Question Connecting Fan to PSU

Sep 1, 2021
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I have an old HP Compaq microtower prebuilt PC and I want to upgrade my CPU Fan. There is only one 4-pin fan header on the motherboard which is a proprietary fan header just for the chassis fan. I heard the wires are swapped and when I unplug it I get an error when the PC is booting. That doesn't look like an option so I found out about connecting fans directly to the PSU, with the only difference being not being able to control the speed. I'm OK with that. I bought a SATA to 4-pin fan cable and connected my fan to the PSU with it but the fan doesn't work at all.
I thought the only thing necessary for the fan to work was the 12V and the ground. I tried connecting the cable in many ways, all four, just three pins on both sides, just two pins on the 12V side, even re-arranging the fan cable itself but the fan never worked. Is connecting the fan to the PSU through SATA even possible? What am I doing wrong here? Thanks for all the help.
 

jay32267

Champion
"I thought the only thing necessary for the fan to work was the 12V and the ground. "
This should be correct.
The sense line (generally green on the fan) should remain disconnected.
The control line (generally blue on the fan) should also remain disconnected.
Notice I said "disconnected" and not "connected to ground".

"According to Intel, if no control signal is present the fan shall operate at maximum RPM. "
This is why the blue line must remain open (disconnected).
 
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Eximo

Titan
Ambassador
Um, you ran all four wires to the fan? Not really sure, but that might have broken it.

Just need two wires to make a fan go, 12V and a ground if you want max RPM. 5V and a ground may not be enough to spin the fan, but if it is, it will run quite slow. 12V with the 5V in the position of the 'ground' will net you 7V and the fan should spin slower as well.
 
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Sep 1, 2021
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Oh no... I've tried many many things and also got some electricity sparks when trying to measure volts :D Well... One last question then. If I broke it, is it broken broken? And is there a way to test that?
 

Paperdoc

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Some hints, and further cautions.

Look closely at the 4-hole female connector on the end of the NEW fan's cable. On one side are two ridges that "surround" the first three of the holes. Hole #4 is beyond the ridges. Hole #1 is at the opposite end right inside one ridge. THAT is the GROUND power supply line. NEXT to is is Hole #2, and THAT is the +12 VDC power supply line. Make those two connections ONLY between the fan and the relevent wires from the adapter. Of those, BOTH of the Black lines are Ground, and the YELLOW is +12 VDC. IF the fan starts easily and runs full speed when connected this way, then it may be OK. If not, it is damaged and needs replacemant - you cannot repair.

That still may NOT solve your problem. When you connect a fan to a mobo header, the mobo receives from the fan a speed signal. MANY mobos - especially the CPU_FAN header - monitor that signal for NO signal, indicating zero speed and fan failure. THAT usually will cause the mobo to refuse to start up at all, to protect the CPU chip from operating with no cooling. BUT when you connect a fan directly to the PSU, there is NO way for the fan speed signal to be fed to that mobo header, and it detects - NO fan speed! So no boot!

IF you can find out the pinout details of the original fan (and hence of the mobo header), you can rig a custom connection for the new fan - maybe even just swapping wires in the fan connector - so it CAN plug into the mobo header. THEN you could run the fan AND gets its speed signal to the header, and make the mobo happy.
 
Reactions: mevcutonur
Sep 1, 2021
4
0
10
0
Some hints, and further cautions.

Look closely at the 4-hole female connector on the end of the NEW fan's cable. On one side are two ridges that "surround" the first three of the holes. Hole #4 is beyond the ridges. Hole #1 is at the opposite end right inside one ridge. THAT is the GROUND power supply line. NEXT to is is Hole #2, and THAT is the +12 VDC power supply line. Make those two connections ONLY between the fan and the relevent wires from the adapter. Of those, BOTH of the Black lines are Ground, and the YELLOW is +12 VDC. IF the fan starts easily and runs full speed when connected this way, then it may be OK. If not, it is damaged and needs replacemant - you cannot repair.

That still may NOT solve your problem. When you connect a fan to a mobo header, the mobo receives from the fan a speed signal. MANY mobos - especially the CPU_FAN header - monitor that signal for NO signal, indicating zero speed and fan failure. THAT usually will cause the mobo to refuse to start up at all, to protect the CPU chip from operating with no cooling. BUT when you connect a fan directly to the PSU, there is NO way for the fan speed signal to be fed to that mobo header, and it detects - NO fan speed! So no boot!

IF you can find out the pinout details of the original fan (and hence of the mobo header), you can rig a custom connection for the new fan - maybe even just swapping wires in the fan connector - so it CAN plug into the mobo header. THEN you could run the fan AND gets its speed signal to the header, and make the mobo happy.
The fan signal part is no problem-- this PC originally only has a chassis fan and no CPU fan and the heatsink of this new fan I bought is enough to make it work, but it still gets really hot when under load.
Today I bought a 9V battery to find out whether the problem is with the power or the fan itself and for the very first time I was able to make the fan work somehow, with the happiness of the moment I didn't really pay attention to how I did it. After that no matter which combination of the 4 wires from the fan cable I tried, I couldn't get it to work again. So even if it wasn't damaged I might have damaged it today. I'll try connecting it the way you said and see if it works. Just to make sure, if it doesn't spin, does it mean that I broke it by trying a bunch of wrong ways to connect it? If so, is there nothing I can do?
 

Paperdoc

Champion
Ambassador
Yes. I'd say of it does not work, it is broken. By the way, a new good fan SHOULD start up and run with a 9 VDC supply, just not full speed. However, a 9V battery does not store a lot of power, so the fan will not run for a long time.

One easily-accessed 12 VDC power supply is an auto battery, with lots of power stored. Remember that the LARGER battery post is +, and that must go to Hole #2 of the fan female connector. Smaller post is -, to Hole #1. A couple short lengths of wire is enough tor a short-term test connection.
 
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