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SCR question

Discussion in 'Electronic Basics' started by Jason S, Mar 23, 2006.

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  1. Jason S

    Jason S Guest


    Bit of a dumb question here.....

    Not having much luck with circuits triggering SCR's.
    What I want to do is trigger an SCR, and I know they require a short burst
    of current. Triggering them manually with a wire works fine.

    Trouble is, how do I get another circuit to trigger it momentarily?
    e.g. Let's just say I have a 555 timer IC (another circuit), and I want its
    555 output pin to trigger the SCR, even if the 555 timer output changes back
    to 'low' again. The SCR must remain on until power is turned off to the
    actual SCR, even if later the 555 was switched off completely. This is just
    an example.

    I thought a non-elec cap and a high value resistor would work (providing a
    pulse), but it didn't affect the SCR at all.. not sure if its due to the
    type of SCR I'm using or not. If I manually trigger the SCR with a lead, it
    works fine, so its not a connection problem.
    The SCR I'm using is a C106Y and have also tried a BT169D (from Jaycar)..

    This is what I tried, assuming would work:

    PWR----||----+-----> To SCR gate

    10n | (pulse)



    | |

    | | 10M





    I know about the electromagnetic induction method, and that a 555 can be
    configured to work like an SCR, but isn't there a much quicker better way,
    similar to the above?

    Thanks in advance.
  2. ehsjr

    ehsjr Guest

    Make sure your load is on the anode side.
    Here's diode OR triggering:

    + ----------+ ---
    | +---|<--- } | |
    [Rload] +---|<--- } whatever --- ---
    | +---|<--- }
    --- |
    \ / | ------
    ----- | | 555 |
    | \ | | |
    | ---[10K]---+--|<----|3 |
    | | |
    | ------
    Gnd ---------+

    Stuff a + square pulse into the gate. There is a turn
    on time and a rise time - check the datasheet.
  3. Jason S

    Jason S Guest

    I think it's working now. I hooked up a transistor between the cap/res and
    SCR to amplify the short 'burst'. It seems like the burst may have been
    extremely weak to trigger the SCR directly previously.


    ____ |/

    PWR----||----+-----|____|--K NPN
    10n | 2K2 |\
    | |
    _ ------> SCR Gate
    | |
    | | 10M

    Any suggestions or comments regarding this are very welcome, as I'm still
    learning! =)

  4. Chris

    Chris Guest

    Hi, Jason. A couple of things:

    * SCRs have a spec called I(gt), the minimum gate current guaranteed
    to turn on the SCR. The typical I(gt) for the C106Y is 50mA, and can
    be up to 200mA. A 555 just can't source that much current.

    * Your transistor booster circuit will work much better for the C106Y.
    You're depending on the current gain of the transistor, though, to
    limit the current going into the gate of the SCR. Not only that, but
    you haven't taken the leakage current of the transistor into account --
    for your other SCR, it could cause false triggering (although that's
    not much of a problem with the C106 line). You might want to add a
    couple of resistors like this (view in fixed font):

    | + + + +
    | | | | |
    | .--o---o--. | |
    | | | | .-.
    | | | | ( X )
    | | |.01uF | '-'
    | | | 3|| ___ |/ |
    | | o--||-|___|-o-| |
    | | 555 | || R1 | |> V
    | | | .-. | ___ -
    | | | R2| | '---|___|-o/|
    | | | | | R3 | |
    | | | '-' .-.|
    | | | | R4| ||
    | | | | | ||
    | '--o------' === '-'|
    | | GND | |
    | === o-'
    | GND |
    | ===
    | GND
    (created by AACircuit v1.28.6 beta 04/19/05

    If you've got a 12V supply voltage, try 1K for R1, 33K for R2, 33 ohms
    1/2w for R3, and 220 ohms for R4.

    Also, there are SCRs which are made to be triggered on low gate
    current. They're called logic level or sensitive gate SCRs. Your
    BT169D is one of those. The NTE equivalent data sheet says gate
    trigger current can be less than 1mA. You should be able to drive it
    from the 555 directly without the transistor, but you may want to bump
    up the cap to 0.1uF or 0.22uF. This gives more time for the SCR to
    turn on. It might not be wise to use the transistor follower to crank
    so much currrent into this SCR -- you are getting close to the peak
    gate current rating.

