Joseph M Gwinn skrev:
[email protected] wrote on 06/10/2009 03:13:19 AM:
In message <[email protected]>, Bruce Griffiths writes:
Bruce,
The thermal time constant (not the thermal impedance per se) is what
matters [...]
That is pretty much exactly the (mis-)definition of thermal impedance.
Thermal timeconstant or thermal corner-frequency had been much
better names.
It is possible to construct an enclosure with a long thermal time
constant together with relatively low thermal resistance so that the
temperature of a GPSDO or similar device within the enclosure only
increases by a relatively small amount.
Nope. This is essentially a thermal low pass filter.
Well, yes you can, but it is not very useful:
A really huge block of metal will do that: It can transfer a lot
of heat (=low resistance), but will take a long time doing so (=high
impedance).
I read somewhere the suggestion to take the cast iron block from an old
automobile engine and put it in a heavy insulated wooden cabinet: ~250 Kg
of iron in an insulated box. Cracked blocks are useless in an engine, and
so are available in junkyards quite cheap.
It is still just approaching a 1 pole filter. Alternating
insulation/low-capacitivity - conduction/high-capacitivity in several
layers creates higher pole systems.
The alternation in impedance creates impedance missmatches and we should
expect reflections...
Oh, an interesting experiment is to try to freeze a watermelon in a
steaming hot desert. Put towels on it and poor water over it. The amount
of energy needed to evaporize the water exceeds the suns heatup effect,
so the watermelon cools down... down... down... a bit of non-linearity
causes what may seem to be a thermal induction effect.
Cheers,
Magnus
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