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Two power supplies, two motherboards, one set of fans.

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April 6, 2014 8:34:00 PM

Earlier this year I built myself a server cabinet. It is about 4 and a 1/2 feet tall and close to 2 feet wide and deep. The upper half of the cabinet houses a 19" flat screen monitor, a power strip, a network switch, a KVM switch as well as a lot of the wiring and what not. The top also has a mini control panel where the power on, reset and KVM switch buttons are located. The lower half of the cabinet houses two bench style motherboard trays (flat down) and two 6 core PCs attached to them.

The motherboard trays are attached to drawer slides that allow me to slide the entire board, power supply and hard drive caddy almost completely out of the case. All the components for each PC are attached entirely to the motherboard tray itself and they move in and out independent of eachother for easy access.

Now, here is my question. The lower case has 6-120mm fans pulling air in to the front of the case and 5-120mm fans attached to the rear door pulling air out. I am trying to think of a way that I could power on all 11 fans no matter which of the two PCs I turn on.

Right now I am using 2 NZXT Grid Ten Port Fan Hubs to connect all of the fans to two 4 pin molex power connectors on the bottom PC's power supply. One for the front fans and one for the rear fans. Problem is, if I want to just power on the top PC, the fans don't run. This rig was built for a media and storage server as well as a small scale rendering farm so there are times I want to only run one of the PCs instead of both.

Running all the fans off of either of the power supplies exclusively is of no concern at all because they both have more then adequate wattage to run the fans or even more if I wanted to.

It may be that I am overlooking a very simple solution but I would rather not use a whole other power supply and rig up a power switch just for it to run the fans. So far though, that is all I have come up with that would allow for running either one or the other PC or running both at the same time.

I am curious if it would be too much trouble to wire up something small that would allow either of the two power supplies to power the 4 pin molex connections regardless of which PC was on. Both PSUs have plenty of additional connections to hook into both wires for the fans.

Does anyone know of an adapter that has 2 molex in and 1 out? If I rigged up my own 2 molex in 1 out adapters and connected a 4 pin molex power wire to it from both power supplies, would there be any issue if both power supplies were supplying power to it at the same time? I would also like to add a few LED light strips to the lower case that come on when the fans are running. They also use 4 pin molex for power.

I guess the bottom line here is, does anyone here know of a simple solution that doesn't require a third power supply just for fans and a few lights? Also, would there be any issue with running power from both power supplies to the same wire if both power supplies were turned on?

Forgive me if these are all completely obvious and silly questions. I fear I am over complicating things but I want to be sure before I splice any wires together and wrap them in tape beside a couple thousand dollars worth of equipment. :) 

Thanks in advance for your time.
a b V Motherboard
April 7, 2014 4:22:07 AM

Its possible but not possible, The only way I can see doing this is with a 3 way toggle switch, EX: up is on when the top server is on, or both, and down is on when the bottom server is on or both.

the way you described it would not be safe. Instaed of feeding fans normally 12v, I think you would end up feeding the fans 24v if both server PSUs were on, that would not be good and possibly blowing up an PSU.

Here's how I would do it (just and idea). I would start by getting a 140watt PSU or what ever I have laying around from a cheap PC. And wire it where it would turn with a toggle switch with a light (easy to do, usually the Green and a black wire turns the PSU on without a mobo). And use it just the PSU to power the fans. That way you could even throw in just 1 fan controller.

Like I said thats just an idea, not sure what or how much room you have in them server boxes, or if this would be even possible for you
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a c 180 V Motherboard
April 7, 2014 9:10:56 AM

I think the easiest solution would be to connect half the fans to each power supply and have them come on when each power supply is turned on. If the air flow is not sufficient then adding fans would be easier then adding a separate power supply and a switching mechanism.
If one computer setup gives off more heat then the other then you could separate the fans accordingly.

There are fans that run off AC and you can connect a few to a regular switch that you could turn on no matter which computer is running.

http://www.performance-pcs.com/catalog/index.php?main_p...

http://www.frozencpu.com/products/6740/fan-ac-15/Fantec...
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April 7, 2014 9:45:46 AM

Viking2121 said:

the way you described it would not be safe. Instaed of feeding fans normally 12v, I think you would end up feeding the fans 24v if both server PSUs were on, that would not be good and possibly blowing up an PSU.


Right, see that is what I figured. Even though I still wanted it to magically work, I guess I just needed to hear someone confirm it for me because now it seems painfully obvious. Of course it would blow up, I'd be connecting two power sources together.

I am pretty sure I have a brand new 20 pin Power Supply sitting here somewhere that I doubt will ever get used again. Perhaps a simple toggle switch would be ideal as long as I always remember to turn it on. The only thing that worries me about that is if the power supply running the fans ever malfunctions or shuts off mid render, a lot of heat would be generated inside the cabinet in a very short period of time. Some of the video that I produce can be well over 5000 frames so it can take over 8 hours to render even with two 6 core processors working on it.

