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How would I amend a 12V fan to take 2*5V?

The 12V fan has 2 wires on it. one would take 12V the other GND.

I'd like to be able to feed it 5V and 5V, instead of just 12V.

What do I need to do it?

I'm a geek but not an electronics geek..so if there is terminology, please use it but tell me what it means..

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  • \$\begingroup\$ Why do you need 2*5V? \$\endgroup\$ – suha Jun 28 '12 at 8:54
  • \$\begingroup\$ The 5V is USB from my laptop. it's for powering a 12V 0.1A 1.2W fan externally from my laptop's USB \$\endgroup\$ – barlop Jun 28 '12 at 8:59
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    \$\begingroup\$ I suggest you shop around for a 5V fan. Far easier than inserting a 5->12 converter. \$\endgroup\$ – Wouter van Ooijen Jun 28 '12 at 18:28
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You can't make 10 V from two times 5 V unless they are completely separated. In that case you can place them in series.

The best way to get 12 V from 5 V is to use a switcher, or SMPS (Switch-Mode Power Supply). Buck types deliver a lower output voltage than the input, boost give a higher output voltage, so you want the latter. 5 V in to 12 V out is a piece of cake for a boost switcher.

Switchers can be very efficient (little power lost), and therefore often very compact as well. Keep in mind that you can't fool the laws of thermodynamics: if your fan would need 200 mA, for instance, that's 2.4 W, then the 5 V supply has to deliver that too, and 2.4 W at 5 V is 480 mA, not 200. Make that probably 600 mA because there's still some losses in the switcher.

example

enter image description here

So check how much current (or power) the fan needs at 12 V, and that the 5 V supply can deliver the required power.

edit
If you want to power a 1.2 W fan from your USB port you'll need more than 240 mA from it, probably around 300 mA. A USB port can deliver up to 500 mA, but that has to be negotiated with the host (part of the USB software protocol). Without negotiation you can only get 100 mA, and that's not enough.

Further reading
Powering electronics from the USB port, TI application note

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  • \$\begingroup\$ see my comment to my question. what's the difference between using a step up voltage regulator, and a step up SMPS? \$\endgroup\$ – barlop Jun 28 '12 at 9:03
  • \$\begingroup\$ They're the same. The SMPS is a voltage regulator, and step up is what the electronic principle behind it is called. (I updated my answer.) \$\endgroup\$ – stevenvh Jun 28 '12 at 9:13
  • \$\begingroup\$ how can I get the negotiation? \$\endgroup\$ – barlop Jun 28 '12 at 9:19
  • \$\begingroup\$ perhaps i should make how to get the negotiation another question? \$\endgroup\$ – barlop Jun 28 '12 at 9:20
  • \$\begingroup\$ You'll need an USB capable microcontroller to talk to the host (your laptop), and I'm not sure if it's worth the trouble. Can't you use a 12 V wall wart or is this for in the field use? \$\endgroup\$ – stevenvh Jun 28 '12 at 9:48
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If you have two isolated power sources, connect them in series and the fan will most likely work fine. So basically connect the positive side of one source to the negative side of another source. I've been running several 12 V fans for years at 10 V and so far, so good.

If you try to do the above and the sources are not isolated, expect a short circuit, sparks and possibly a loud bang and melted cables.

The simplest way to determine if the sources are isolated is to test them with a multimeter. There should be no connection between their negative pins. If there is, you can't use them directly. If you're not certain about the sources, provide us with more information and we might be able to help.

If the sources aren't isolated, you may have to use a boost converter to increase the voltage. If the single 5 V source doesn't have enough current, you may be able to connect the two sources in parallel to provide more current, but we'll need more information to be able to tell you how safe the connection is.

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  • \$\begingroup\$ see my comment to my question. there's no -5V. just +5V and GND. or +5V,GND and +5V and GND. \$\endgroup\$ – barlop Jun 28 '12 at 9:02
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(I'm guessing this is a PC style fan? Why are you trying to do this - what effect do you want to achieve?)

Power doesn't really work like that. You might try feeding it 5V on the 12V wire, this may cause it to turn slowly. Otherwise you'll need a suitable DC-DC converter module ("boost converter") - these are available already built.

If your computer has a -5V rail which is capable of supplying suitable current, then connecting the fan across the +5V and -5V should make it turn. But I can't think why you'd do that.

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  • \$\begingroup\$ The -5V rail was removed from the ATX spec in v2.01 (2004); so it's unlikely that any recent PSUs will provide it. \$\endgroup\$ – Dan Neely Jun 28 '12 at 12:36

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