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I am working on a circuit that is completly powered by 5VDC from a USB port. The circuit uses SSR's to turn on and off 120VAC from a standard wall plug; the 120VAC powers a high-power heater of sorts. Now it appears as though I am also going to need to add a PC fan into the mix. Most of the PC fans that I have seen run off 12 VDC. So what would be the cheapest and easiest way to get 12VDC to my fan. I am thinking that it doesn't have to be well regulated and smooth becuase the ONLY thing it will be used for is powering this fan. I will likely use another lower power SSR to turn the 12VDC on and off from my MCU. Extra points for a space saving solution as well!

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  • \$\begingroup\$ The first couple fans that I have looked at require about 250mA @12VDC \$\endgroup\$ – PICyourBrain Feb 17 '11 at 0:48
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    \$\begingroup\$ You could just give the fan 5 V and select a larger one to compensate for it spinning about half as fast. \$\endgroup\$ – Nick T Apr 1 '11 at 23:26
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    \$\begingroup\$ Just find a 5V fan! That's the cheapest and easiest way to get power to the fan. Digikey has 75 5V fans in stock right now, starting at a little under $5. \$\endgroup\$ – Kevin Vermeer Apr 2 '11 at 16:29
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    \$\begingroup\$ If you need a big fan, 120VAC fans aren't hard to find. \$\endgroup\$ – markrages Sep 1 '11 at 19:17
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Looks like MPJA.com has 12V, 300mA power supplies for $1.25 and if you buy 50 they're $0.99. That is hard to beat! But they aren't the smallest solution.

http://www.mpja.com/prodinfo.asp?number=18295+PD

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You could just get a 12VDC wall supply. Has the transformer, rectifier, filter and regulator built in. can get fans cheap also. Check these links, $8.50 for all.

http://www.allelectronics.com/index.php?page=search&search_query=dctx-1227&x=39&y=7 http://www.allelectronics.com/index.php?page=search&search_query=CF-387&x=29&y=12

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Why would you not just use a 5 volt fan?
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=259-1345-ND

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Ok here's my best guess.

I can get a transform at Radioshack for $4.49 that will drop the 120VAC down to 12.6VAC. (Suprising I couldn't find a transformer of this type cheaper on digikey or mouser) Transformer: http://www.radioshack.com/product/index.jsp?productId=2102494#inTheBox

Then I could use four 1N4007 diodes to make a full wave rectifier. Cost of diodes is about a dollar total.

Finally stick a large capacitor at the output to smooth out the signal.

This puts the total cost around $7.

I know the output won't be prefect and it will be a little higher than 12VDC but I think it should be fine for a PC fan...

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    \$\begingroup\$ instead of diodes you could use a Rectifier radioshack.com/product/index.jsp?productId=2062580 \$\endgroup\$ – jsolarski Feb 17 '11 at 1:03
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    \$\begingroup\$ "a little higher than 12VDC"? More like 40% higher at least. You're smoothing the peak of the ac, not the rms, so it has the 1.414 factor for a start. Then add on the fact that transformers are specified for the output voltage at the full load, so if you don't fully load them, the output voltage is higher (see the regulation part of their spec). Then subtract the diode drops. \$\endgroup\$ – Martin Feb 17 '11 at 13:53
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I'd think outside the box on this one: Go get a 12VDC wall wart from Goodwill and switch with a relay on the DC side (or a solid-state solution to taste). For your $2 you may even be able to find one with an on/off switch.

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