[Elecraft] Safe power output to maximize finals lifetime

[Don Wilhelm]

Andreas,

The correct answer is --- it all depends ---

What will damage semiconductors (the K2 finals included) is heat.  Actually
the device junction temperature is the thing that is important, but the
device case and the heat sink on which it is mounted conduct heat away from
the junction to keep the temperature down - this takes a finite length of
time, so the 'secret to success' is to add power (heat) no faster than it is
dissipated.  The device case and heat sink also average this heat 'soaking'
over time too, so you can run more power for short intervals with the same
effect as running less power over a longer interval.  As an extreme example,
radar (pulse modulation) can develop a LOT of power from a device because
the pulses are short and widely spaced, and no more harm is done to the
device than a much lower power applied continuously (assuming that the
voltage and current limits of the semiconductor are not exceeded).

If you are running SSB, you can usually run the max - unless you arealso
using max compression settings to boost your average power (that will heat
things up a bit).

CW has about a 50% duty cycle and can usually run at max power unless you
are into long winded transmissions.

But digital modes like PSK31 create a 100% duty cycle, so when using these
modes, it is good to throttle back the power control to 50 watts or so to
keep the finals cool and happy.

If you operate with full power over extended periods of time (and with high
ambient temperatures), an auxiliary cooling fan to move more air over the
heat sink fins may be a good measure.
If you use the heat sink temperature as a guide, that will give you a good
indicator to tell you whether you should reduce power or not.

Last, but not least, the finals in the K2 are conservatively rated at 100
watts - they could just as well be rated at 150 watts using the same devices
(probably with a bigger heat sink), so just keep tabs on the temperature of
the heat sink and all should be well.  Semiconductors (unlike tubes) will no
t be 'used up' less by operating them at lower power -  semiconductors work
happily with no degradation unless they fail, and that failure point is
abrupt - they just stop working.  Tubes on the other hand can have their
life extended by running them at lower ratings because they do degrade over
their operating life.

73,
Don W3FPR

----- Original Message -----
>
> Here is what I have been always wondering about. This is not a specific K2
> question but since my main and only rig is a K2 I think this is the right
> place to ask. Since my K2 has a specified output of 100 W, is it safe to
> operate 100 W all the time?  Or will the finals last longer if I operate
> with, say 80 W?  Also, if I crank up the Power control, I could get up to
> 111 W out. Safe to do so?
>