PICdude wrote:
> Here's what's confusing me... I tend to think of the wattage rating as th=
e
> ability (or rate) of the device to dissipate heat (given it's package siz=
e,
> surface area, materials, etc), which is why I imagine higher wattage bein=
g
> no problem as long as the heat is being sucked away.

Yes, but it isn't as simple as you think. The manufacturer comes up with
the overall steady-state power rating with full knowledge of the internal
structure and materials in the resistor. They know that if the external
surface is at the specified temperature, the internal parts won't be stress=
ed
to failure at the specified power level.

When you overdrive the resistor, the internal temperature distribution
changes, and some spots may become hotter much faster than the external
surface does.

You really need to consider all of the paths and materials that lie between
the spot where the heat is produced and the spot where it is dissipated to
"ambient". It's all about thermal resistances -- and, in dynamic situations=
,
the specific heat of materials (thermal capacitances, if you will) comes in=
to
play as well.

-- Dave Tweed
--=20
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