Expanding generously (gusting on):

When I
designed the Flight Recorder, the FAA mandated a heat budget for its
acceptance.

Aircraft electronics lives with a common airduct. Your design must
not load the cooling air such that it becomes a flame thrower into the
next instrument in the stack. I won't go into issues of crash
survivability.

Returning to our concerns, for certain phase combinations that caloric
solution can arrive suddenly in the form of an arc.

I'm sure most readers who run tube rigs will recognize this situation
immediately. However, there is more than one combination of phases
and currents/voltages. I have also seen heat soaking arrive at a tube
to watch the plates glow cheerily. This, too, is probably an
experience borne by several tube rig operators. In fact, it can be
tolerated far more than a solid state amplifier, and tubes are noted
for their resilience. However, I have also seen the glass envelopes
turned into a taffy consistincy and the vacuum draw them like
heatshrink around the internal structure. Surprisingly, I have also
witnessed that these tubes still worked!

For other phase combinations that caloric solution can arrive
gradually (heat soaking); and catastrophe arrives through thermal
runaway. Operators rarely observe this until it is too late.

The latest generation of solid state components have survivability
design into them such that they are specified to operate into an
infinite mismatch (or some such similar claim). This is suitably
taken care of by being able to withstand more voltage. Other issues
of current crowding, the original thermal disaster for transistors,
has been long solved. That solution revealed how the problem was in
heat confined to a small volume.



Finally, my measurements were never pushed to the point of failure.
All may well anticipate that this sudden arrival would preclude any
accuracy in the heat determination to demonstrate a quid-pro-quo of
returned power. Further, once the failure occured, heat is usually
removed by the very failure it brought - it usually removes the source
too. ;-)

73's
Richard Clark, KB7QHC