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Retriggering TRIACs — Parallax Forums

Retriggering TRIACs

TinkersALotTinkersALot Posts: 535
edited 2011-06-09 13:50 in General Discussion
I am driving one of these : http://www.futurlec.com/SSRAC.shtml

with a propeller based board. Any channel is enabled whenever a logic zero is put on to the channel's control line. That behvavior is already demonstrable (control the output channel on the board, no loads are attached yet).

When I connect the loads to each channel (a 90 Watt flood), will the lamp "go out" when the AC crosses zero?

And if that is true, In order to keep the 120 VAC flowing, should I:

Send a pulse-train to the channel control line to keep the triac triggered?
If I don't care about switching noise and or the phase synch of the pulse train, does it matter if I enable/disable the control line completly out of synch with the zero crossing of the AC?

Any advise that anyone has would be greatly appreciated.

Thanks in advance

Comments

  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2011-06-09 09:33
    AC Crossing zero generally shuts off the TRIAC. How to keep it going is a bit more complex because you have to stay in synch with the AC at 60 cycles (or 50). I don't see how not caring about phase synch is going to make this easy.

    A schematic would be helpful. One might use some logic chips and opto-isolation to provide a 60 cycle source and that would somewhere be included.
  • TinkersALotTinkersALot Posts: 535
    edited 2011-06-09 09:38
    Well, what I was wondering is if I send a pulse train of maybe 200 Hz and just keep re-triggering the TRIAC if that would just keep the load lit.
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2011-06-09 09:51
    It'll conduct if its gate is kept biased "ON", just as an SSR will.

    The sync stuff is only for fading and the like.

    The 200Hz idea will result in harmonic generation (broadcast.)
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2011-06-09 09:52
    Well, maybe a bit hit and miss. Since the triac defaults to an off, you can try it and you will likely get an odd flickering light of some sort.

    Tracy Allen seems to offer a better solution.
  • TinkersALotTinkersALot Posts: 535
    edited 2011-06-09 10:31
    @PJ -- so are you saying that if I just hold the control signal for the channel low, the load will stay on?
  • kwinnkwinn Posts: 8,697
    edited 2011-06-09 10:45
    Holding the control signal low will keep the channel on assuming it is turned on by a low to start with. You do not want to send short pulses on the control line unless you can synchronize them to the AC zero crossings. With a zero crossing reference signal and a microcontroller you can use timed pulses to control power to the loads. For example, you could use it to dim lights.
  • TinkersALotTinkersALot Posts: 535
    edited 2011-06-09 10:50
    @ALL-of-you : thank you for the replies! Very helpful. So, it sounds like I don't need to retrigger the triac at all then.

    Thanks again!
  • Tracy AllenTracy Allen Posts: 6,667
    edited 2011-06-09 11:05
    That was PJ Allen not Tracy Allen, but I do agree with what PJ said. Keep current flowing in the triac gate and it will refire at every zero crossing. That controller board likely on-board circuitry that sits between the optoisolator and the triac, so you couldn't use it as a dimmer even if you wanted to. It looks like it is meant only for on-off control.

    Hey! Is this for the Bear detector you showed me at UPEW?
  • TinkersALotTinkersALot Posts: 535
    edited 2011-06-09 11:16
    @Tracy -- no -- I've been asked to build a pair of race-staging start lights (aka christmas trees) for this event: www.ncderby.com
  • kwinnkwinn Posts: 8,697
    edited 2011-06-09 11:25
    Tracy Allen wrote: »
    That was PJ Allen not Tracy Allen, but I do agree with what PJ said. Keep current flowing in the triac gate and it will refire at every zero crossing. That controller board likely on-board circuitry that sits between the optoisolator and the triac, so you couldn't use it as a dimmer even if you wanted to. It looks like it is meant only for on-off control.

    Hey! Is this for the Bear detector you showed me at UPEW?

    In most cases boards like this use a ULN chip or individual transistors to drive the leds in the optoisolators. The optoisolator is a triac output, and the only circuitry between the optotriac and the gate of the power triac is a small capacitor. Can't be absolutely sure that is the case for this board but it certainly looks like it. If that is the case it should be possible to trigger the triac at any point in the ac cycle by powering the led at the appropriate time.
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2011-06-09 13:03
    Now that I've offended PJ and Tracy, i just wish you good luck and be on my way.
  • Tracy AllenTracy Allen Posts: 6,667
    edited 2011-06-09 13:50
    Hehe. No offence taken. It happens. No relation, I think, but us Allens (there are more of us you know) have to make a better effort to distinguish ourselves.
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