Question about opto-triacs

Discussion in 'General Electronics Chat' started by daviddeakin, Nov 30, 2009.

  1. daviddeakin

    Thread Starter Active Member

    Aug 6, 2009
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    Hi,
    I am about to start a project which I think will require a random-phase opto-triac (probably MOC3022 http://www.farnell.com/datasheets/94947.pdf )
    It will be used to allow a full-wave rectified signal pass to the load. Ideally I want the triac to trigger on the peak of each rectified signal, and remain on until the voltage drops to zero again. I also hope to add a control to allow the triac to trigger at some time after the peak.

    My question is, do opto-triacs remain on only while the LED is lit? Or, if you apply a quick pulse to the LED, will it still remain on until the MT2-MT1 voltage drops to zero again?
     
  2. Duane P Wetick

    Active Member

    Apr 23, 2009
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    Do you want to control power to your load (phase control) or turn on the triac for 1 cycle or let a number of cycles pass; (heating or burst control)? All of these are possible with triacs. It depends upon the triggering methods used. Usually the triac stays on until the current goes to 0.

    Hope this helps, DPW [ Spent years making heaters out of op-amps ].
     
  3. daviddeakin

    Thread Starter Active Member

    Aug 6, 2009
    207
    27
    It is for a curve tracer- I want to sweep the voltage across the load, but keep load dissipation low. So basically I want to turn full-wave rectified pulses into a kind of saw-tooth. Varying the phase of the signal sent to the LED would allow me to reduce the max voltage fed to the load, for testing lower-voltage devices.

    What I want to know is, do I have to keep the LED on all the time I want load voltage, or can I just apply a quick pulse to the LED and the triac will remain on for the rest of that half cycle? It sounds like only a quick pulse is required, from what you say.
     
    Last edited: Nov 30, 2009
  4. daviddeakin

    Thread Starter Active Member

    Aug 6, 2009
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    Here's what I mean:
     
  5. Duane P Wetick

    Active Member

    Apr 23, 2009
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    If your input ia a full-wave rectifier output (120 Hz.), then a SCR will work for you. There are devices called Diacs which will establish a gate trigger point which can be varied by RC network and potentiometer. You can probably find such a circuit on this forum by searching for phase control.

    Cheers, DPW
     
  6. daviddeakin

    Thread Starter Active Member

    Aug 6, 2009
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    Thanks Duane, I know the circuit you mean. I was a little concerned about it though, as I would have almost 400Vpeak on the pot, relative to the body, and I wasn't sure if I can readily obtain pots with that kind of insulation rating. I will have a look around though!
     
  7. daviddeakin

    Thread Starter Active Member

    Aug 6, 2009
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    Does anyone know of a high-voltage photo-transistor? Something I could use to switch on-and-off roughly 500Vdc at a few tens of milliamps?
     
  8. Wendy

    Moderator

    Mar 24, 2008
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    2,536
    Have you ever looked up or studied SSRs? (Solid State Relays)
     
  9. daviddeakin

    Thread Starter Active Member

    Aug 6, 2009
    207
    27
    I have now, and I think I have found something suitable, thanks!
     
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