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Christmas Light Fader

Discussion in 'Electronic Design' started by Jerry Girard, Dec 12, 2003.

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  1. Jerry Girard

    Jerry Girard Guest

    Okay, so I asked anyone a few months ago for a schematic of a Christmas
    light crossfader using discrete components. Instead, I took several
    people's advice and built it using a PIC chip. I did it for the capability
    of added features. Anyway, I have the circuit functioning except for one
    problem. Please see link to schematic:

    There are two channels that crossfade back-and-forth between each other
    evenly. When cross fading, I am getting cross firing of the adjacent triac
    which is causing a random flicker. When I disable one triac or pull off its
    load, the other fires perfectly at a nice steady fading rate. With both
    loads enabled, I get random flicker. They seem to be doing it on the
    negative phase cycle.

    I am using Teccor L4004L3 sensitive gate triacs. In the code, I either
    tri-state of logic-high the port pins to fire. My layout seems sound. Are
    there some additional components I should be using to prevent cross talk?
  2. I assume the word after tri-state should have been or. You certainly
    do not want to be leaving the TRIAC gates floating if you want to
    avoid misfiring. So do not tri-state the outputs. Swing them between
    high and low, only.

    I would be interested in seeing your layout. I would run the neutral
    wire to the TRIACs first and then to the supply capacitors for the PIC
    chip. This will keep any inductive ringing in the load circuit from
    getting between the TRIACs and the PIC.
  3. Jerry Girard

    Jerry Girard Guest

    Yes, the word is "or". I tri-state then logic-high the outputs to fire.
    I've tried logic-low to logic-high with no better results.

    The problem is not with the power supply and the PIC. The problems is with
    false triggering of the TRIACs. I've been doing some research and this
    seems to be a common problem. The cure is a RC snubber network across MT1
    and MT2. I've tried 100 ohm and .1uF and that seems to only make it worse.
    So I don't know what the deal is.

    The L4004L3 is a very sensitive part needing only 3mA to turn on. I think
    they are just too sensitive for my application. I might consider ordering
    some other versions that aren't as sensitive.
  4. I've done this kind of thing several times. Get a MOC3012, which is a triac
    output optoisolator. You can run it directly from the PIC, and it will
    control the triac quite well.

    You can get it from or locally, I'm guessing.

    The other issue is that when the PIC's gate is on, you are subjecting the
    protection diodes to quite a bit of voltage. The gate on a triac seems to
    follow the line voltage. You'll fry the port if you aren't careful.

    Bob Monsen
  5. Robbie Banks

    Robbie Banks Guest

    Sorry, can't offer more than has been given below. I take it you are using
    the ADC to determine the voltage input from the pots? Would you be prepared
    to share this code with me, as I'm looking for something like that.

  6. Jerry Girard

    Jerry Girard Guest

    Yes, I could share the code, though it isn't any of my better work. It's
    written in all C. I would include the HEX file, so if you want to flash it
    with that, you won't have to worry about recompiling.

    How it works is this:

    Fade pot - Controls the crossfading speed from 100mS to 5S.
    Period pot - Controls the period between crossfades from 100mS to 5S.

    When the pots are turned all the way up past their 95% position, they switch
    to pseudo-random timing.

    It's actually a nice little device.

    I'm still refining everything so I will post is when I'm complete in the
    next few days.
  7. I read in that Jerry Girard <>
    I predict December 26. It always takes longer than you think. (;-)
  8. Fred Bloggs

    Fred Bloggs Guest

    You can make it insensitive by dropping a shunt resistor off that gate,
    and you would do well to listen to Popelish- despite what /you/ may
    think, your layout is crap. When you fire a TRIAC there is a surge
    current on the neutral, and during the negative cycle this will make the
    main terminal of the OFF TRIAC negative with respect to the gate- so it
    fires. This is fixed by running two separate wires from the VCC filter
    capacitor neutral connection to each TRIAC terminal. You also need to
    unbundle and keep separate the hot side wires from the TRIACs to the
    bulbs to avoid capacitively coupled cross-talk into the OFF gate- if
    this is not possible then a small inductance like 10uH in series with
    each hot side lead at the TRIAC-in combination with parallel TRIAC
    snubber 0.1U + 100R- to kill the slew-rate- will fix the problem.
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