When I did my zero cross light dimming I ran into the same issue, and
it turned out to be my timing was off.  When I went to 'full bright'
the triac was actually triggering just before the zero cross, and then
turning back off again, instead of triggering just after the zero
cross.

Be aware that the zero crossing scheme you implement gives you a
positive pulse after the actual zero cross.  You might instead start
your timing on the falling edge of that signal, which will start
timing just before the zero cross, and put the "light out" condition
at the dark end of your scale.

Either way, if you have an oscilloscope then you can view the timing
relationships easily enough (zero cross, triac trigger, AC line) but
if not you can calculate everything and find out where the timing it
off.

You can see a little bit of what I did here:
http://www.ubasics.com/adam/electronics/doc/phasecon.shtml

Good luck!

-Adam

On 3/10/08, Gayan Soyza <gayan1.lk@gmail.com> wrote:
> Hi all
>
> I'm fading a 230V bulb. Using zero cross on an interrupt pin.
>
> The operation is it starts from full brightness & fades down to zero
> brightness & starts to increase the brightness to its maximum & continue the
> operation.
>
> I wrote a coding & its works the only problem is when it's reached maximum
> brightness the bulb is turning off for a moment (its turning off for a very
> small time) & after its working nicely. When in the next full brightness
> again bulb is turning off for a moment & after its working nicely.
>
> My TRIAC is BT136 on GP0 via 390R resister.
> Zero cross detecting via 1M resister on GP2 interrupt directly on AC mains.
> Interrupt on every 10mS for (50Hz) using only positive cycle to trigger the
> gate.
> Using 256 steps to dim the bulb
> [CODE]
> ;********************
> ;detecting zero cross
> ;********************
>
>        org    0004h
>        btfsc    INTCON,INTF    ;is it a zero cross
>        goto    Select        ;yes
>        goto    Exit        ;no
>
> ;*************************************************************
> ;turning on the timer 36uS (Delay) X 255 (Steps) < 10ms (9180)
> ;*************************************************************
> Select
>        btfss    Change,1    ;select inc or dec the brightness
>        goto    Decrease    ;
>        goto    Increase    ;
>
> Decrease
>        incfsz    S_Count,F    ;fade from fully brightness to dim
>        goto    $+3
>        bsf    Change,1
>        goto    Increase_L
>        movf    S_Count,W    ;backup dim level
>        movwf    W_Count        ;//
>        call    Delay
>        decfsz    W_Count,F
>        goto    $-2
> Fire
>        bsf    GPIO,GP0    ;makes a short pulse
>        call    Delay        ;about 33uS delay
>        bcf    GPIO,GP0    ;to latcht the triac
>        goto    Exit
>
> Increase_L
>        movlw    .255
>        movwf    S_Count
>        goto    Exit
>
> Increase
>        decfsz    S_Count,F    ;fade from dim to fully brightness
>        goto    $+3
>        bcf    Change,1
>        goto    Fire
>        movf    S_Count,W    ;backup dim level
>        movwf    W_Count        ;//
>        call    Delay
>        decfsz    W_Count,F
>        goto    $-2
>        goto    Fire
>
> Exit
>        bcf    INTCON,INTF
>        retfie
>
> Delay
>        movlw    .9        ;36uS delay including call
>        movwf    d1
>        decfsz    d1,F
>        goto    $-1
>        nop
>        return    [CODE]
>
> --
> Electronics God
>
> --
> http://www.piclist.com PIC/SX FAQ & list archive
> View/change your membership options at
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>
>
>


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