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This question already has an answer here:

I am trying to install this miniature computer fan in a heated incubator that operates at 50 Celsuis 24/7 in order to encourage air flow throughout the box.

Small fans like this run on 12 DCV power, but because it will be running constantly, a battery wont do -- I need constant power, forever.

I understand that I could use a step-down transformer to convert 120 ACV to 12 DCV, but I'm not sure about how to match the specifications of the transformer with the specs of the fan. This fan says it's "rated current" is 0.06A.

Given this info, I'm not really sure how to shop for a transformer that will work and is safe. Do I have to perfectly match power supply to the fan, or just make sure that it's powerful enough? So if I bought a 144 Wt transformer, does that mean there will be 12 Amps (or more like 1 Amp?), and is it OK to wire this to a fan with rated current = 0.06 Amps?

Can I power two or more fans with this same transformer? I would like to wire these in parallel, so that I can unplug one or the other. Would that work, or do I have to wire them in series?

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marked as duplicate by Passerby, JRE, laptop2d, Daniel Grillo, Tom Carpenter Nov 8 '16 at 21:27

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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In general a consumer device will only draw as much current as it needs, as long as the supply voltage is correct.

In this case you need to buy a DC plugpack (transformer, rectifier and regulator all in one package - much like your mobile phone charger) that will provide 12V and can provide at least 0.06A. Most plug packs will manage this.

So you could connect your fan to a 12V plugpack which is 1200Watt and could deliver 100A, but your fan would only pull 0.06A, so this would be a waste.

You can power multiple fans off the same power supply, just make sure that you can supply the sum of all the current draws when selecting the plugpack. Wire them in parallel.

In summary, you do not need to match exactly. Just match the voltage exactly and make sure it is powerfull enough to supply the sum of all the currents you need.

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  • \$\begingroup\$ thanks! I heard we must "protect the circuit from motors" (I guess because of induction that could burn some components. It doesn't apply for the charger electronic component? Why ? \$\endgroup\$ – JinSnow Apr 25 at 4:21
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A transformer can step the voltage down, but it's still AC. Since you need DC, it will take a few additional steps to get what you need.

I recommend buying a 12VDC power supply from your local box store (or Radio Shack or Amazon). You should be able to find a wall-wart-style supply for under $10.

This is simple and cheap, and also has the advantage of being tested and certified.

When choosing a supply, you need:

  • The correct voltage (12VDC), and
  • At least as much current as your fan(s) will take. So, at least 0.06A per fan.
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  • \$\begingroup\$ Note that cheap wall-warts that supply 12V DC output often specify a rated current, like "1A" or "1 amp". With a light load like 0.06 A, their output voltage can be higher than 12V (easily 15v or a bit more). This may reduce the lifetime of a continuously running fan. Yes, at least 0.06A per fan , but don't go overboard. \$\endgroup\$ – glen_geek Nov 3 '16 at 17:22
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If you connect the fan to a voltage of 12V, it will draw automatically the correct current at steady state. The back EMF will increase to say, 11.5V, and then if it has one ohm of internal winding resistance, it will draw 0.5 a.

V=IR+E I=(V-E)/R=(12-11.5)/1 = 0.5A

Fanstastic! The back emf is proportional to the speed. If it goes half as fast, it will have half the back emf. Then it will draw more current. Therefore, don't stick your fingers in the fan, as this will slow it down, increase the current, and exceed the current rating.

Thus you just need to give the fan 12VDC.

A transformer will not give you DC. It will give you AC. What you need is called an AC to DC inverter. You can't just feed a DC motor AC. If you search for power supply, you will find many.

It's okay to buy a power supply that can source up to 12 amps at 12VDC. At half an amp per fan, you could hook up 24 fans in parallel! Fantastic. As is, that's overdesigning. Pick a power supply that can source an amp or two, and that'd be more than enough.

Honestly, if there's a power supply in your computer giving 12V, just wire it up to that. Be careful, of course!

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The DC motor will pull as much current as it needs as long as you supply it the correct voltage (12VDC in this case). Why not use a 120VAC to 12VDC converter specifically for getting the 12V? You can wire the fans in parallel like you want as long as your power source can supply enough power to turn as many as you use on.

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