At 01:10 PM 07/12/2009, you wrote:
>I've always wondered why soldering gun tips like this work so well:
>
>http://www.cooperhandtools.com/brands/CF_Files/model_detail.cfm?upc=3D0371=
03478818
>
>The material appears to be made of the same alloy all the way through,
>and it doesn't get thinner near the tip, so why is the very tip of it
>the part that gets really hot?
>
>Is it because the tip is somewhat equidistant from the fat connections,
>so it starts off a little hotter, but then the temperature/resistance
>coefficient causes a "thermal runaway"? Is that why when you are
>soldering something big that's hard to heat up it helps to get the gun
>hot first before plunging in?
>
>Cheerful regards,
>
>Bob

Yes, I think so. If you've ever taken apart a soldering gun (about the
only thing they are good for in electronics, aside from EMC testing)
you'll have seen that the transformer secondary is a fat copper tube (IIRC)
that will suck the heat away from the connections. They are designed to be
used with a small duty cycle (< 25%) so the connections never get
really hot (hot enough to burn flesh, though, IIRC).

If we do the math, the resistance of the part of the "tip" near the
connections (say at 100=B0C) is going to be about half the resistance of
the part of the tip at (say) 400=B0C, so most of power will be dissipated in
the hotter section (and the hotter part will get tend to get hotter). It's =
not
a true runaway because the heat loss with temperature increases fast enough.

Maybe someone with a really good (very small spot size) optical IR meter or
FLIR camera can map the temperatures for us.. I would expect the temperature
would drop much faster than linearly (despite the constant cross-section)
towards the connections. Probably have to spray the tip with stove paint
to control the emissivity.

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the rewar=
d"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com




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