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Action of SCR & Triac?

Discussion in 'Electronic Basics' started by Louis Bybee, Dec 29, 2003.

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  1. Louis Bybee

    Louis Bybee Guest

    Could someone explain the reason why a SCR or Triac need a minimum load or
    current level to conduct?

    I understand (at least I think I do :-] ) the circuitry necessary to
    utilize the component, but I'm having a little difficulty understanding why
    there needs to be a minimum load in addition to a trigger circuit to cause
    the device to conduct.

    Thank you.

    Remove the two fish in address to respond
  2. John Fields

    John Fields Guest

  3. Think of an SCR as a relay with one set of contacts that continues to
    energize the coil when the input is removed.

    V V d
    | +----+---+ | |
    | | | | o / o /
    | o | C| | / /
    |=|> | C| | ./ ./
    | o | C| | o o
    | | | | | |
    +--+ | +---+-----+

    Closer to the truth, an SCR is a pair of complementary bipolar
    /| |
    | |
    | |
    | |
    | |/
    Gate o-------+---|

    When a current is applied to the gate, the NPN (lower) transistor turns
    on causing current to flow in the PNP (upper) transistor. The
    collector current in the upper transistor continues to supply base
    current to the NPN transistor (keeping it turned on) until there is no
    current available in the upper transistor (no voltage across the Anode
    to Cathode).

    If there is no current in the anode then there isn't any current in the
    PNP transistor to keep the NPN transistor turned on (thus keeping the
    PNP turned on).
  4. Louis Bybee

    Louis Bybee Guest

    It makes sense now.

    Some of the electronic instructors that I've had wern't very good at
    breaking concepts down for students!

    Thanks to all who replied.

    Remove the two fish in address to respond
  5. James W

    James W Guest

    Keith, your drawing implies that ALL of the current that flows is thru
    the base of either transistor. I suspect there must be more to this.

  6. Yeah, well sorta! ;-) This is an "equivalent circuit" (I.e. *closer* to
    the truth ;-) used to understand the SCR's operation. Indeed the
    circuit will function as an SCR, as you note, perhaps sub-optimally).
    In reality, the PNP's base and the NPNs collector are the same slab of
    diffusion, as are the PNP's collector and the NPN's base. An SCR is a
    three junction PNPN device.

    Perhaps this will help some more:

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