Understanding a 555 w/ SCR circuit.

Discussion in 'General Electronics Chat' started by Wendy, Sep 28, 2010.

  1. Wendy

    Thread Starter Moderator

    Mar 24, 2008
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    OK, this is a spin off of this thread...

    2 led timed circuit

    It is funny, what the OP needed was actually much simpler than what I came up with. However, I came up with a circuit that shows how a basic SCR works.

    I'm thinking of using this for one of the articles. I'd like to understand when something don't go the way I think it should. The first circuit doesn't work on the alarm/red light, the second does. I'd like to understand why.

    [​IMG]


    [​IMG]
     
  2. gerty

    AAC Fanatic!

    Aug 30, 2007
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    Looks like the Sonalert is shunting the led, maybe the fact that it's polarized has something to do with it. I don't know whats in the driver circuit for them.
    It's probably something simple that'll make us slap our foreheads.
     
  3. BillB3857

    Senior Member

    Feb 28, 2009
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    Just a guess, but could the 1K resistor in Ckt #1 limit the current to a point below the "holding current" requirement?
     
  4. lmartinez

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    Mar 8, 2009
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  5. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Take some voltage and current measurements of that Sonalert. That and R5 seem to be the only difference.
     
  6. bertus

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    Apr 5, 2008
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  7. Wendy

    Thread Starter Moderator

    Mar 24, 2008
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    Actually R5 is the same part on both, I just forgot to update the schematic. If you look at the print it is 330Ω.

    Review this and you will see the connection...

    http://www.allaboutcircuits.com/vol_3/chpt_7/5.html

    Thanks Bertus, I hadn't realized the difference. Subtle, but there. I needed to trigger on the negative side of the pulse.
     
    Last edited: Sep 30, 2010
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