12VDC cooling fan
12VDC cooling fan
(OP)
Hi, I've designed a pressure bottle that houses some electronics for an underwater robot. Although I predict that the electronics will remain sufficiently cool without much forced convection, I would like to install a 4" 12VDC computer fan inside the bottle to circulate the air. The fan would pretty much run all the time the DC to DC converter would be in use so I won't need to switch the fan on and off. I am tempted to just connect the fan across the 12V output of the DC to DC converter. However, I would like to know if there is a better way than just connecting the fan to the output (i.e. putting a capacitor across the output). I want to remain simple, but i would also like to learn a little too since i'm an ME.
Thank you in advance for the suggestions!
windell
Thank you in advance for the suggestions!
windell





RE: 12VDC cooling fan
RE: 12VDC cooling fan
Dan - Owner

http://www.Hi-TecDesigns.com
RE: 12VDC cooling fan
I have seen fans used for similar purposes as windell's in military gear. Internal fans moving the air around.
Too bad the ocean full of cool water couldn't have been used more directly for cooling; but I understand the difficulties in trying to make such a design practical in the face of overriding considerations to deal with ultra high water pressure and ease of maintainance.
RE: 12VDC cooling fan
I totally agree with macgyvers2000 comment. In a small closed space a fan moves almost no air but rather becomes a static heater.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: 12VDC cooling fan
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Sometimes I only open my mouth to swap feet...
RE: 12VDC cooling fan
RE: 12VDC cooling fan
However, you might simply use passive heatpipes directly to the outer shell. They can draw large amounts of heat out, without the extra heat load of the fan.
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RE: 12VDC cooling fan
RE: 12VDC cooling fan
I understand that the capacitor would prevent voltage variations across the dc to dc converter output, but what purpose does the 10ohm resistor play? Is it just to drop the voltage?
Also, would it be a good idea to put a diode in series with the input to the fan to prevent back current?
Lastly, for reference and documentation do you know if there is a typical application note or schematic that illustrates the wiring (yet very simple) for connecting the fan with the resistor and capacitor?
Thanks!
RE: 12VDC cooling fan
No. There is no "back current".
The 10 ohm resistor just reduces the voltage to the fan and therefore reduces power consumption. The recirculating fan works well if the convection cooling dominates the conductive cooling. For example, if you have used small heatsinks on the pcb there might be indequate cooling on those components without a fan. The fan reduces hot spots (if they exist).
Fans draw very "unpleasant" spikey current waveforms. The capacitor reduces the current spikes from the power rail.
If the DC-DC converter is 70% efficient and your fan is a 1W load, the battery has to supply 1.43W. Therefore you put the fan on the battery and save the 0.43W. Also the DC-DC converter will have worse output ripple when loaded by a fan, even with a large smoothing capacitor.
RE: 12VDC cooling fan
The only problem you could run into in adding a fan,is that the wattage of your supply may be tapped to the point where ripple increases to the detriment of the circuits. Additional capacitance may be required to mitigate this condition.
RE: 12VDC cooling fan
Dan - Owner

http://www.Hi-TecDesigns.com
RE: 12VDC cooling fan
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: 12VDC cooling fan
I suggest trying it as a coffee stirrer.
RE: 12VDC cooling fan
RE: 12VDC cooling fan