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Thyristor and op amp circuit

  1. Jan 20, 2012 #1

    I have tested this circuit on the computer and it works but when I try to make it both on a pcb and breadboard the circuit will not work at all. If i swap the thyristor - TIC106D - for a transistor - TIP121 - the circuit works fine so I think the problem can be isolated down to somewhere between the Op Amp output and the thyristor?

    The other unusual thing is that if I short the Cathode pin to the Gate or the Anode pin to the gate, then the buzzer will sound but the thyristor doesn't latch?

    Any ideas what is causing the fault.


    Attached Files:

    Last edited: Jan 20, 2012
  2. jcsd
  3. Jan 20, 2012 #2
  4. Jan 20, 2012 #3
    Sorry I got the codes the wrong way round.

    Transistor I current have in it is TIP 121

    Thyristor I tried was a TIC106D
  5. Jan 20, 2012 #4
    Try connect 470R resistor parallel to the buzzer.
  6. Jan 20, 2012 #5
    What exactly will that do as I can't see it making that much difference?
  7. Jan 20, 2012 #6
    We increase the load current so that SCR can latch.
    So we ensure that
    I_load > I_holding_current
    Last edited: Jan 20, 2012
  8. Jan 21, 2012 #7


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    Staff: Mentor

    Is the 9v supply smooth DC, or are you using a wall wart? Try adding a large electrolytic across the power supply to make sure it is smooth.
  9. Jan 21, 2012 #8
    Its a 9V PP3 battery so the DC supply should be smooth.
  10. Jan 21, 2012 #9


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    Staff: Mentor

    Shorting the cathode to the gate should not turn on a thyristor. Something seems amiss here. Have you got a second one you could try? Check the connections. The gate would be the smallest lead, I think.

    For testing, you could connect the buzzer from the op amp output to ground, to make sure the comparator is working how you think it should. How long are the leads to the LDR? They may be picking up some mains hum.
  11. Jan 21, 2012 #10
    I tried 2 TIC106D Thyristors - by 2 different makers - in the circuit and both behaved the same.

    I've test the comparator part of the circuit by putting an TIP121 transistor in instead of the thyristor and the circuit works fine. I also checked the Voltage from the output pin wit a multimeter by putting one probe on the pin and the other on the ground rail and the Voltage was exactly as I had simulated it.
  12. Jan 21, 2012 #11


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    Staff: Mentor

    Let's focus on the SCR. You are not able to turn it on by supplying gate current? In place of the piezo buzzer can you connect a 560 ohm resistor + LED (or 9v or 12v light bulb). Maybe the buzzer is a weird load. Return the 10k gate resistor directly to the +9v instead of relying on the op-amp during this testing phase.

    Steering a separate course, it should be possible to do away with the thyristor and convert the comparator to one with hysteresis so it latches on and can drive the buzzer directly with no need for the SCR. Or are you committed to demonstrate use of a SCR?

    Can you double check your pin connections: http://www.datasheetsite.com/extpdf.php?q=http%3A%2F%2Fpdfdata.datasheetsite.com%2Fpdf1%2FPhilips%2FC106D.pdf [Broken]
    Last edited by a moderator: May 5, 2017
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