Arfa Daily wrote in message
...
"N Cook" wrote in message
...
DaveM wrote in message
. ..
"whit3rd" wrote in message
ups.com...
On Sep 11, 8:29 am, "N Cook" wrote:
I was not convinced that for an existing used amp with 4x TOP66
power
output
devices that the silicone pads were better than mica.
The previous post, that said it was the transistor temperature
that would be variable, was correct; remember, the whole
heat output is dumped into the heatsink, and that isn't affected
by the thermal resistance of your mica. Heatsink-to-air temperature
difference is whatever it takes to drain that heat.
It is possible to fit a second heatsink onto the TOP of the
transistor,
if you really need better thermal transfer (using both bottom and top
surfaces of the metal plate).
The interface between the device and the heatsink is probably the most
critical
piece in the puzzle, and is the piece that a builder/repairer has most
control
over. The device temperature is what you're trying to control. You
have
no
control over the internal thermal impedances of the device... they are
determined by the manufacturing process and materials.
Given a heatsink has adequate capacity for dissipating the heat
(heatsink
to air
thermal impedance), then that only leaves the device to heatsink
interface
to
manipulate. In comparison between greased mica and silpad insulators,
the
mica/grease combination has the better thermal characteristics.
Mica can be problematic though, mainly because there seems to be no
quality
control on the manufacturing process. The thickness of a mica slab can
vary
from 0.010 to 0.050 or more. Mica can withstand about 1000 volts per
mil,
so it
really doesn't need to be so thick. You can usually split a mica slab
several
times and still be safe in using it. The thermal impedance goes down
somewhat
with thinner slabs, too.
Overall, mica/grease can exhibit almost half the thermal impedance of a
silpad.
Given the same device power dissipation, lower thermal impedances mean
lower
device temperature.
--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate
characters
in the
address)
"In theory, there isn't any difference between theory and practice. In
practice, there is." - Yogi Berra
I think I will rely on the evidence of my own observations and not
performance tables produced by the manufacturer's with an obvious vested
interest.
I've no reason to believe the original silpads had aged, been affected
by
WD40 or anything.
I will assume they are , all manufacturers, all generically bad until a
similar personally conducted experiment, in a real situation, proves to
me
to be otherwise.
--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/
Surely a degree of faith in what manufacturers say, has to be assumed,
otherwise we would all go around our whole lives with our heads up our
collective arses ! Do you for instance do a complete analysis of every
piece
of processed food you eat to see if the manufacturer is lying about how
much
salt it contains or how much calorific value it has ? Do you check your
bottles of beer to see if the ABV that the manufacturer quotes is true ?
Do
you run a complete set of tests on a resistor type before stocking it as a
spare to see if what the manufacturer says about its tolerance stability
is
correct ? No? Neither do I.
I can understand scepticism about 'waved in the air' figures that appear
to
have little or no foundation in reality, but figures for these washers
that
quote specifics on the applied pressure to get the results that are
published, don't seem to fall into that category to me. I can also
understand manufacturers of some products getting into 'mine's bigger than
yours' specmanship contests, but silicone heatsink pads ? Do you really
believe that these are such a big part of the manufacturer's total output
of
products, that he needs to engage in the sort of behaviour that involves
lying, or at least distorting, the 'true' facts about the product, just to
sell a few more than the other guy ?
As it happens, I cannot remember seeing pads distorted in the way that you
describe, so maybe they were from a bad batch or something. Maybe, the amp
manufacturer torqued them up too much, and the heat did the rest. Maybe
the
amp was used on midsummer's day in Ibiza at full volume with a blanket
over
it, and a fault ...
I agree that many amps have output devices that appear to run hotter than
we
might like, and that the thermal transfer across the insulators seems less
than ideal, but if that's the way the manufacturer made it and is happy
with, and the item is not on my bench with failed output transistors, then
I
tend to just note the fact with interest, and work on using the "If it
ain't
broke, don't fix it" principle.
And to Graham, I HATE amps that have live heatsinks. Just a few days ago,
I
had a big AV amp on the bench that nearly threw me off my chair when I
laid
the edge of my hand across the two heatsinks ready to take some
measurements
with the meter probe in the other hand. This thing had rails of about +/-
80v, so it dumped 160v across my hand. Basstud thing ...
Arfa
Unless big G has a defendable patent on mica then why do I not see
like-for-like , in real situations, comparison tables of BigCorp GeeWizz
SiilyPad versus mica in the performance figures.
"Live" heatsink amps would be ok if the damn things were labelled with big
fluorescent stickes stating such. But of course such amps should only be
serviced by their own service agents is what they would say.
--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/