low power SCR?

Discussion in 'General Electronics Chat' started by tattee, Dec 8, 2008.

  1. tattee

    Thread Starter Member

    Apr 24, 2008
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    Hi,

    I'm planning to use SCR in my circuit but im having a problem of trying to find a low power SCR. What I have is a circuit with 5v supply and current consumption of 20mA maximum. Is there any SCR suited for this operation? If not, what can you suggest for replacement? What ive come up so far are SCR's for high voltage/amps circuits.

    By the way, here's what my circuit will do:
    • A pushbutton will be push momentarily, thus enabling the current flow to the circuit.
    • A PIC will disable the SCR by applying '0' to the gate of the SCR
    Question: Will a '0' from the output of the PIC disable the SCR?
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    SCR equivalents can also be made with 2 BJT transistors.
     
  3. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    No, at least not for the usual SCR. Read about them in the AAC book.

    If you are using a PIC anyway, why not have the switch input to the pic and have the PIC control some other switching device?

    John
     
  4. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    From memory a 2N5060 might do,60V @ about 1/2 A, used a bunch in alarm systems, 1972+_.
     
  5. tattee

    Thread Starter Member

    Apr 24, 2008
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    0
    Thanks for your help guys.. I will try to look at your suggestions and update you on what I've come up with.
     
  6. tattee

    Thread Starter Member

    Apr 24, 2008
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    By the way, here was my initial plan.

    Attached is the circuit im planning to do:
    • The pushbutton will be pressed to start the operation
    • The circuit will run for sometime after button is pressed
    • The PIC will send a signal to the gate of the SCR to disable it (if possible)
    • The circuit stop conducting
     
  7. Wendy

    Moderator

    Mar 24, 2008
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    Here is the transistor version. The 10MΩ resistor help raise immunity from noise. It drove me crazy when I used it and didn't have the resistor, a 1MΩ. It kept false triggering.

    [​IMG]

    A SCR is a PNPN device, this is a direct equivalent. It really is a true SCR.

    The only way to stop a SCR conducting once it is on is to stop the current flow. With AC this happens automatically on the alternate cycle, in my design (which was a DC alarm) I used a simple push button to break the current flow.
     
    Last edited: Dec 10, 2008
  8. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    I saw this circuit (as best as I can remember) on a forum recently. Can't remember which one. It uses R2, sized to maintain the hold current, pulled high to hold the SCR on, once started. To shut down, the μC could pull R2 low, depending on the rest of the working circuit load, or go into a low-current sleep mode to starve the SCR off.

    Ken
     
  9. nanovate

    Distinguished Member

    May 7, 2007
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    Most commercial SCRs have a gate-cathode resistor that ranges from a few tens of ohms to a couple hundred. There are also "sensitive-gate" -- which you created ;) -- that have very high gate-cathode resistance but even they are actually spec'd with a 1k resistor externally. One thing to watch out for is the dv/dt spec. Sometimes applying a very fast pulse to the anode can trigger it on.
     
  10. Wendy

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    Mar 24, 2008
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    Yeah, mine needed to trigger off a pulse. It was a water monitor for the factory, I wanted an alarm mute. It amazes how many companies don't install that simple feature, once people are responding then you don't need loud audio driving you crazy.
     
  11. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Add another npn@pnp transistor and you have a reset ?
     
  12. Wendy

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    Mar 24, 2008
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    Don't know, it might work. You have to divert 100% of the current, or it will stay on.
     
  13. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    10-15Ω s between scr and load, with reset at load end might help and only cost .15 V., a Ge diode ok at about .3V.
     
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