Mistake in The Silicon-Controlled Switch (SCS)

Discussion in 'Feedback and Suggestions' started by Kvarner, Nov 16, 2012.

  1. Kvarner

    Thread Starter New Member

    Nov 16, 2012

    Please correct mistake in The Silicon-Controlled Switch (SCS)
    On page: http://www.allaboutcircuits.com/vol_3/chpt_7/9.html
    Two picture SCS: Motor start/stop circuit, equivalent circuit with two transistors.
    Mistake is in schematic; Anode and Anode Gate of SCS mast replace place.

    Best regards
    Igor Bura ing.
  2. Georacer


    Nov 25, 2009
    You are right. Thank you for your observation.
  3. Dcrunkilton

    E-book Co-ordinator

    Jul 31, 2004
    I plan to leave the circuit as it is. If you ignore the scs symbol on the left and look at the transistor level diagram on the right, the theory of operation in the text makes sense.

    If there is a question about the correspondence of the scs terminals to the correct terminals on the 2-transistor model, See "GE Transistor Manual", 1964, figures 16.1a, d, j , pg 392. This reference can be found as a pdf scanned image on the internet.

    I have built up a working version of the transistor equivalent on a breadboard with V=3.8Vdc, R1=470K, R2=100 Ohm, Motor=180 Ohm + LED. I suspect this circuit will work as is with low beta power transistors. However, I used npn=2n2222, pnp=2n2907 and interchanged the collectors and emitters to simulate low beta transistors.

    What I mean to say is that it did not work with high beta transistors-- normal polarity. But it works just fine, latching ON and OFF by the push buttons, with low beta transistors. If I were writing up a lab for Experiments in Vol-6, I would try it with power transistors for the lower beta.

    Dennis Crunkilton
  4. Georacer


    Nov 25, 2009
    Is the transistor equivalent circuit for the SCS an actual transistor-level depiction of the SCS? Or is an actual SCS built in another manner.

    What I 'm getting at is, even though the transistor circuit does work, would the SCS anode gate be able to draw the motor current via the control lead in a real-world part?
  5. Wendy


    Mar 24, 2008
    I have build equivalent SCRs out of transistors, I have to agree with Dennis on this. It took me a minute to realize the OP was trying to say the Anode and Anode Gate must switch places.
  6. Georacer


    Nov 25, 2009
    My question isn't if a pair of transistor behaves like an SCS. It's if you can expect the gates of an actual SCS to source so much current.
  7. Georacer


    Nov 25, 2009
    Okay, I browser a few datasheets and I realized that an SCS is actually made out of two transistors. It's not just a way to simulate it.

    I rest my case.