On Fri, 14 May 1999 08:16:16 -0400, Jason Wolfson wrote:

>Today I'm trying to control a 1800W electric heater with a PIC.
>About 16A @ 110VAC, I was thinking of using a 25A Triac and
>optocoupler/driver.
>My question is how hot will the Triac get? is there a better way? cheaper?
>thanks

1) Don't for get about the inrush currents.  Within the 1st
half cycle you will have a current of approx 2.5 times the
rated load or 40 amps in your case.  Make sure your device
is rated for this surge current or you will cause
pre-mature failure.

2) How much of an inductive load is the heater going to
present?  Triac's have a habit of not shutting off under an
inductive load unless the snubber cap is designed properly.
 According to the Motorolla Thyristory databook snubber
desing is kind of an art form.  The equations get you close
but the rest is trial and error.

Perhaps you should consider an inverse parallel SCR design.
No inductive problems and as a bonus you share the load
across 2 devices which should ease your heat concerns.

I am interested in seeing whatever PID code you come up
with.  I just finished a project where I control a 192kW
cheston heater.  It will heat a steel bar from ambient to
bright glowing red hot in 6 seconds! The original spec did
not call for any kind of PID control but of course it
really did require it.  So I had to hack in a kind of
bastard PID to keep the inrush from blowing fuses.



--
Richard A. Smith                         Bitworks, Inc.
rsmith@bitworks.com               501.521.3908
Sr. Design Engineer        http://www.bitworks.com