyristor(SCR)

    I'm not sure I understand the question.

    We are talking about forward bias on the device as a whole. (Anode to Cathode)
    This means forward bias on two junctions and reverse bias on one.

    The reversed biased junction therefore acts to block current as the voltage is increased, it does not remain the same. So the device is not in conduction.

    At some point the voltage is sufficient to breakdown this blocking junction and the device rapidly enters conduction. At the same time the voltage across the device falls rapidly to around 1 volt.

    This is what happens when we use the anode - cathode voltage to switch the device (normally an SCS).

    We can also precipitate a self latching switching by injecting current into one of the two inner layers via a gate connection. there are two possible gate connections known as the anode gate and the cathode gate. Only one will appear or be used in any particular device, making an SCR a three terminal device.
     
    Last edited: Jun 21, 2011
  17. Jun 21, 2011 #16
    Re: Thyristor(SCR)

    Thank you, this clear things up. Going back to Zener thread
     
  18. Jun 27, 2011 #17
    Re: Thyristor(SCR)

    You could give these guys a shout, they seem to know a bit about http://www.mitsco.co.uk" [Broken].
     
    Last edited by a moderator: May 5, 2017
  19. Jun 27, 2011 #18
    Re: Thyristor(SCR)

    Thank you for you link, but I finished my exams. With flying colors too! :)
     
    Last edited by a moderator: May 5, 2017
  20. Jun 27, 2011 #19
    Re: Thyristor(SCR)

    Aceing exams is a great feeling! Well done..
     
  21. Jun 27, 2011 #20
    Re: Thyristor(SCR)

    Congratulations
     
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