Running 12v computer fans on a 24v power supply-- just run them in series?

[redfishsc]

I have a used 24v, 2.2A power supply and a few computer fans.

They are all 12v and draw around 0.25A, so I have plenty of amperage to work with.


Is this as simple as it seems? Just wire them in series (two fans)?

Could I do two strings of two fans (4 total)---- running the two strings in parallel to each other, assuming I don't exceed the current rating of the power supply?

Any advice is appreciated. I know this is low voltage but I cannot risk making a :furious::furious::furious: mistake.

 

[stugray]

Yes, you can try it.

Just use the DMM to verify that it is not over/under-driving them.

I just built a setup with 2X 12VDC @ 1.3A fans.
I put them in series & ran them with a 18VDC ( 9VDC each ) Wallwart, and it is perfect
They run at under full power and start consistently.

Stu

 

[redfishsc]

Do I simply place the multimeter contacts before and after the fan I'm checking to see the voltage that it's receiving, or do I check the input/output of the wall wart-- or does it matter?

Many thanks!!!

 

[sjm817]

Yes, that should work. Put the meter across the fan and it should read ~12V.

 

[redfishsc]

Well. I just did that and got some unusual numbers, but the fans are running just fine.


Voltage across one fan-- 6.5 to 6.75v

Across both fans together (ie, power supply - and +), I get just noise. With the DMM set to 20VDC this reading bounces around 0.5v, which oddly is about double the noise that I got from checking the single fan (0.25v noise).


Any idea what I did wrong? Thanks for the help.

 

[funkman262]

I have 2 fans running in series over my lights but then I'm running a single 12V fan at 24V over my sump water for cooling (I want the extra power and the fans were free so I'll just see how long they last).

 

[sjm817]

Are you sure it isn't a 24VAC supply?

 

[redfishsc]

I assume the greatest risk you're taking by overdriving the fans is maybe the fan will blow a little magic smoke and be done with, since the 24v power supply most likely is rated for much more than the fan current draw. Right?

 

[redfishsc]

Are you sure it isn't a 24VAC supply?

Um. Actually, it IS an AC supply. Somehow I misread it (from the shop's junk pile).


So I'm running two fans for the past couple of hours, as I'm studying hebrew vocabulary for class, and they've been running very well on this AC supply.

I assume it's not a good idea to keep them on this supply right? The power supply is only mildly warm to the touch and the fans seem to be working....


But that certainly explains the voltage noise.

 

[funkman262]

I assume the greatest risk you're taking by overdriving the fans is maybe the fan will blow a little magic smoke and be done with, since the 24v power supply most likely is rated for much more than the fan current draw. Right?

Correct. My power supply is rated at 6.5A so I never even bothered to check the amperage of the fans. The only other things connected right now are two 1A buckpucks for my LEDs.

 

[sjm817]

Um. Actually, it IS an AC supply. Somehow I misread it (from the shop's junk pile).


So I'm running two fans for the past couple of hours, as I'm studying hebrew vocabulary for class, and they've been running very well on this AC supply.

I assume it's not a good idea to keep them on this supply right? The power supply is only mildly warm to the touch and the fans seem to be working....


But that certainly explains the voltage noise.

Turn it off till you get the correct DC supply. You dont want to burn something up.

 

[redfishsc]

Done. I ran them for nearly 3 hours, lol.... they ran fine. Fortunately I didn't have them mounted in place, they were sitting on the cement shop floor where I had set up a little "study".

Looks like I'll be going to WalMart tomorrow, I think they carry 12v wall warts, hopefully with some reasonable amperage. I'll just plumb them in parallel

Thanks for your help!

 

[sjm817]

If you can, get the ones with the adjustable voltage. I use one that has 12, 9, 7, 5 VDC or something like that. If the fans are too noisy, you can run them at 9 or 7V to quiet them down (with a loss of flow).

 

[der_wille_zur_macht]

I assume the greatest risk you're taking by overdriving the fans is maybe the fan will blow a little magic smoke and be done with, since the 24v power supply most likely is rated for much more than the fan current draw. Right?

The only way you can overdrive a DC fan is to provide it with more voltage than it's rated for. The fact that the fan only draws X amps and your power supply is rated for more than X amps is not going to overdrive the fan - it will only pull X amount of current, regardless of the rating on the supply.

RPMs on DC fans are more or less directly related to the voltage you supply, as evidenced by:

If you can, get the ones with the adjustable voltage. I use one that has 12, 9, 7, 5 VDC or something like that. If the fans are too noisy, you can run them at 9 or 7V to quiet them down (with a loss of flow).

While I agree this is a useful practice, make SURE the fan operates OK on the lower voltage. Some cheap PC fans don't behave well at lower voltages (they'll stop after a few minutes, or won't actually start running when they're switched on). So, test several start/stop cycles over several days before relying on it to cool your LEDs. In practice, it seems like pretty much every cheap 12v fan will run fine on 9v, almost all will run fine on 7 or 8v, but below that, reliability is pretty spotty.

 

[sjm817]

I prefer to run the fans @ 12V, but if you happen to own noisy fans and want to quiet them down, undervolting is a way to do it. As above, 9V or 7V is pretty much a given. Below that they may not start.

 

[funkman262]

I have 2 fans running in series over my lights but then I'm running a single 12V fan at 24V over my sump water for cooling (I want the extra power and the fans were free so I'll just see how long they last).

I assume the greatest risk you're taking by overdriving the fans is maybe the fan will blow a little magic smoke and be done with, since the 24v power supply most likely is rated for much more than the fan current draw. Right?

The only way you can overdrive a DC fan is to provide it with more voltage than it's rated for. The fact that the fan only draws X amps and your power supply is rated for more than X amps is not going to overdrive the fan - it will only pull X amount of current, regardless of the rating on the supply.

I think the OP understands that concept because he asked in response to my comment about applying 24V to the 12V fan. He only mentioned the current to state that burning out the fan is the only risk because the fan won't draw more current than the power supply can supply, therefore not affecting the power supply, only the fan

 

[der_wille_zur_macht]

Agreed! Just wanted to make the point clear, in case others read the thread and missed that subtlety. Seems like every other thread about powering fans, people get worried because their power supply is rated at "too many amps."

 

[funkman262]

As far as overpowering fans (in case anyone was interested in doing so), for me it was hit or miss. My first fan did exactly as redfishsc stated:

maybe the fan will blow a little magic smoke and be done with

But then I tried another one and it's been running fine for about a week now. Again, I got my fans for free and didn't care but take caution if you have a nice fan that you just dropped a few bucks on

 

[der_wille_zur_macht]

take caution if you have a nice fan that you just dropped a few bucks on

ALSO take caution and/or include failsafes or backups in your design if the fan is providing vital cooling to an LED array or other equipment - it would really suck to cook a bunch of LEDs because a single cheap fan died!

 

[redfishsc]

Agreed, DWZM, on the failsafes. I have two heatsinks (2X24" long) each with 13 LEDs, and active cooling is an absolute.... so I'll probably have two of the small CPU fans on each and possibly another Walmart personal fan blowing across them both.



So two questions.

1) Why did the DC fans work so well on the AC power supply? Seems to me like they'd just get warm and not really move much.

2) What would happen if you connected a 12v fan to a 12v automotive battery....... I have no reason to do that but... the kid inside wonders....