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12v to 7v Molex Fan Switch

Picture of 12v to 7v Molex Fan Switch
When you have high performance fans [high CFM and static pressure] your fans tend to be pretty noisy [over 20dBA] when you use them at 12v. I will show you how to make a simple 12v to 7v switch so you can silence your fans on demand but not lose the 12v for those high performance moments. Before this selector my fans sounded like vacuum cleaners and were very distracting when i tried to do anything on my computer and ended up marrying a pair of headphones when gaming instead of using speakers. Afterwords I couldnt even hear my fans.
 
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Step 1: Tools You Will Need and Supplies

You really do not need much for this.

Tools
*dikes
*digital multimeter
*crimpers 

What you need
*a molex to fan adapter or some time of female molex connector
*a selector switch [single pole double throw if you are going to make one, double pole double throw for 2 in one]
*some terminals

Optional
*a proper molex mod tool
*soldiering iron
*soldier
*heat shrink
*a drill to mount the switch externally
*extra wire

Recommended
*3 way fan splitter

Step 2: Molex Principles

Picture of Molex Principles
Standard Molex wires are 12v [yellow], ground 1 [black], ground 2 [black], 5v [red]

fans typically run off of 12v, but if you use 12v positive and connect the 5v to the negative it acts like this 12v + [-5v] = 7v. The 5v is used as a negative voltage in this case which results in 7v. This is pretty basic stuff. What I'll show you how to do is make a selector switch between 12v and 7v. There is another post on here for a 12v to 5v selector switch, but 5v is almost useless, doesnt have a high enough CFM and static pressure to do any good and some fans wont even spin up at 5v. 
macwhiz1 year ago
On most power supplies, the grounds are all common. The only negative bias voltage is -12v.
waldosan1 year ago
wouldn't the voltage be higher if you were following the equation you gave? also to make sure that i'm understanding this: you attach the negative terminal of the fan to the five volt rail and the positive to the twelve volt rail and it's the difference between the two that makes seven volts? (12-5=7) what keeps the circuit from blowing up the power supply when you connect the five volt rail to the twelve volt rail? or is that not something to worry about?
vanwazltoff (author)  waldosan1 year ago
There is such a thing as a negative voltage, basically its like having a bunch of batterys in series, then you flip one around so instead of 3v + 3v + 3v + 3v = 12v, you flip one around and it acts as a negative voltage so [-3v] + 3v + 3v + 3v = 9v [wouldnt recomment doing that with batteries, it is simply to help understand the principle] if you have a descent quality power supply this wont be a problem. anything that is an 80+ will handle it no problem, if you dont have an 80+ you are asking for problems with or without this mod

quote from tech powerup: average computer has a +/-12, +/-5, and 3.3v rail, and it is simple to power fans off of these rails by changing the power lead for the fan from the normal 12v to the 5v rail, as most fans won't start with 3.3v. While these rails are common knowledge, many people don't know that there are also several "virtual rails", the 8.7v, 7v, and 17v, as well as the 24v and 15.3v rails. These rails are not true voltage rails, they are accessed by using one of the real voltage rails instead of the common ground as a ground for that rail, so the 12v=12vrail+ground, 5v=5vrail+ground, and 3.3v=3.3vrail+ground. Now, if you switch the ground to one of the lesser voltage rails, such as the 12vrail+5vrail, you end up providing 7v, which is the popular 7v fan mod
In your scenario with the batteries, you would get 6V.
-3V +3V = 0V, then you get 0V +3V +3V = 6V.
A better analogy would be moving the ground from the negative of the fourth battery to the negative of the third battery - which would give you 9V since you have raised the ground plane up by 3V.

Waldosan - what he is doing is technically raising the ground voltage up by 5V.
Since the 12V is measured with respect to where ground is, if we raise ground by 5V, then the "12V" line is now only 7V above the new "ground".
This can be hazardous IF the equipment has any other ties to the casing or other true ground sources, but these fans are all plastic so that isn't an issue.
However, I wouldn't recommend this procedure at all because depending on the power supply, you are creating (an albeit small one) bridge between 2 outputs, and could cause the circuits to perform erratically in cheap power supplies. A better solution would be:
Swap the fan from the +12 to the +5 volt line. Most fans will operate at 5V, and it would be even quieter than at 7V - although you are sacrificing quite a bit of cooling.
.
OR

Put the switch as a series resistor bypass. You may need to experiment to see what value of resistor works best for your fan type - but once you have a resistor in series (make sure it can handle the current) with your fan that makes it quiet enough without sacrificing too much cooling, then all you do is wire the switch in parallel across the resistor. When the switch is on, you get the full 12V, and when off, the fan runs in series with the resistor.
It might be easier if I added a diagram.
Q-Fan.jpg
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