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Help needed with 'back to back' scr's

Discussion in 'Electronic Design' started by Rekcirednos, Sep 13, 2004.

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  1. Rekcirednos

    Rekcirednos Guest

    I am making a 1kw microprocessor controlled lamp dimmer that uses two
    back to back scr's. I am having trouble finding a way to trigger them,
    especially the one that switches the negative half of the ac sinusoid.
    I have to use dual scr's in this design.

    could anyone point me to a schematic or verbally tell me how to do

    any help would be greatly appreciated.
  2. Jim Thompson

    Jim Thompson Guest

    WHY do you "have to use dual scr's in this design"?

    Use a TRIAC instead... it behaves as back-to-back SCRs, but is much
    easier to trigger.

    ...Jim Thompson
  3. CFoley1064

    CFoley1064 Guest

    Subject: Re: Help needed with 'back to back' scr's
    Another homework assignment (sigh).

    For a lamp load, a TRIAC is the obvious choice. Back-to-back SCRs are usually
    used in switching inductive loads, where the current and voltage are out of
    phase, meaning that the TRIAC won't turn off at voltage zero crossing. Since
    B2B SCRs have only one thyristor conducting at a time, you can step past this

    But, since "teach" specified B2B SCRs, you might have wanted to try a Google
    search on "Back to back SCR" schematic. Page one of search results would have

    Look at page 4 of this appnote to get an idea of what to do. Note that the
    opto specified, the MOC3041, is a "zero-crossing" opto-TRIAC, which means it
    won't do for a lamp dimmer. You need phase control, meaning you have to turn
    on the SCR in the middle of the AC cycle. We'll leave the voyage of discovery
    to determine the right opto-TRIAC for the job to the OP -- he needs some
    "ownership" of this design, I suppose. As a hint, I suppose I could say that
    almost all of the alphanumeric characters of the IC you want are correct.

    As part of your education, also try:

  4. You can connect an opto-triac between the gates of the SCRs. Or use a
    dual-secondary pulse transformer with zero voltage or picket-fence
    firing. Or use an alternistor. Or use a triac, perhaps with snubbing.

    Best regards,
    Spehro Pefhany
  5. Ken Smith

    Ken Smith Guest

    Get an anode gated SCR.
  6. Dana Raymond

    Dana Raymond Guest

    One way would be to use a fet output optocoupler.
  7. Bill Bowden

    Bill Bowden Guest

    I made a 1KW SCR controlled space heater. Schematic is

    It uses a small trigger transformer (3 windings) to gate 2 back to
    back SCRs.

  8. john jardine

    john jardine Guest

    Years ago wasted a whole hour figuring it out from scratch :-(
    It's simply a case of diode steering.

    Diode | |
    ___ .--------o-.|
    Drive -|___|-.-->|-|Gate - ||
    | | ^ || Thyristor #1
    | '--------o-'|
    | | |
    | | |
    | .--------|--o-.
    '--|<-|Gate | V | Thyristor #2
    | | - |
    Diode '--------|--o-'
    | |
    | |

    created by Andy´s ASCII-Circuit v1.24.140803 Beta
  9. Hi Bill,
    Nice little site!
    What software do you use for doing the circuit diagrams?


    Serious error.
    All shortcuts have disappeared.
    Screen. Mind. Both are blank.
  10. Bill Bowden

    Bill Bowden Guest

    Hi Martin,

    The drawings are done by hand using a pixel editor similar
    to Windows Paint. It was time consuming at first, but now
    I have a good supply of templates for new drawings. I can
    usually find something close to what's needed and just move
    things around, and reconnect the lines.

  11. I read in that martin griffith
    >) about 'Help needed with 'back to back' scr's', on Tue, 14
    Sep 2004:
    Indeed. I have caused consternation and incredulity by describing as a
    'passive amplifier' a version of the 'long loopstick' using a frame
    antenna instead of the loopstick.
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