Connect with us

Silicon-Controlled Switch SCS

Discussion in 'Electronic Design' started by Aaron, Oct 22, 2003.

Scroll to continue with content
  1. Aaron

    Aaron Guest

    Does anybody know a source for a really inexpensive SCS chip? The only one
    I can find on google is an NTE239 and BYP62, but digikey doesn't carry
    either and I wouldn't know where else to look.


  2. Leon Heller

    Leon Heller Guest

    Must be 35 years since I saw an SCS. :cool:

  3. Aaron

    Aaron Guest

    Crap! What is a better (newer) technique for achieving the same
    functionality of an SCS? What I am trying to do is use a momentary switch
    to turn on a system that can time itself out (power itself down).


  4. R.Legg

    R.Legg Guest

    A real SCS or four-layer diode has been a rare animal for a while.
    Part numbers that used to be seen are illustrated at:

    ITT still listed the 4Exx types in the 80's. The inventory of Clevite
    was distributed by 4LD Inc as late as 1992. I believe these were

    Cheapest replacement used to be a Diac, a bidirectional version with a
    break-over voltage of ~ 32V. These were common as surplus.

    ST-2, BR100, DA-4, IN5411.

    Sidacs can perform the same function with breakover voltages extending
    above 200V, similar to a four layer diode. They are not cheap.,4533,,00.html?searchString=sidac

    A PUT is much more versatile. Unless you actually need the peak
    current capabilities of the older parts, I'd advise one if all you
    want to do is substitute the part location in the old timing circuit.

    Otherwise, there's probably another way to do what you're talking
    about, if you'd care to explain.

  5. Robert Baer

    Robert Baer Guest

    As someone else mentioned, the SCS is a 4-layer device, it is really
    an SCR with the anode gate also available; i think GE may still make
    In any event, they can easily be made with two transistors: an NPN and
    a PNP; connect collector of one to base of the other (two symmetrical
    connections); the NPN emitter becomes the "cathode", the PNP emitter
    becomes the "anode", the NPN base becomes the cathode gate (SCR gate),
    and the PNP base becomes the anode gate - which is like a PUT gate.
    Note PNPs tend to have a larger B-E breakdown, allowing a larger
    stand-off voltage for PUT type applications.
    You can use small signal transistors, power transistors, or a mix as
    you see fit and depending on tha application requirements.
    If you need very sensitive gate requirements, Zetex has very high beta
    transistors (300 minimum on PNP and 500 minimum on NPN if i remember
    corretly) that could be used. Do not use a darlington, as thsy cannot be
  6. Leon Heller

    Leon Heller Guest

    Why didn't you say that in the first place???

    Devices like the SCS can probably be simulated using a pair of BJTs. 40
    years ago I needed a UJT when they were very new. I couldn't get any so
    I used a couple of ordinary transistors. My little circuit actually went
    into production on the LEO III computer.

    I'd be inclined to use one of the 8-pin MCUs like a PIC or AVR for your

  7. Aaron

    Aaron Guest

    OK, let me start from the start, then you guys can send me in the right
    direction. My (probably mis-) understanding is that SCRs and SCSs have very
    little current leakage. I am building a battery operated toy (the core
    module is a PIC chip) that I want to turn on when the toy is moved (or
    shaken). The idea is that the PIC chip will act as a timer and then kill
    power to itself in order to save the battery life.

    The device I have to wake the toy up has two states: on/off. It is a
    mechanical device. What I really need is the device to wake up when the
    mechanical devices changes states from on-to-off or from off-to-on. When
    the device is at a "steady state" it should not allow current to pass so as
    to save battery life. I was thinking about using a capacitor in line with
    the DC current so that when the device changes states, current can flow and
    trigger an SCS ... then the PIC chip can shut it off using the fourth lead.

    I could use the technique below with two transistors but I was worried about
    current leakage draining the battery pretty quickly. Maybe that is an
    unfounded concern?

    One of the other posters has mentioned a PIC chip. Perhaps there is
    something built it to do what I am doing already and I didn't even realize


  8. Look at using sleep mode, wake on port pin change, that sort of thing.
    If there's no regulator draining the battery you don't need to cut
    power to the chip, just have it shut down the oscillator and other
    analog circuitry on the chip. It should only draw a few uA under those
    conditions, if you do everything right.

    Best regards,
    Spehro Pefhany
  9. Aaron

    Aaron Guest

    OK I will really go over the PIC specs a lot deeper then.


  10. Robert Baer

    Robert Baer Guest

    A SCR or other SCR-like device is not too easy to turn off with a DC
    load, so i do not think you want touse that for controlling the load.
    You could cross-couple two transistors or FETs as a R-S flip-flop and
    one could control the load.
    If bipolars were used, you may need to drive the power transistor to a
    forced beta of 10 or less to guarantee saturation for a heavy load. That
    would require a fair current for the driver (low power side of the
    flip-flop), also hurting your concerns.
    If you use FETs, there is no steady-state power for controlling them
    (no gate current), which can be a factor for better battery life.
    The lower power side could be set to run at low currents; for FETs one
    could use microamps for the ON, and the OFF current can be picoamps.
    This all assumes slow switching times are OK.
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day