[Elecraft] K3 SHUTDOWN.

[Adrian]

The K3 can run 100w RTTY all day long without excessive PA temperatures.
Just do it with 15v and a decent power cable/PS.
Going to 110w is not recommended as the power consumption wasted rises
considerably for just the extra 10w.

-----Original Message-----
From: elecraft-bounces at mailman.qth.net
[mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Lyle Johnson
Sent: Sunday, 30 September 2012 6:53 AM
To: elecraft at mailman.qth.net
Subject: Re: [Elecraft] K3 SHUTDOWN.

In the end, it is a THERMAL problem.

The K3 PA heatsink+fan has a limited, finite thermal resistance to the
ambient air, as well as a limited thermal mass of its own to absorb heat.
The heat dissipated in the PA transistors has to go somewhere, and by design
and construction it goes into the heatsink.  The fan uses moving ambient air
to reduce the thermal resistance of the heat sink (or perhaps you can view
this as increasing its heat capacity).

Now, if your ambient air is cold and dense, and perhaps a bit moist, it has
relatively larger heat capacity than if it is hot, thin and dry.  
Put another way, you can run more power in high duty-cycle modes if your
operating environment is cool and your altitude low than you can in a high
desert.

To find out how much power you can safely run in a high duty cycle mode,,you
should run the PA (preferably into an adequate dummy load) and monitor its
temperature rise, rate of rise and final stable temperature for various
power levels and bands (since the efficiency typically varies from band to
band) while noting your ambient air temperature (and perhaps humidity).  If
you do this, you'll be able to figure out the temperature difference between
the PA and the ambient air, and how it is affected by the power you are
running or wanting to run.  Or if you need an external fan, or redirect an
air conditioning duct towards the radio, or...

Then, BEFORE the contest, while your mind is still clear, you can determine
what power setting you can safely run on each band, and set up the K3 for
that power level.  You can now run the contest with one more piece of
station engineering firmly in hand for a competitive edge (either by having
a bit more power, or by not having the radio shut down when working that
rare multiplier).

As a bit of encouragement to spend a little time doing this, you may be
surprised at the PA power dissipation versus power output. And you may learn
something about thermal design, resistance, and so forth.

73,

Lyle KK7P

> ...I have not seen any caveats either in the Elecraft manual, or in 
> Fred's book about not being able to run at 100 watts...
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