Re: Controlling Holiday Lights

[Marc_F_Hult <MFHult@xxxxxxxxxxxxxxxxxxxxx>]

Not to worry ;-)

A TRIAC controlling 60hz AC can only be switched on (it turns _itself_ off) at
a maximum rate of 120 hz no matter what the input. So regardless of the
frequency of the input, and regardless of the waveform, there will be a
maximum number of 120 transitions per second. (IOW, what sylvan is trying to
do does not actually work.)

The maximum output slew rate (and potential noise) occurs when the AC waveform
is at 90 and 270 degrees (= peak voltage of 170 volts, rms output voltage for
the entire half cycle = 85Vrms, and ~33% output for typical tungsten lamp
compared to full output). These calculations assume 120VAC and neglect the
band-gap and IR drop (loss owing to resistance) of about 1-1/2 volts.

See http://www.econtrol.org/dimmers/TRIACDimmerCalcs_MFHult.pdf

... Marc
Marc_F_Hult
www.EControl.org


On Thu, 02 Nov 2006 23:30:07 GMT, "Max" <nospam@xxxxxxxxxxx> wrote in message
<3cv2h.78624$E67.37158@clgrps13>:

>If I am understanding what you are trying to accomplish I would
>have two possible concerns, 1) What kind of noise would be
>generated by switching that fast and 2) what kind of heat would
>be generated by having your control device switching on and off
>that much?
>
>
>"sylvan butler" <ZsdbUse1+noZs_0611@xxxxxxxxxxxxxxxxxxxxx> wrote in message
>news:slrnekkfah.8g4.ZsdbUse1+noZs_0611@xxxxxxxxxxxxxxxxxx
>> On 2 Nov 2006 03:43:55 GMT, Dan Lanciani <ddl@danlan.*com> wrote:
>>> the specific triac trigger waveform you use for 50% power.  Could you
>>> describe it in terms of frequency and duty cycle or such?
>>
>> It isn't a simple wave.  :)
>>
>> I have not measured or attempted to calculate 50% power out of the
>> triac.  I don't have the scope out right now, so I just played a bit
>> with software...
>>
>> Full speed alternating on/off (a 1:1 duty cycle, which produces a fairly
>> nice square wave on the gate) has no obvious effect on intensity.  A
>> full speed 1:4 (on:off) duty cycle noticably dims the light, maybe a 10%
>> power reduction.  At full speed with a simple wave (e.g. all on then all
>> off for regular periods) I cannot get down to a 50% power output without
>> flicker -- the off times are too long.
>>
>> (This system was a C633 the last time I had the scope on the output, and
>> at that time full speed 1:1 produced a square wave of about 150khz,
>> IIRC.  Now it is PIII 750, but I expect that bus waits will keep the
>> output about the same since a k6-2/400 was also about the same.)
>>
>> Crude rate limiting to approximately two samples per tick (reprogrammed
>> the timer to 1000 ticks per second) I can get lower power but I start
>> getting flicker somewhere around the 1:4 on:off ratio.  I expect
>> interrupt overhead is becoming significant and my delay loop between my
>> two samples per tick is very crude.
>>
>> So now I go to a sample table where I can generate more complex waves...
>>
>> Full speed using a 200 sample table I get an estimated 50% power (light
>> very orange, but still bright enough to fill the globe) when I do a
>> pattern with a duty cycle of about 1:2 (on:off) somewhat randomly
>> distributed within the table (it was actually 64 on, 136 off).  Using
>> the same table at 2 samples per tick (default frequency 18.x/sec)
>> results in very dim light, sometimes a bit of flicker.
>>
>> Using a PIC it might be easier to sync with the zero-crossing and get
>> some determinism.  I've googled a bit, and syncing a PIC seems pretty
>> simple.  But I don't want to input that back to my PC parallel port.
>> Plus the real-time 120x/second is a bother.  I maybe should have saved
>> that data table...  :( Just keyed it all in directly to memory with
>> debug.)
>>
>> sdb
>>
>>
>> --
>> Wanted:  Omnibook 800 & accessories, cheap, working or not
>> sdbuse1 on mailhost bigfoot.com
>>
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