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Detecting triac failure whether under load or not

  1. Jul 20, 2012 #1
    [First post here, please make allowances..]

    I want to use a triac as a solid-state SPST switch for a normal 120VAC receptacle. The on/off state will be controlled by a low-voltage DC circuit through an opto-isolation MOC3020.

    http://ken.coar.org/images/opto-triac.png [Broken]

    So far so good. However, I want to be able to detect faults in either the diac or the triac -- faults being defined as:

    • the diac conducting when TP1 is low, or failing to conduct when TP1 is high; and/or
    • the triac conducting when the diac isn't (VTP2 = 0), or failing to conduct when the diac is.

    I'm basically a low-voltage DC kinda guy, so I'm unclear on how to detect the lack of signal at TP2 and TP3 in general, or at TP3 when nothing's plugged into J1 in particular.

    Any/all pointers appreciated..

    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Jul 20, 2012 #2


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    Staff: Mentor

    Welcome to the PF.

    First, your switch triac D1 needs to be in series with the Hot lead, not the Neutral lead as you have it shown now. What does the symbol in the Hot leg represent? Is it a motor or something?

    To tell if you have a fault, I think the best/easiest thing would be to see if you have the high voltage after the switch D1 (after you move it over to the Hot side). If you can sense that voltage and process it, you will know if your circuit is working correctly.

    To sense that line voltage after the triac, you can use a large value resistance and an analog optocoupler or similar arrangement. Do you have a microcontroller (uC) that you are going to be using to control the circuit? Does it have an ADC on-board?
    Last edited by a moderator: May 6, 2017
  4. Jul 20, 2012 #3
    Circuit diagram updated. It was just meant to be illustrative, anyway, not exact. :)

    As I mentioned, it's a receptacle. Whatever is having its power switched by the remote DC circuit.

    Indeed; it's the 'how to sense it' part that's my problem.

    How do you mean? I'm not visualising this.

    Nope, just a simple momentary SPST switch, a 555 monostable, and TTL signals.
  5. Jul 22, 2012 #4


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    Staff: Mentor

    Hmm, if you don't have a uC, what are you going to do with the fault information? A small PIC with an internal ADC costs less than $1...
  6. Jul 23, 2012 #5
    Simply enough, the fault signal is just going to red-light an indicator.

    Yeah, a µC is cheap enough, but this entire circuit is going to be triplicated; I have three receptacles to be controlled by three separate circuits. I may end up using one eventually, but for now I'm just trying to sort out the sensing details.

    This is where I've gotten so far. Aside from putting a couple of Zeners across the input to V02 for overvoltage protection, I'm now trying to figure out the rating/part number for the sensing transformer.

    http://ken.coar.org/images/opto-triac3.png [Broken]

    And I have no clew about the transformer. I'm used to step-down and step-up power transformers, not this sort of thing. So I'm still feeling my way.
    Last edited by a moderator: May 6, 2017
  7. Jul 25, 2012 #6
    Meh. I just noticed I put the transformer in backwards.

    http://ken.coar.org/images/opto-triac4.png [Broken]
    Last edited by a moderator: May 6, 2017
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