RE: Dimmers

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Daniel Barrett" <dbarrett1-at-austin.rr-dot-com> 

	I Agree completely- I meant that the *leading* edge of the pulse is
modified. The trailing edge is always the zero crossing of the AC sine
waveform. So the pulse begins at 0-90 degrees on the positive half cycle and
180-270 degrees for the negative half cycle, depending on the knob setting.
db

-----Original Message-----
From: dhmccauley-at-spacecatlighting-dot-com
[mailto:dhmccauley-at-spacecatlighting-dot-com]
Sent: Monday, November 24, 2003 9:32 PM
To: Tesla list
Cc: dbarrett1-at-austin.rr-dot-com
Subject: Re: Dimmers


 > Hi Jim-
 > The rate doesn't change, the phase angle changes. If the dimmer is at 50%,
 > the output will be off for the first 90 degrees of the of the AC waveform,
 > on for the second quadrant, off for the 3rd, etc. As you vary one from
full
 > on to full off, the firing angle becomes later in the cycle.
 > db

Thats not entirely correct either.  The phase of the output of a dimmer
switch is identical to the phase of the input.  A dimmer switch merely
truncates a portion of the
original AC sinewave beginning at the first zero crossing.  And since the
TRIAC is a latching device, once the TRIAC is turned on and conducting, it
will remain on until
the next zero crossing at where this process will repeat.

Dan