I think if I do go with the 3rd power supply option, I wonder how complicated it would be to have some sort of alarm or fail safe built in. Any ideas? Maybe it would be ideal to just lower the shut off temperature from the UEFI?

Not sure why but it still amazes me how finding a solution to one problem usually ends up creating a completely different problem.

Thanks for the reply.
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April 7, 2014 10:52:10 AM

inzone said:
I think the easiest solution would be to connect half the fans to each power supply and have them come on when each power supply is turned on. If the air flow is not sufficient then adding fans would be easier then adding a separate power supply and a switching mechanism.
If one computer setup gives off more heat then the other then you could separate the fans accordingly.

There are fans that run off AC and you can connect a few to a regular switch that you could turn on no matter which computer is running.


Interesting. I hadn't even thought of AC fans as a possibility. Didn't know such things existed.

To be honest, a big thing I don't like about the idea of adding in another power supply is how, since there is no motherboard for it, no matter what I'll have to splice something into existing wiring. I like to use things as they are intended to be used whenever possible. I think it somehow, inside my own head, convinces me the component or device will last the longest.

Thing is, as far as running only some fans with either of the two PCs, I'm not sure I like that idea. The front fans are meant to create slight positive pressure and move a ton of air directly across all the components inside. With all of them running I have seen fantastic temperatures being maintained. 19c or less at idle and 40c under heavy load on the CPUs and that is with just air cooling inside a wooden box.

This thing is all custom made from top to bottom so adding any fans is not possible. Built it in my backyard actually, lol. There is just no more room. Here is a page I just put a few pictures up on. Excuse my messy desk. Maybe this will give you a better idea of what I'm working with though.
Server Build Pics

Both the top and bottom PCs are almost exactly the same. ASUS M5A 99FX Pro boards with AMD FX6350 6 core CPUs and 8GBs of GSkill 1866 Sniper Series RAM. Both have Cooler Master CPU coolers with dual fans and an exhaust fan on the motherboard tray itself all pushing air towards the back so maybe you are on to something here.

Curious though, if I were to purchase 11 AC fans wouldn't that mean I'd need 11 outlets to plug them into?

As I mentioned to the post above yours, I do have a new 20 pin power supply here that I doubt I will ever use for anything else. It is 430W and I am pretty sure I could easily get it into the top of the cabinet and then wire it down to the 4 pin connectors for the fans and add in the LED strips. Maybe that would be the best idea at this point but the only thing that worries me about that is if that power supply stops running, the fans stop and the PCs could go on running with significantly less airflow.
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a b V Motherboard
April 7, 2014 11:28:05 AM

Well its not hard to get the PSU to turn on, I usually use a paper clip to turn on a PSU to test DVD/BruRay drives or bearings/bushings in fans. I think even with a cheap PSU, It would be vary unlikely to to fail, Fans don't require much, not like what your servers would.

If your worried about the PSU possibly failing over time. I agree with inzone, Hook the fans up where some are in the 1 or the other. If you have a server that runs hotter then the other, You may just need to adjust it where more fans are on in the hotter server when its powered on.
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April 7, 2014 12:25:38 PM

Viking2121 said:
Well its not hard to get the PSU to turn on, I usually use a paper clip to turn on a PSU to test DVD/BruRay drives or bearings/bushings in fans. I think even with a cheap PSU, It would be vary unlikely to to fail, Fans don't require much, not like what your servers would.

If your worried about the PSU possibly failing over time. I agree with inzone, Hook the fans up where some are in the 1 or the other. If you have a server that runs hotter then the other, You may just need to adjust it where more fans are on in the hotter server when its powered on.


Yeah, I think you're right. A 3rd PSU will allow me to run all the fans and a controller plus all the LEDs on one switch. I would imagine it would be pretty simple to wire in a toggle switch between the two wires that turn the PSU on right? So, essentially, when you stick a paper clip into the 24 pin connector you are closing a circuit and I should be able to do the same thing but use an actual switch.

Not sure if you saw it on the reply I just posted to inzone but here is a link to a few pictures of the cabinet.

Server Build Pics

Not super easy to tell but both motherboard trays are in the bottom and right on top of eachother with their own drawer slide mount. They slide straight out the back of the cabinet so I can access them easily. Anyway, what would be ideal would be to wire a toggle switch to control the power supply turning on and off and to have that toggle switch mounted up on the control panel just under the monitor.

Would a wire from the toggle switch positive to the green of the 20 pin and a wire from the toggle switch negative to the black of the 20 pin do this you think? I am not all that educated on wiring if you can't tell. Could you link me to a toggle switch you would use for this perhaps? Just a simple on and off.

I really appreciate you taking the time to try and help me with this.

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a b V Motherboard
April 7, 2014 9:10:48 PM

Green wire in the 24 pin activates the PSU, Black is just ground, Here is diagram on how to switch on a PSU without a motherboard, vary simple.

http://www.techpowerup.com/articles/other/22


Thats a nice setup! I like it.
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