Best way to run PC fans from outlet?

Submitted by Tamariniak t3_11cfzah in DIY

I have a couple spare PC fans lying around that I'd like to repurpose to give a bit of active airflow to the cabinet that holds my networking stuff (no fancy racks, just a few home-grade boxes on wooden shelves).

My understanding is that I'm going to need an AC/DC converter and some sort of step-down to 12V / 0.3A, but that's where my knowledge ends. I'm also looking for ways to run more than one fan from one outlet. I do have a soldering iron on hand.

Does anyone have any experience with this? How do I not burn the house down doing this?

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idk_01 t1_ja2whuh wrote

12v wall wart ac/dc adaptor... splice n' dice

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idk_01 t1_ja2wuyf wrote

here ya go:

https://www.youtube.com/watch?v=evQtPGJe7xM

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Tamariniak OP t1_ja30ptt wrote

What about the voltage and current ratings though? What does it mean when a fan says "12V/0.3A"? Is that the maximum, the minimum, or something else? It makes me a bit scared to use lower voltages.

I also saw people saying that the fan "will always draw 0.3A regardless of the voltage," but I find that a bit weird. Surely a 12V/0.3A rating would mean that the fan has a resistance of 40 Ohms and the current would just be I = V / R.

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SomethingIrreverent t1_ja3cq3n wrote

You need a 12V adapter that can supply at least 0.3 A. If you have a 12 VDC adapter that can supply more current than that, it will work fine. If your adapter can only handle 0.2 A, you'll have trouble.

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5degreenegativerake t1_ja3fy5q wrote

And if you have “a couple” fans, you need 0.3A per fan on the wall wart spec.

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Diligent_Nature t1_ja3vqbp wrote

>people saying that the fan "will always draw 0.3A regardless of the voltage,"

That's nonsense. 0.3A is the nominal current. Lower voltage=lower current. It could be a little higher if airflow is impeded.

>Surely a 12V/0.3A rating would mean that the fan has a resistance of 40 Ohms and the current would just be I = V / R.

Ohm's law applies to DC in resistors. Fans use coils which have inductance as well as resistance. They commutate the DC into AC for the coils. Plus most DC fans are brushless and you can't measure the resistance directly because of their electronic commutation.

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discotim t1_ja4ae0o wrote

You want 12 volts and go overboard on the current, 2 Amp or more wall or inline warts are tiny. The fans will only draw the current they need and won't tax the power supply.

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kyrsjo t1_ja58joo wrote

You don't want to go too far overboard tough, switch mode power supplies aren't really stable without load.

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idk_01 t1_ja5imz7 wrote

12V/0.3A means that it requires 12V and .03A

The fan manufacturer has a datasheet out there somewhere that will have the tolerances of the device (i.e max & min levels etc.)

But for practical usage any 12v source, be it a battery or an ac/dc adaptor, that can supply at least .3 Amps should power your fan.

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Old_Geek t1_ja3drgy wrote

Totally ok to run a PC fan at a lower voltage.it will use less current. The wall wart suggestions are the cheapest and easiest as you probably have one laying around. I'm an ee and do this all the time. It's actually hard to find a 12v wall wart that is less than 1amp these days. A 5v cell wart will run it silently but with much less air flow. I use this combo for my bench fan for soldering to keep the fumes out of my face

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Tamariniak OP t1_ja3izr0 wrote

So please let me know if I'm understanding this right:

  • The stated voltage (12V in my example) is the maximum voltage
  • The stated current (0.3A in my example) is the maximum current
  • The current going through the fan can be calculated using Ohm's law, I = V / R, where R = Vmax / Imax (40 ohm in my example)
  • I just need to make sure the sum of the currents going through each of my fans is less than what the PSU is rated for

I saw some people elsewhere suggest that the current flowing through the fan is constant and not dependent on the supplied voltage, but that doesn't really make sense in my head since I don't think the fans could just magically reduce their resistance.

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aldol941 t1_ja3u4zg wrote

The equation I = V/R shows that current is proportional to voltage.

Applying more than 12V to the fan will force more current thru the fan and could melt the tiny wires in the fan motor (or at least shorten its life span).

You are correct, do not try to draw more current from the PSU than it is rated for. Using a PSU that has much higher current capacity is OK.

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Dysan27 t1_ja7evqe wrote

Yup.

Couple of additional things to take note of. You do not have to run the fans at 12 volts, if you run them at lesser voltage they will spin slower, an thus be quieter (if that's important to you). Though there is usually a minimum voltage that they will not operate under.

You will also see talk about 3-pin and 4-pin fans. Don't worry about it. For your use case, ie running it of a separate power supply, it makes no difference. The 4th pin is use for PWM (pulse width modulation) which is a different way to modulate the fan speed. If you're not using it 3 and 4 pin fans operate exactly the same way.

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jarnvargr t1_ja2txvl wrote

The easiest way?

Buy a cheap PC power supply. Gives you 12v, 5v, in a contained, small, safe enclosure.

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AlcoholPrep t1_ja38j1c wrote

These show up used for cheap at computer flea markets. Probably available at any computer store as scrap -- as whole computers get scrapped out when they're not worth refurbishing.

