Tom wrote:
There have been several discussions over the years about variable fan speed based temperature control. I can't explain it, but I've always been suspicious of this technique. It seems to me still air is inherently better than moving air. Passive (no fan) is better than active (fan). And constant velocity is better than turbulence is better than variable velocity. But I don't know for sure.
There is no such thing as "still air," unless there is no temperature gradient. If there is any temperature gradient (typically due to power dissipation), there will be convection currents. In a closed space (for example, internal to a TBolt or in a sealed box that encloses a TBolt), these convection currents will set up a flow pattern that may be benign or malicious with respect to keeping a particular part of the device at a constant temperature. If the part you are particularly interested in is a creator of thermal gradients (as the OCXO in a TBolt is), analyzing this gets very complicated very fast.
Fans (speaking here of fans that circulate air internal to a closed volume, not fans that exchange air between the inside and outside of a volume) tend to mix up the air and reduce thermal gradients. Then, the question becomes whether the circulation due to the fan has a patterned or a random thermal flow. Typically, a random (diffused) pattern is best -- but it is relatively hard to achieve. With careful design, active circulation is usually better than passive convection. However, "careful design" is not easy. Also, fans raise a concern about vibration, which is a real worry with any precision oscillator.
One other possibility is to use passive techniques to randomize (more or less) the passive convection. This can be achieved (to a degree) by filling the internal volume with low-density, very porous insulation. On a larger scale, a sealed box of, say, 2 cubic feet can be filled with common packing peanuts and the isolated object placed in the middle. Air will still circulate by convection, but in a more random manner. (There will also be less bulk circulation, so the thermal gradient will be somewhat larger than before.) Applied to a TBolt, one might fill the inside of the TBolt itself with smaller pieces of styrofoam (irregular shapes perhaps 6 or 7mm in size). [Spheres (styrofoam beads) may pack a bit too tightly for this, impeding airflow more than desired.] The same can be done for a sealed box that encloses a TBolt or other oscillator. I have achieved very good results with this method, when properly applied.
I have done a fair amount of experimenting with and without fans (but one must recognize that there are so many variables, even a lot of experimenting really only scratches the surface), and have always found that passive circulation (within sealed volumes) works very well when the object ultimately being controlled is an ovenized oscillator. For tight control, which is needed for precision voltage references, DAQ circuits, and other precision process-control applications, I do use a thermostatically operated fan to exchange air between the outermost sealed volume and ambient -- but even this I usually find unnecessary if the ultimate object is minimizing the frequency drift of an ovenized oscillator.
Finally, re.: fan control. For a brushless DC fan to run slowly, you need to feed it full voltage with pulse-width modulation ("PWM"). Even then, they will not run all that slowly. The Microchip TC642B fan controller (8 pin IC, about $1.20) is a very handy part when you need a wide range of fan speeds. It uses commutation noise to sense fan rotation, and has a "stall routine" that gives the fan a kick if it stalls (NB: this is a feature of the 642B, absent on the 642). So, not only will it run the fan at its lowest possible self-sustaining speed, you can also run the fan much slower than its self-sustaining speed by letting it stall and be restarted periodically. The fan looks like a windmill with three sheets to the wind below its self-sustaining speed, but it works extremely well and this operation does not damage the fan or the controller.
Best regards, Charles _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
