Question on Triggering a Switch and holding in on

Discussion in 'The Projects Forum' started by RiG615, Jun 8, 2009.

  1. RiG615

    Thread Starter Active Member

    Nov 13, 2008
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    So heres my problem


    I have voltage on a wire @ 40khz. [ DC, just pulsed @40khz ]

    Once the above happens I want an LED to go on and stay on even after the pulses stop.

    SO, heres what I want to happen

    [nothing]...[nothing]...[wire pulsed]...[LED on]...[pulse ends]...[LED on]...[LED on]...[I kill power and reset circuit]



    --I can't seem to conjure up anything in my head w/o using an IC, which I perferably dont want to use to keep cost low--
     
  2. DC_Kid

    Distinguished Member

    Feb 25, 2008
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    pulse the gate of a scr.
     
  3. RiG615

    Thread Starter Active Member

    Nov 13, 2008
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    So if I put a transistor in place of the LED(from above) and there was varying current through the transistor would the SCR shut off?

    what about slightly varying current? whats the tolerance?
     
  4. CDRIVE

    Senior Member

    Jul 1, 2008
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    If your thinking that you can use the pulse train to supply power to the Led then you misunderstand how you would use the SCR. Here's a SCR equivalent circuit. In lieu of an actual SCR you could make one like this from two small NPN & PNP general purpose switching transistors.
     
  5. RiG615

    Thread Starter Active Member

    Nov 13, 2008
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    No I wasn't planning on doing anything with the pulse train besides to trigger the SCR

    my question was about the in general usage of the SRC

    1. Once the voltage to the gate is cut off, their is still flow of current through the anode and cathode, right?


    2. What causes the SCR to unlatch or turn off?


    3. Can there be varying current/voltage through the anode-cathode without the SCR going off
     
  6. CDRIVE

    Senior Member

    Jul 1, 2008
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    SCR,s have a holding current parameter which vary depending on the type of SCR. A small low current SCR will hold down into the microamp region. If the current through the SCR fluctuates it is not an issue as long as polarity of the current is not reversed. A SCR can be also shut off by shunting the Anode & Cathode. Tomorrow I'll spice the drop out current of the circuit that I posted. It's late now.;)
     
    Last edited: Jun 9, 2009
  7. RiG615

    Thread Starter Active Member

    Nov 13, 2008
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    thanks CDRIVE
     
  8. DC_Kid

    Distinguished Member

    Feb 25, 2008
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    if you wanted AC to hold you could trip a SCR with your pulse, then that SCR could drive the gate of a TRIAC (aka, solid state AC relay). then when you want it off you short the SCR out.

    note: shorting scr cath to anode means you need to do it after the load. otherwise you run the risk of shorting out your V supply, etc.
     
  9. CDRIVE

    Senior Member

    Jul 1, 2008
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    I want to make a correction here. In an earlier post I stated that some SCR's could have a holding current in the microamp region. Well I did a search of SCR types and didn't find one the held below 2.5mA. Now this doesn't mean that they don't exist. It just means that I haven't found one. To cover my butt, one could say that 2.5mA = 2,500UA! :rolleyes:
     
  10. DC_Kid

    Distinguished Member

    Feb 25, 2008
    638
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    but to us physics nuts that would be several magnitudes of uA :)

    so you could say "holding current near several magnitudes of uA" :)

    lol
     
  11. RiG615

    Thread Starter Active Member

    Nov 13, 2008
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    Whats the difference between these SCR types:
    -Sensitive Gate
    -Standard Recovery
    ?
     
  12. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
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    That works for me too! :D
     
  13. CDRIVE

    Senior Member

    Jul 1, 2008
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    A Senitive Gate SCR requires very little Gate current to trigger it. In fact some of them will fire by touching the Gate with your finger.
     
  14. CDRIVE

    Senior Member

    Jul 1, 2008
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