[Elecraft] Thermal control for Q1 / Q2 in KPA100?

[Ron D'Eau Claire]

The question of grease vs. pads came up before and Eric answered it as
follows:

We officially recommend that you use the thermal pads supplied with the
KPA100 kit instead of grease. They consist of a very efficient thermally
conductive compound coated on both sides a a very thin aluminum sheet. The
compound reflows when the transistors get hot the first time and fills all
of the small air gaps between the devices and the heatsink. It is very
effective and provides a better thermal bond in most cases than manual
application of thermal grease.

73, Eric  WA6HHQ
Elecraft
=========

You aren't the only one to be bothered by what I call the "whizzer" on the
back of the K2, although I confess I'm getting used to it. Sort of...

The fan doesn't just blow air into the "box", it forces air into the space
between the KPA100 shield and the KPA100 heat sink, which includes the
KPA100 PCB of course. The air exits at the end of the heat sink nearest the
front panel, flowing out through the spaces between the heat sink fins. 

One thing some builders have done is to put a large muffin fan on top of the
heat sink. A larger, slower speed fan should be a lot quieter. 

Of course, how quickly the fan comes on depends upon the ambient temperature
in the shack and the duty cycle as well as the power level and efficiency of
the finals. I don't know if anyone has run tests to see what the minimum fan
action has to be, but I know that I've run tests with my K2/100 that
involved keying it so much I couldn't leave my hand on the heat sink. No
harm was done to the KPA100 in those tests.  

Ron AC7AC


-----Original Message-----
As I'm thinking about trying to operate the K2/100 so the fan doesn't run
(or doesn't make noise) I wonder some things.
 
First, blowing air out of a box cools the box better than blowing air into
it.  I'm half surprised the fan blows in.  I suppose it is to direct the
stream of air on the finals.  Still, I wonder about doing it differently.
 
What about thermal grease?  There is only a non-grease pad between the
transistors and the heat sink.  Would a dab of grease conduct more heat away
from the transistor?  I have some fancy "silver" stuff left over from my
computer assembly activities.
 
What power level should I run if I don't want the fan to come on?  75 Watts?
50, 30, 12?
 
- Keith N1AS -
- K2 5411.ssb.100 - _______________________________________________