[HomeBrew] Heatsinks

[Dick Foster]

There is a thermal analogy to Ohm's Law.  It's a useful approximation for a
lot of applications.  The "voltage drop" becomes a temperature drop, and the
"current flow" becomes an energy flow in watts.  So, R=V/I (Ohm's Law)
becomes Rthermal = deltaT / Power.  Or, deltaT = Rthermal X Power.  For
example, if you have a heat sink with a natural convection (without fan)
rating of 10 degrees C per watt and your TO-220 device dissipates 5 watts,
the temperature of the TO-220 case will rise 5x10= 50 Centigrade degrees
above the ambient temperature.  If the heat sink is sitting in an air
conditioned room at 22 degrees C, the TO-220 will heat up to 22+50= 72C.
You want this temperature to be well under the maximum rated temperature for
the device in the TO-220.

This explanation is simplistic, because there are other temperature rises to
be concerned with.  If the heat sink is inside a cabinet, the other devices
will heat up the air in the cabinet raising the ambient temperature that the
heat sink "sees".  Also, there are temperature rises between the heat sink
and the TO-220 case (use thermal grease to minimize this), and between the
case and the device inside.  Manufacturers often spec the "maximum junction
temperature" of the device.  The junctions usually are on the top side of
the chip, so a detailed thermal evaluation would include the temperature
rise from the back to the front of the chip as well.

If they specify "forced convection" for the heat sink, see how much air flow
they are assuming.  You'll have to have a fan that provids that much air
flow in that case.

Very 73,
Dick  W5TA

----- Original Message ----- 
From: "Durwydd MacTara" <durwydd@hotmail.com>
To: <Homebrew@mailman.qth.net>
Sent: Wednesday, September 03, 2003 10:59 AM
Subject: [HomeBrew] Heatsinks


> ** Please do NOT cross-post messages to multiple mailing lists on the
"To:" or "CC:" line of the e-mail message. **
>
> what does the term "thermal resistance" mean in the catalog
specifications?
> I am looking for a heavier/more efficient heatsink to use for a TO-220
> Mosfet in an amp...
>
>
>   "Communications" is a People skill, the rest is technology.
>              Durwydd MacTara