I'd like to point out that you can buy a new windbox fan for $20. Granted, that might not be the best choice, but it sets a price point.

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robertjpjr t1_ja3dilz wrote

Buy a cheap PC PSU. Jump the green wire to black so it turns on. Connect your fans to the power supply. If your fans use molex, that's easy. If your fans use a 3,4 pin connector you can buy adapters and or a hub type splitter.

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kyrsjo t1_ja58o5w wrote

Eh, a wall wart is cheaper smaller and less noisy.

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Senior_Cheesecake155 t1_ja6cjpp wrote

I’m in the process of doing that to power an old car stereo and amp in my garage. It’s totally pointless, but fun regardless.

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robertjpjr t1_ja6d12o wrote

Ya I've done that too. Wired up 4 old truck speakers to a homemade enclosure. I got about 50 old PC PSUs for $50 once. Made a bunch of benchtop power supplies for me and my buddies. You can get 12v 5v and 3v or hook up a buck controller and get variable voltage.

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Senior_Cheesecake155 t1_ja7da72 wrote

Yeah. This started when I stripped down an old Dell desktop that I kept moving around my garage while waiting for an e-cycling day. I’m still trying to decide what to do with the fan (possibly cooking for the enclosure everything is going in).

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crypticthree t1_ja2tqcq wrote

Just search for "rack mount fan power supply" lots of companies make them for AV and Network racks. Some of them even have speed control or thermostats

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Sea-Manner-9238 t1_ja33ugj wrote

Almost… You need a 12v transformer, power supply, wall wart, or whatever you want to call it, rated for at least the total current drawn by all the fans you plan to power. (Really should shoot for at least double). So if your are running 5 fans, aim for 3A or greater on the p/s.

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Tamariniak OP t1_ja34giq wrote

So the voltage on a power supply is a constant supplied voltage and the current rating of the PSU is just its maximum current? Meaning I just have to match the voltage and get a higher (maximum supplied) current than the sum of the fans?

Also, would you happen to know how I could regulate the speed? Is 12V the slowest or the fastest? Is it safe to run the fans at a lower voltage than what their rating is?

EDIT: one more thing, at a lower voltage, would the current flowing through the fans be lower than the rating (via Ohm's law) or does the current somehow stay the same regardless of the voltage?

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Sea-Manner-9238 t1_ja367ai wrote

Yes

That should be fine, though a PWM controller might be better.

Theoretically yes? But I’m not sure if there is something funky about the way motors work.

Only one way to know for sure…

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Temporary-Ad-8201 t1_ja3ddgd wrote

An analog fan controller (if you can find a usb powered one) or something like this: https://amzn.eu/d/be1TKwS would be the easiest way without having to cut cables and do some crazy wiring.

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GideonD t1_ja3qrlm wrote

There are several items like this available: https://www.amazon.com/Controller-100-240vAC-Adjustable-Splitter-Multi-Functional/dp/B0BBSBV9CF

Also several that can plug into a USB charger. Something like this would be my choice as you can control fan speed via voltage so you can get your airflow to noise ratio dialed in to your preference. I got a similar setup in my AV rack, though it turns on and off automatically with a temperature probe.

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ThreeBlurryDecades t1_ja48463 wrote

If one of your boxes has a decent power supply you could pull twelve volts for the fans from that. Plug into your harness and out the back of a case. I cooled a small network cupboard with extra fans like this for years without problems.

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SatanLifeProTips t1_ja4t2s5 wrote

Under-volt the fans so they run quieter. If 12v fans, try 5-8V and see how the sound is.

Just repurpose old wall warts. Get a cheap dc/dc converter if they are too high. You can also buy adjustable voltage universal wall warts and dial in the voltage (fan speed) that you like. Make sure it has a amperage rating of 0.3 A or higher. Higher is fine. Lower is not fine.

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Mildly_Angry_Biscuit t1_ja3toa2 wrote

I've done a few things with spare fans - you can use pretty much any power supply that puts out 9-12V and you'll get good airflow - and yeah - like you and a lot of other posts are thinking, you'll want to make sure it puts out good current.
On my larger boxes where I'm using fans and TEC's, I use an old tower power control - even the relatively dinky ones are 350w and will drive your fans no problem with room to expand. You can get them new for 25 bucks and up. What I like about them is you have a switch on them - gives you control of whether the fans are running or not.

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LazerWolfe53 t1_ja3x4xt wrote

I literally just did this last week for my solar inverters. I just used a 12V DC wall wort I had for my son's old hot wheels car. I wired two fans in parallel. Each fan uses 0.25A so 0.5A total

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knowitallz t1_ja3i27q wrote

USB charger puts out 5v use that.

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samuraipizzacat420 t1_ja3j1iw wrote

you need something to convert DC to AC

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Tractor_Boy_500 t1_ja42obf wrote

Huh? Home computer fans are 99.97% of the time DC.

For years, I've used a "wall wart" type supply to run a computer fan for various needs; just make sure the amps that it can supply are equal to, or greater than the amps that the fan will draw.

FYI... I have a bin full of "wall warts" to choose from - when I get rid of old gear, I always keep the power warts so I will always have a handy supply. Solder on a different end, if needed, heat shrink tubing to finish the job right.

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