Running 3-Pin Fans on 7V The Easy Way

With fully illustrated insructions. – Owen Stevens

CORRECTON:

Too bad I’m a Mechanical Engineer, not an Electrical one – HA! I received several emails telling me how I incorrectly made reference to a -5V line which is really a +5V line. It seems that what I am doing is raising the ground from 0V to 5V. The real equations would be more like +12V – (0V) = 12V for normal operation and +12V – (+5V) = 7V for 7V operation. Thanks for the all the feed back!

I discovered this while working on another cooling project but it came in so useful I thought I would make it a separate article. I may not be the first person to think of this, so if someone else has done this, I don’t mean to take all the credit – I just haven’t read about it any where else myself.

This idea I had was how to power a fan with a 3-pin plug at 7 volts (V) without having to clip the wires and re-wire it to a 4-pin Molex connector. I have had past issues with soldered together connectors reliability and they just look too “shade-tree” for me (As if! Tongue firmly in cheek on this one! Most of my projects are definitely shade-tree!). This method IS very clean and reliable if done correctly.

I have a few of these 3-pin to 4-pin adapters from a computer show. The adapters only cost $1.25 each, so I figured if I ruined one it was no terrible loss.

Adapter

In their original state, these things use the +12V yellow wire and the adjacent black ground. The idea is to move the ground wire from where it originally is to the location of the red wire +5V.

When you have +12V – (+5V) you get +7V.

At 7V, most fans still run fine but are much more quiet. Of course, your power supply (PSU) has to be OK with the amount of current you are sending down the +5V line or problems may arise. My Enermax 431W PSU seems to be fine with this.

Here are the tools that I used: A $2 small screwdriver kit from the Home Depot checkout line display (hey – it was an impulse buy that actually comes in really handy!) and a utility knife.

Tools

Aha! It includes some tweezers! These are the real ‘find’ in this kit. The thin tweezers allow me to take apart the Molex style connectors.

To take apart the Molex male connectors you need to bend these tabs back in toward the center. You can squeeze both at once (you may have to rock the tweezers back and forth to get both sides compressed) with the tweezers like so…

Plug

Do this for both the red +5V line and the black fan plug ground. So it now looks like this:

Plug

The tabs need to be bent back out before re-insertion – here is where the utility knife comes in. Be careful with utility knifes because they’re razor sharp! Of course, you have to put in a sharp blade every once in awhile or they can get pretty dull. This one is pretty new and sharp but it’s predecessor looked more like a saw blade. Did I ever mention how cutting metal nicks these blades?

Cut

Re-inserted, you can see how the red and black wires cross-over now:

Switch

Our next task is to switch the female connectors. Their tabs can be pushed in like the male ones. But this time use the thin edge of the tweezers. If these have two tabs you have to rotate the wires inside the plug to get to both.

Push

Don’t worry if the tabs get pushed all the way in and look like this:

Tab

You can insert the tweezers into the center and push the tabs back out like this to recover:

Push

And of course, you can use the utility knife blade to bend out the tabs also:

Knife

You need to remove the black and red wires and then switch them like before. You should end up with some thing like this:

End

The beautiful thing about this method is that this 3-pin to 4-pin adapter will power a fan at +7V, yet still be a straight pass-through for the other wires so the next device can be a hard drive, CDROM, etc. The only difference is that the outer black wire is really the red wire from the PSU and the red wire is really a black wire (confused yet ;). I hope this comes in handy for those of you who want a clean looking 7V fan power solution.

PS:

Oh yeah, I will update this later with how to separate out the fan RPM signal wire so you can still get fan speed. I am not sure if it is as accurate because I think the RPM signal uses the ground which is now +5V? I tried it once but I don’t remember how well it worked (too many new ideas to keep track of historical data, HA!)

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