On 02/02/14 20:16, Charles Steinmetz wrote:
Magnus wrote:
My point was, the thermistor is never "happy." It always wants more
cooling. So it spins the fan up to full speed and is still too hot to
reach equlibrium.
Which only means that the thermistor setup is shifted and needs to be
adjusted to achieve the goal.
* * *
So, the question is, do we just replace fans when
they go bad and live with the poor thermal design, or do we try to
improve the thermal design? If we want to improve the thermal design,
the methods available to us are:
I think a sub-set of these are needed, but you also need to include
(v) change the balance-point for the thermistor stabilization.
Well, sure, you can raise the setpoint until the fan slows down and
regulates the temperature, but then the regulated temperature would be
too high. In other words, if the fan cannot keep things cool enough by
running at full speed, raising the set-point so the fan runs slower is
not the right answer even if it does result in some approximation to
isothermy. [Note that adjusting the setpoint *down* will have no effect
-- the fan already cannot cool the instrument enough to reach equilibrium.]
I'm not discussing it as a single measure, but rather a thing you can
look at only after you got air-flow working much better.
I think the single biggest problem is too little air inlet, which
forces the fan to run in stupidly high speed without actually do much,
as the air input has too low cross-section and with several small
holes you need to create a large pressure difference in order to
achieve the air flow wanted. Also, those holes help to create noise as
they are not shaped to avoid turbulence. Best way is just achieve
large enough cross-section.
I have experimented with cardboard top covers that have various holes in
them. My conclusion is that even with more air inlet area, you still
want more cfm of air flow (i.e., a stronger fan). You also need to
adjust the airflow to the various sections of the interior to keep
everything sufficiently cool. As it is, there would be way too much
temperature variation from one part of the interior to another even if
you did reach a regulated equilibrium for the thermistor location
(wherever you put the thermistor).
I agree. You need both, but just tossing in a stronger fan isn't going
to cut it either.
Cheers,
Magnus
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