Roger,
> You do need a certain amount of metal in there above the minimum
> required for heat transfer, or thermal inertia to take care of cooling
> failures.
If you want that thermal mass as a safety measure against failure of the
cooling circuit, why not use water for the thermal mass too? After all
water has a specific heat per weight that is more than ten times better
than that of copper, and even when comparing specific heat per volume,
water is still better than copper! And water is quite a bit less
expensive than copper...
You can have a certain amount of water enclosed in the cooling block,
and this water would be separate from the cooling circuit, just for
thermal mass. You can leave a vent, so if the cooling circuit fails, the
reserve water will alert you by boiling and hissing - that is, as long
as the transistors can survive operating with their mounting surface a
bit above 100 degrees Celsius, which depends simply on the power they
are dissipating, compared to their ratings. Or better, have a
temperature sensor shut down the beast if it overheats, and the enclosed
water will give it lots of time to do that.
> BTW when you figure 20% duty cycle for SSB AND the typical 10 minutes
> out of 30 transmit time the required water becomes miniscule.
That's right. I calculated that a 10 liter bucket under the desk would
be enough for even the most intense operating I ever do. I wouldn't need
any active cooling. Just starting the water from room temperature (so
there is no risk of condensation), and having it warm up a few degrees
during an operating session, then cool down naturally over the next
several hours.
Manfred.
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