piclist 1998\10\21\132339a >
Thread: Light / Lamp dimmer
www.piclist.com/techref/index.htm?key=light+lamp+dimmer
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face BY : Harri Suomalainen email (remove spam text)



>>        Another poster almost said this, but not quite so I will.  The
>>thing to keep in mind is that there is only one frequency to worry
>>about synchronizing to and only one zero-crossing to look for no
..
>>just keep a particular bit low until the counter ran out and then go
>>high.

I've been thinking on this one too and my desided solution has been
this: Detect zero crossing the usual way. However, this may have some
noise with it if power line is very noisy. So, it is not too reliable.

The solution seems to make timer track zero crossings. When the frequency
is roughly fixed (or exact, depending on where you are located) it should
not be too difficult. All you need to do is to measure time from one
zero crossing to another. Adjust timer *slowly* to keep the timer in
sync. (Now it's like a PLL.)

After a few cycles timer interrupts should be coming syncronously with
zero crossings. So, to make sure triacs are triggered you may take a bit
advance and fire the triac a bit earlier to take account of the
time it takes to conduct. This should give you a very clean switching
at the exact point you want.

This approach would be a very well behaving one. No misfire at wrong
point should occur ever. This is very important if the load is something
nasty which might cause triacs to pop for every wrong firing point. Not
everyone is "dimming" just lamps. Things like a transformer as a load
would work more nicely with this approach.

Yeah, to keep things *not* flickering, make sure you have the firing
pulse that works. It should have enough current. It should be long
enough to enable current rise to a level where triac keeps conducting.
Short pulses near zero-crossing might actually make triac conduct and
turn off as soon as pulse is removed because there is just not enough
current flowing yet. With small loads you might sometimes need to
consider this too. (Depending on triac, values like 50-100mA mininum
current to keep device conducting are not uncommon!)

The result would be a triac fired never (conduct only near zero voltage
during the firing pulse). As the device has unsymmetrical nature
smaller current might keep it conducting at one half cycle while a bit
more current is required for the other half cycle. At some point the
result would be half-power like someone described here earlier. (Device
would conduct half the time.)


--
Harri Suomalainen     KILLspamhabaspamspam_OUTcc.hut.fi

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