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What is Operating Mechanism of Thyristor?

Discussion in 'Electronic Basics' started by Walter Contrata, Apr 4, 2004.

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  1. Greetings,

    Please explain the physical mechanism for thyristor function. Why
    does current continue to flow from anode to cathode after current to
    the gate has been turned off?

    I have found a diagram of the thyristor structure at

    It shows an p+/n-/p/n+ stack, with the following ohmic contacts

    top p+ layer = anode;
    p layer = the gate;
    bottom n+ layer = the cathode.

    The bottom three layers, n-/p/n+, look like the collector, base, and
    emitter of an NPN bipolar transistor, but instead of an n+ collector
    ohmic, the thyristor has p+.

    According to several sources, the thyristor is normally biased with
    the anode positive and cathode negative, like an NPN bipolar
    transistor. Like the transistor, negligible current flows from anode
    to cathode until a positve current flows into the gate. However,
    unlike the NPN, this current does not stop after the gate current
    returns to 0. Why?

    My apologies for multiple postings.

    Best Regards,
  2. CFoley1064

    CFoley1064 Guest

    Subject: What is Operating Mechanism of Thyristor?
    All your questions are answered in Teccor AN-1001, Fundamental Characteristics
    of Thyristors Files/Power/an1001.pdf

    Good luck
  3. John Larkin

    John Larkin Guest

    It's just equivalent to a PNP transistor connected regeneratively to
    an NPN...

    A ------+
    e < P
    PNP b----------+ < N
    c |
    | c
    G-----+----------b NPN < P
    e < N

    This has two stable states: both transistors off, or (with current
    flowing) both on. If either transistor starts to conduct, it feeds the
    other, which feeds the first... If the current drops to the point that
    the product of the two betas falls below 1, the whole mess turns off.

    The PNPN SCR structure just merges the PNP and NPN transistors by
    sharing layers. Often there's a diffused resistor added to each B-E
    junction to reduce the feedback gain; this reduces spurious triggering
    from noise or high-temperature leakage. "Sensitive gate" SCRs don't
    have the resistor on the NPN side.

    If you bring out the PNP base as the gate, you get a
    negative-triggered SCR. This is the PUT (programmable unijunction)

    Lots of linear bipolar and CMOS IC structures have junction isolation
    designs which create unwanted PNPN structures, and have nasty latchup
    problems in consequence. The old CD4000A series parts were notorious.
    LM35 is still very bad, and Bob Pease won't fix it.

  4. Chris,

    Thanks for the speedy reply. This reference looks great.
  5. John,

    Thanks for clearing that up so well. I wish I had figured it out.

    Sorry to hear about Mr. Pease... Should he fix it?
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