It is interesting that most of us don't know how important the heat sink
goop is, and how much of the heat transfer takes place through it.
The two metal surfaces (heat sink and transistor) even if machined quite
flat and smooth actually touch microscopically at a very few tiny points.
That is the limit of the metal to metal transfer. Even if you work really
hard at it, that is the best you can do.
The rest of the space is filled with air, and air is an excellent insulator.
To fix this problem, one approach is to use conventional heat sink compound.
These common compounds use a passivated aluminum oxide extremely fine powder
suspended in a petrochemical. This compound fills the air gaps and the
aluminum oxide conducts the heat far better than the air would have.
In extreme applications, a better heat sink compound uses extremely fine
silver in the place of the aluminum oxide, for many times better results,
but at the expense of having the compound be electrically conductive which
can cause many issues.
So, generally we use the aluminum compound and it works really well.
To go beyond that, you have to turn to the heat sink itself. Make it
bigger. Make it Copper. Give it more and larger fins. Blow air on it.
Or - if you really want to do it right, use a TEC for active cooling and
make that heat sink really really cold. Just short of the dew point of
course. And much more practical than liquid cooling.
TEC's really rock when used properly in this manner.
73 de Gary, AA2IZ
----- Original Message -----
From: "Shawn Upton" <kb1ckt@yahoo.com>
To: "Discussion of Ten-Tec Equipment" <tentec@contesting.com>
Sent: Tuesday, March 06, 2007 7:15 PM
Subject: Re: [TenTec] Is there a beefier 2N5301?
> What's wrong with a fan, or a couple of small ones?
> The cheap PC ones won't last long, but should do alot
> for continous duty heat dissapation.
>
> Responding to an earlier post: it's better to use too
> little than it is to use too much. I forget the exact
> numbers now, but in terms of relative heat conduction,
> if glass is a 1, and metal 1000 (metal allowing heat
> to flow 1000 times easier), that heatsink goop is like
> a 3. Its use is to fill in the valleys left from
> machining the metal surfaces; by filling those gaps,
> somewhat better heat conduction occurs. But the
> majority of the heat needs to flow from metal to
> metal.
>
> When using the stuff, it's best to apply it, and then
> wipe it off each surface. You should see lots of
> metal, and just small dots of white.
>
> Why are the transistors dying so often? Is the supply
> tripping out too often? Maybe a 10W resistor, say 0.1
> ohm, on the collector side? I'm thinking of its
> purpose to limiting fault current when the crowbar
> fires.
>
>
> Shawn Upton, KB1CKT
>
>
>
>
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