24v on an ATX Power Supply?

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uzor

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I tried searching for this, but didn't find anything. I rescued a 24v DC fan from an old, dead DEC Alpha 2000. It is rated at 24v .75A, and I wanted to know if and how I can run this off of my standard ATX power supply. If you need anymore information, let me know, and I will try to answer for you.

Thanks

 
Technically you CAN, by using the -12V (blue wire) as your ground and any +12V (yellow wire) as the hot. However, I'd check the amperage rating on your -12V, since on most power supplies it's not very high at all.
 
Well as far as i know i don't think you can make 24v from a atx psu. If you do some hardcore modding you can but you well more then like'ley kill your self. Anyway just buy a 110v ac to 24v dc inverter that would be ALOT easy'er.
 
Running a fan off -12V isn't a good idea...

You could step 12V up to 24V using a quick switching regulator circuit, but it would be cheaper to buy a 12V fan...
 
Yeah, but that would defeat the whole purpose. This thing is about 10" across and kinda looks like a turbine. It's a BEAST!! I don't have a camera at work with me today, but I'll bring one tomorrow, and post a pic for you guys (if anyone even cares).

Would any 24v ac/dc inverter work? I have a box of bricks from old dead hardware, since I always figure that they will work with something else in the future eventually. Assuming I went that route, I'd just need to find one with an amperage rating higher than my fan right?

 
If it's a beast as you say, you definitely shouldn't run it using the -12v line (btw, that rail is used for signals in older tech, I believe serial ports and ISA slots).

Inverter? It sounds like you're describing a regular 120vac to 24vdc wall transformer, but inverter refers to a DC to AC converter. Just find a wall wart with an equal or higher amperage rating, and 24vdc output. The fan would also probably run fine at lower voltages, maybe as low as 12v, at reduced speed and noise.
 
When I got my hands on a 24v 7” 250cfm Rotron fan I was faced with the same dilemma… My fan pulls 1.7 amp @ 24v, so there is no way I would hook it up to the 200ma (spec’d on the side of my PSU) -12v line. What I did was get a 2amp 120v-24v transformer, a 12v relay and a bridge rectifier from www.allelectronics.com (also where I got the fan) for $6. I put the transformer, relay and rectifier in a Radio Shack enclosure, and mounted it on the outside of my PSU. There are 4 wires that come out the back of the box, and go into my PSU (120v h, 120v n, +12, ground) and a 2 pin connector on the front that the big fan plugs into. I soldered the 4 wires right into the PSU PCB.

24v.JPG


When the computer is off, the fan is off. When the computer +12v becomes active, the relay closes, and the transformer steps the 120v AC down to 24v AC, which the bridge rectifier turns into 24v DC.
 
so presuming my wart goes from 120v ac to 24v dc (I'm pretty sure I have one of those), the only extra bit I would need is the relay, correct? Got my camera today. I'll take a pic when I get into the office.


Thanks
 
Looking at it again today, I think we might have the same fan...

Here's mine:

fan1.jpg


Closeup of the label:

fan2.jpg


::edit:: not quite. this is the closest I found to mine on their site. Model number is different, but they are rated the same. BTW, do you know what the white lead is for?

 
try running it at 12v and see if it'll spin... you dont want that thing at full speed, i doubt :p
 
uzor said:
Yeah, but that would defeat the whole purpose. This thing is about 10" across and kinda looks like a turbine. It's a BEAST!! I don't have a camera at work with me today, but I'll bring one tomorrow, and post a pic for you guys (if anyone even cares).

Would any 24v ac/dc inverter work? I have a box of bricks from old dead hardware, since I always figure that they will work with something else in the future eventually. Assuming I went that route, I'd just need to find one with an amperage rating higher than my fan right?

Click to expand...
One thing - the inrush current on those fans is insane. It might only pull half an amp while it's running, but when you first plug one in, they can draw 5A or more. This is why you *don't* put it on -12V.

I designed a switching power supply at work for powering the fans in a radio transmitter - driving 22 almost-identical fans (they're 48 volts). The power supply bounces off its current limiting for about a second until the fans start going full speed.

If you use a wall brick, short the thing's output several times and make sure it still works. If it does, then it's overcurrent protected and it'll work fine for the fan.

And if you can't find a suitable wall brick, I still suggest pricing a few 12V fans of the same size. Delta makes some awesome fans around that size, and they're probably the cheapest on the market compared to NMB, Papst, Rotron, etc...
 
Sounds like you almost should have designed a staged design, so they don't all spin up at the same time. I'll look through my box, and see what I come up with.
 
gee said:
One thing - the inrush current on those fans is insane. It might only pull half an amp while it's running, but when you first plug one in, they can draw 5A or more. This is why you *don't* put it on -12V.
Click to expand...
this is exactly why I do not recommend putting it on the -12v atx line… even with a hefty capacitor across the rails. A transformer rated at 2a off 120v mains can easily start a motor that pulls 10a+. “wallworts” are not as forgiving, they usually have extra (and in this case, unnecessary) filtering components that will not take the hit as well.
uzor said:
so presuming my wart goes from 120v ac to 24v dc (I'm pretty sure I have one of those), the only extra bit I would need is the relay, correct?
Click to expand...
yeah all you need is a relay ($3 at radioshack) unless you want the fan to be on all the time
uzor said:
Looking at it again today, I think we might have the same fan...
BTW, do you know what the white lead is for?
Click to expand...
the white lead is an RPM, it goes to ground every rotation. you can hook it to the rpm pin on one of the 3 pin fan connectors on your motherboard so that you can see the speed readout on a program like Mother Board Monitor. If you use a separate transformer, you’ll also have to run a ground wire to the motherboard. it works well, and that is how mine is hooked up.
diredesire said:
try running it at 12v and see if it'll spin... you dont want that thing at full speed
Click to expand...
actually, i do want it at full speed. :rolleyes:

but yes, it does run at 12v, and it feels like a heavy 120mm fan at full speed. this thing can take off a finger at 12v, at 24 volts its deadly.
 
theshadow27 said:
the white lead is an RPM, it goes to ground every rotation. you can hook it to the rpm pin on one of the 3 pin fan connectors on your motherboard so that you can see the speed readout on a program like Mother Board Monitor. If you use a separate transformer, you’ll also have to run a ground wire to the motherboard. it works well, and that is how mine is hooked up.
Click to expand...

Could you explain what you mean a little better? Where would I run the the ground to/from? Wouldn't I just do red-->wart+, black-->wart-, white-->mobo tach?


 
There must be a common ground between the fan and the mobo if the fan's signal is to be understood by the mobo. This is accomplished by taking the (-) lead from you wall wart and connecting to a ground wire from your PSU, or even to the metal case.
 
mattg2k4 said:
There must be a common ground between the fan and the mobo if the fan's signal is to be understood by the mobo. This is accomplished by taking the (-) lead from you wall wart and connecting to a ground wire from your PSU, or even to the metal case.
Click to expand...
right.

just for clarifcation: looking at the mobo 3 pin connector with the latch down, we have:
(gnd)(+v)(rpm)

EDIT... gosh lol
fanconnector.jpg
 
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