2N6027 PUT/UJT based SCR control circuit

Discussion in 'General Electronics Chat' started by tech1962, Nov 28, 2016.

  1. tech1962

    Thread Starter New Member

    Nov 28, 2016
    Hello all,
    My first post here ...and thanks for all the helpful info on this forum. Its excellent.

    I am simulating a circuit before building as shown in attachment. I've never used PUTs before but have now read about them a bit.
    The PUT part is driven by a sinusoidal signal, and the SCR is also. The circuit aims to control the brightness of the LED/lamp via phase control switching. Nothing too trickey.

    What I dont understand is how to avoid multiple triggering of the PUT and SCR within one cycle.
    Ideally, it should fire once per cycle and then switch off until the next cycle.
    but since everything is driven by the sinusoidal input, it seems that the threshold drops with the input signal and re-triggers.

    I was hoping someone with experience of these devices could fill me in on how its done.
    • sim1.PNG
      File size:
      318.2 KB
    DickCappels likes this.
  2. DickCappels


    Aug 21, 2008
    Do us a favor -change your LED to a light bulb because we cannot discuss LEDs being powered directly by the mains on this site.

    It is not clear that multiple triggering during a given half cycle is a problem since the SCR would continue to conduct from the first firing until the zero crossing.

    If you are still concerned about it, derive the triggering voltage from the voltage across the SCR. That way, the triggering voltage won't be able to be greater than the voltage across the SCR until the end of the half cycle and a second trigger pulse cannot be generated during the half cycle.
    tech1962 likes this.
  3. tech1962

    Thread Starter New Member

    Nov 28, 2016
    Thanks for your reply Dick,
    I should have emphasised...the input sinusoid signal is only 8V peak...as in the simulation. This is an low voltage educational circuit...not a high voltage circuit.

    Thats a good point that the SCR itself does not re-trigger after the first firing. I had not comprehended that.

    As far as deriving the voltage from the SCR, I will give that a go to see if I can understand that better.

    Thanks for the tips.
    Any other insights much appreciated.
    I am still curious as to whether the multiple triggering of the PUT is "normal" for these devices in this type of circuit..since I've never used them before...so any more insights from experience would be much appreciated.

    Usually when you see phase controlled signal switching signals in descriptions..they turn on at one point in the cycle..and are adjustable across the entire cycle.
    I'm not sure that this circuit can achieve that level of resolution...so Im wondering if its a "good circuit" or not ?
  4. tech1962

    Thread Starter New Member

    Nov 28, 2016
    Still on this same topic..here are some other signals and further questions.

    Why does the SCR switch OFF at the point marked by arrow ?
    Is it because the load current becomes too low (its 9mA at that arrow point) or because the SCR forward voltage is too low ?

    Shouldnt I be aiming to keep it ON until the end of the half cycle , like in the textbooks ?
    tech1962 UJT-SCR-Attachment 1.PNG
  5. OBW0549

    Distinguished Member

    Mar 2, 2015
    Most likely because its anode current falls below the minimum needed to keep the SCR on. Try reducing the value of R6 and see what effect that has.
  6. tech1962

    Thread Starter New Member

    Nov 28, 2016
    Having a closer look at the datasheets suggest that is the case.
    I've reduced R6 and it has the appropriate effect.