HomeRoast Digest


Topic: triacs and airflow (4 msgs / 243 lines)
1) From: Kevin DuPre
The heat dissipated by a triac is a result of the fact
that semiconductors do not instantaneously turn on and
turn off. There is a brief period of time during turn
on and turn off and vice-versa during which it appears
resistive due to rise and fall times.  It is this
apparent resistance causing a voltage drop and
multiplied by the current results in a power
dissipation.
The THERMAL TIME CONSTANT of the heat sink is
significant (thermal time constant is a result of the
VOLUME and SURFACE area of the aluminum), and why one
would think that it's simply a function of size.  
Typically a heat sink has a thick base (read higher
volume to heat up), and thin fins (read lower volume
with high losses into the surrounding air).
There are two thermodynamic behaviors here involved -
the triac's heat attempting to reach equilibrium with
the heat sink and the heat sink's absorbed heat trying
to reach equilibrium with its ambient environment.
If you reduce the area and volume of the heatsink, you
need to increase the air flow and if you reduce the
airflow  you need to increase the surface area.
As far as what causes a dimmer to have a lower or
higher rating - it is partially a result of the
capacity of the triac and partially a result of how it
is controlled.
Triacs have 2 halves, each of which looks like a
silicon-controlled rectifier (SCR)and each of which
conducts one half of the line cycle.  Semiconductor
SCRs are rated by their ability to handle current
density or I-squared T (I^2 * t), where t is the
amount of time the SCR is switched on before it
reverse biases and shuts off due to the other half of
the line cycle.  Reduce the maximum time it is
switched on and the current handling capability goes
up by a squared proportion - to a point. That's where
the switch-on and switch-off times reduce the
efficiency. 
That's the semiconductor science behind a triac. From
the standpoint of the heat-sink the above
recommendation holds true.  If you could calculate the
surface area of your new heat sink and see how it
compares to the original, all things being equal,
you'd have a pretty good idea of its cooling capacity.
Exact science on the selection of heat sink however is
pretty much non-existent, at least what is available
to the general and engineering public. Often the heat
sink selection results from a manufacturers
recommendation or choosing one so big that it can't
possibly saturate and result in semiconductor
meltdown.
If you're looking at a CPU heatsink with a built-in
fan you're probably far better off than original
equipment for the dimmer.
Regards,
Kevin
<Snip>
=====
--
Kevin DuPre
obxwindsurfhttp://profiles.yahoo.com/obxwindsurf"The real voyage of discovery consists not in seeking new landscapes but in having new eyes -- Marcel Proust"
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2) From: Ben Treichel
Kevin DuPre wrote:
<Snip>
Just don't forget to use 'thermal paste' between the triac and the 
heatsink. It really is important!
<Snip>
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3) From: EskWIRED
<Snip>
Good point.  I've got a tube from when I upgraded an old cpu in an old
'puter.  That's why I have an old heat sink laying around too.
Anybody need a xx486/33?
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4) From: R.N.Kyle
This is a multi-part message in MIME format.
Dang Kevin, that made my head hurt. I wish I understood all that. I'm =
impressed.
Ron Kyle
a coffee roaster from South Carolina
rnkyle


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