    I hope this has been of help.

    Good luck
  5. ehsjr

    ehsjr Guest

    Ummm.... take a look at
    The very first page says "Output can source or sink 200 mA".
    You could drive a bucket full of C106 scr's from one 555 and a
    bunch of current limiting resistors. The C106 line is senitive

    And where on earth did you get the Igt for the C106Y?
    According to the spec sheet:
    or here
    click on datasheet

    The *maximum* trigger current I(gt) is 200 *micro* amps at 25 C
    The *typical* I(gt) is 15 *micro* amps. Your 50 - 200 mA
    is way out of line.

    Maybe both of those datasheets are wrong? That's why I'd
    love to see where you got the specs!
    That point is valid and important. The gate current *must* be
    limited to < 200 mA.

    The circuit below is not necessary. His problem is dv/dt.
    The 10 meg resistor has got to go. He should try his
    cap/resistor circuit (no transistor) at 10nf and 10 or 15K
    instead of 10 meg. But he hasn't posted a complete circuit,
    so who knows what is connected to where.

  6. Jason S

    Jason S Guest

    Thanks for your comments guys.

    The 555 was mentioned only as an example of triggering the SCR.... instead,
    it could be a logic gate output, or maybe just a transistor, or push button.
    Sorry to confuse the situation with the 555 example and the lack of
    diagrams, but valid points were raised regarding the float, etc. which I'll
    keep in mind.

    Ed, I've tried taking away the transistor and replacing the 10M with 15K and
    the ceramic cap and is working fine! I didn't have a 10nF ('103') like you
    said to use, but I only had a cap marked "151"... It's working, that is the
    main thing. I will be getting more caps this weekend and I'll try the 10nF
    like you said.

    I've hooked up a 22K drop-down resistor from the SCR gate pin to ground so
    that it doesn't falsely trigger at power-on (thanks for that Chris....
    although the 220R you suggested wasn't high enough a value to trigger).
  7. Jason S

    Jason S Guest

    Oh hang on, I just realised I don't need the 22K drop-down anymore now that
    I removed the transistor, so just ignore that paragraph. Cheers.

  8. Chris

    Chris Guest

    Hi, Ed. Thanks for catching that. Since the C106 is no more, I went
    to the NTE website, crossed to NTE5452, then misread their data sheet,
    which does say Igt is 50uA typ. / 200uA max. Thinking about those old
    clunker SCRs, I misread uA as mA.

    ON semi also has this data sheet for the C106D:

    The rest of your observations are also correct. I just missed that
    one, which is odd, because I worked with the part in days past. Thanks
    again for the spot.

  9. ehsjr

    ehsjr Guest

    The link you posted to Onsemi is helpful! I had to
    "back into" getting there when I did my "fumble-fingered"
    searching. And my result is a link that is convoluted.
    Yours is straightforward. Nice.

  10. ehsjr

    ehsjr Guest

    Hi Jason,

    I really didn't think about the value of the cap - I
    just thought that was what you had on hand based on
    your diagram. The issue is dv/dt, or from another
    perspective, tc. dv/dt refers to the speed at which
    the voltage level at the gate of the scr changes. tc
    is the time constant of the R/C connected to the gate.

    You can use a "diode or gate" to experiment with multiple
    cap/resistor combinations, logic drives, switches,
    whatever to turn the scr on. Chris made the important
    point about protecting the scr from too much gate
    current - be sure to use a series resistor to limit
    gate current.

  11. Jason S

    Jason S Guest






    \ /

    ____ -----

    TRG >--||---o--|____|--o------/| C106Y

    | R2 | |

    R1 | |C2 |

    .-. --- V

    | | --- GND

    | | |

    --- |

    | |

    V V


    C1 = 10n

    C2 = 2n2

    R1 = 15K

    R2 = 2K2

    C2 prevents false triggering at power-on.

  12. Jason S

    Jason S Guest

    yup, noted.
    Thanks again for your help Ed and Chris.

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