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Question about scr's

Discussion in 'Electronic Basics' started by bench, May 18, 2004.

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  1. bench

    bench Guest

    My question is about the simulation of an scr with two transistors,
    one npn, one pnp. Will it always work this way, ... because scr's
    can conduct large currents, whereas with the two transistors the
    whole current must flow through a base-emitter junction, isn't this
    so, and are transistors capable of conducting such large base
    currents ?
  2. CFoley1064

    CFoley1064 Guest

    Subject: Question about scr's
    The modelling of an SCR as back-to-back transistors is something to give you
    the idea. The dies are optimized in SCRs for the circuit action, and actual
    SCR and other thyristor specs are based on die topology. Take a look at the
    Teccor AN-1001, "Fundamental Characteristics of Thyristors, for a basic

    (Teccor is now part of Littelfuse, I guess.)

    Good luck
  3. Mantra

    Mantra Guest

    Yes, using a .subckt this is often done. The tricky part is that an
    SCR involves positive feedback between the two transistors a so small
    parameter (e.g. BF, etc.) shift can cause the overall behavior to
    change significantly. Things like trigger currents and such. If you
    can find an existing subcircuit model for your device that someone
    else has debugged that will probably help to avoid premature
    loss/graying of your hair. Personally I wouldn't put faith in my own
    home-made SCR model without having the real thing (SCR) there and
    charactering my model against actual measured data.

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