Just a note, any voltage above 100VAC RMS and 150VDC  I use grater than 1W
resistors regardless of power, as they have a higher voltage rating and do
not arc over, been caught on strings of 1/4W, one fails and they all go
flash.

Just remember all good component suppliers supply the working and breakdown
voltage for their resistors, use the catalogue.....

May the Data sheet always be with you ;o) <said in best dathvader voice ;o)>

Regards,

Kat (in a silly festive mood)
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----- Original Message -----
From: "Augusto de Conto" <augusto@AUTOMACAO.ENG.BR>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Saturday, December 15, 2001 10:56 AM
Subject: Re: [EE] question about light dimmer circuit


Eben

No. It is wrong.
The 1k Resistor is a current limitator.
The equation you must use is P = R.(i^2). Consequence of P=i.U and U=R.i
"R" is easy: 1k
"i" is almos equal the Triac Gate current. It depends of the Triac you
choose.
    You must see in it's DataSheet. Generaly it is very low.
I am not shure, but I guess that a 1/4W will work ok.

Augusto de Conto
augusto@automacao.eng.br

  ----- Original Message -----
  From: Eben Olson
  To: PICLIST@MITVMA.MIT.EDU
  Sent: Friday, December 14, 2001 10:24 PM
  Subject: [EE] question about light dimmer circuit


  Hi all. I'm trying to build some PIC-controlled light dimmers for my
school theater. After looking at some schematics from the internet, I pieced
together the attached basic diagram. But if anyone has experience with this
kind of thing, I have a few questions. I'm very confused about how to
calculate the wattages required for the resistors. Using V=IR, and P=VI, it
seems like the 1k resistor between AC supply and the MOC triac trigger would
have to take about 15 watts. Is this correct?
  TIA,
      Eben Olson

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