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Old 9th November 2008, 09:58 PM   #1
reiver is offline reiver  United States
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Default How to convert 120VDC to 12VDC?

My current project is a small monoblock amplifier based on a Hafler 220. Because the chassis is so small I'd like to mount a small computer chassis fan to the rear of the case to keep things cool. Like this one:

http://www.directron.com/ec8015m12ca.html

Can anyone tell the ohm values of a rheostat that I could install to convert my 125V supply to convert it down to the 12VDC range so I can vary the speed of this fan?

-Bryan
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Old 9th November 2008, 11:54 PM   #2
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Instead of using a rheostat, you might look into a DC-DC converter that can switch 120VDC down to 12VDC (12VDC is a common output voltage for most commercial families of DC-DC converters). This would be much more efficient than using a simple resistor to dial in the right voltage. Unfortunately, a DC-DC converter will probably cost you more than the fan itself.

--> Just a suggestion, have you considered using two or three fans in series (for instance, maybe two 48VDC fans, and then a 24VDC fan, all in series)? These fans might be too big for your case, I'm not sure.

For the calculations, looking at your fan, it is 12VDC at .16A (max current). If your input source is 125VDC, then your rheostat would have to dissipate:

(125VDC - 12VDC) / 0.16A = 706 ohms

Power = V*V / R = 18 watts
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Old 10th November 2008, 12:20 AM   #3
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Why not just use a small 12vdc wall unit (1x1" adaptor) or a
9volt unit for a quiet fan. I find them for .50c at flea markets
/thrift shops all the time.
Click the image to open in full size.

There should be a place to fit it inside your case and
you would have the safety factor ,too!!
(most of them are fused and thermally protected)
OS
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Old 10th November 2008, 01:04 AM   #4
infinia is offline infinia  United States
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Hi
There are small case fans that run on 120VAC as well. Mostly better than crap PC stuff with all the added cling-on's anyway.
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Old 10th November 2008, 01:09 AM   #5
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why not use a relay? you could have the power supply across the coil of the relay and then have the car's 12VDC across the switching part so wen the power supply turns on, it will kick on the relay and then will connect the cars 12VDC to the fan.

I am not sure about relays for the power supply's voltage, iv only worked with 12VDC coil relays before.
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Old 10th November 2008, 01:15 AM   #6
reiver is offline reiver  United States
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Default re:

Great ideas, thanks everyone. I like the idea of having it self-contained, I'll see if I can find a 120V version...


-Bryan
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Old 10th November 2008, 01:27 AM   #7
infinia is offline infinia  United States
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Another idea is to control the 120V fan using a small thermal (normally open) switch. They are sold with various temp profiles.
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Old 10th November 2008, 08:59 AM   #8
dangus is offline dangus  Canada
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How about a 12V or 24V fan with temperature-controlled variable speed? Dropping from 12V to 7V makes a big difference in noise level; that could be done with a 5V zener and a thermostat (for 2 speed operation), or maybe there's some off-the-shelf fans with temp sensors.

NMB (Minebea-Matsushita) has some quiet fans, although they can cost almost as much as an entire PC power supply. Or, old name-brand computers from the '80s and '90s often had excellent quality fans that are well worth salvaging.
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Old 10th November 2008, 11:22 AM   #9
infinia is offline infinia  United States
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Here's a small 120V fan. http://www.allelectronics.com/make-a...G-FAN/-/1.html

thermal switch looks similar to http://www.electronicsurplus.com/com...oduct_id=90936


catalog for thermal swiches
http://www.thermodisc.com/loadMedia....f&ReturnPage=8
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