Thermal compound can be highly conductive if you buy the right kind.
Hal
--- On Tue, 10/19/10, Kevin Normoyle <knormoyle@surfnetusa.com> wrote:
From: Kevin Normoyle <knormoyle@surfnetusa.com>
Subject: [Amps] diode junction temps and thermal resistance
To: AMPS@contesting.com
Date: Tuesday, October 19, 2010, 10:34 PM
Carl: "There is an ample front to rear aluminum shield that is more than
adequate to dissipate the 8W or so in normal use."
All heat gets dissipated eventually. The question is: what is the junction
temperature inside the diode and is it over
specification as a result of the thermal issues?
The 1N2971A data sheet says 10 deg C/W thermal resistance for junction to stud.
That can't change since it's the part
packaging. The maximum allowed junction temp for the part is 175C (from the
spec)
The stud to aluminum thermal resistance is probably at least 2 deg C/W. So
that's 8W * (10+2) = 96 deg C rise (for the
diode's junction)
But we have to add the temp of the aluminum chassis wall (the max temp of the
wall relates to it's ability to radiate/be
cooled by air). I'll guess the aluminum wall around the stud can be measured
with an IR thermometer to be 150 deg F.
(after some hours of 15M RTTY contesting) In the summer, with room temp at 90F,
that's easily imaginable?
150F is 66 deg C, so the junction temperature would be 66 + 96 = 162 deg C.
(compared to 175C allowed)
A poor path from stud to aluminum wall would be maybe 3 deg C/W. So that would
add 8 deg C. to get you to 170C.
(remember max allowed is 175 C)
I think it's fair to say that there is marginal headroom, if 8W occurs in
normal use. Does 8W get dissipated by each
zener? Since they are right next to each other, they locally heat the aluminum
wall that's doing the dissipation. My 150
deg F guess is just based on probable heating within the tube cavity (this is
just memory..may be wrong)
The main point: you can do numbers on all this, and IR measurements..it's not
just guessing.
Now: It may be true that none of these common zener failures are due to
overtemp. If so, then what is causing them? Is
RF causing them? or what?
I also noticed that Ameritron put the thermal compound between the case and the
solder lug washer..not just between the
case and the insulator+washer. I Thought the thermal compound was insulating?
If so, than that interferes with the
electrical connection between the diode case and the solder lug/washer.
-kevin
AD6Z
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