Poul-Henning Kamp skrev:
In message <4a2efc6d.4020...@xtra.co.nz>, 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.
Thermal impedances can be divided into thermal resistance and thermal
capacitivity, these forms filters which in a 1D representation would be
a number of RC-links of various values.
Thermal time constant and total thermal conductance is aspects of the
static and dynamic properties that the structure has.
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).
Depends on the metal.
Adding just a little insulation to the added thermal mass can
dramatically increase the thermal time constant combined with a modest
increase in operating temperature.
Isn't that exactly what I explained initially ?
A huge block of aluminium, encased in 1" of styrofoam ?
There are many ways to say the same thing, don't argue with the guy just
phrasing it differently. Discuss the various merits of different
approaches instead.
Readily available inexpensive aluminium foil is a cheaper alternative to
expensive noble metal foils.
But it does not stay as reflective. By the time you add this layer to
the construction I mentioned, you care about the difference between
0.98 and 0.99.
It is interesting to see you being more extreme then Bruce. At the same
time, you are discussing two different approaches, not a single one.
I think discussing a general hints and tips that can apply to many
pratical cases is more useful at this stage then discussing a detailed
extreme solution only.
Nobody having a thermal inductive material around? That would be very
usefull to handle temperature shifts.
Cheers,
Magnus
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