March 26, 2002
by Mike Chin
; updated Dec 14, 2003 (to include 7V information)
As soon as you begin trying to quiet your PC, you realize how useful it is to be able to provide lower than 12 volts DC (VDC) to fans in the system. Most PCs don't provide any way to control the speed of the fans. There may be accessories you can buy, but sometimes, things like fan bays are overkill. Sometimes all you want is to be able to switch between high and low speed.
This simple project describes how to rig up a switch that will let you feed 5 VDC to your fan(s) for low noise operation, and 12 VDC when more cooling is needed for hot weather or a bout of serious overclocking. It's also useful to check whether the fan is working if you have one that's silent at 5V -- like I do in some of my systems. :) Only 2 parts are required: a small toggle switch that cost $1.30 and a male IDE power connector (with pins pre-wired) that I had kicking around. A soldering iron is recommended but not necessary if you use a switch with screw terminals. It took me less than 15 minutes to build.
You probably know by now that the standard IDE power connector from the power supply contains 2 voltage lines and a third which is derived from their difference: 12V, 5V and 7V. Running 7V CAN sometimes be potentially stressful for the PSU for technical reasons I won't go into here. (It is usually quite safe for a low wattage fan or two, but if you want to be perfectly safe, you may want to avoid the 7V tap.) The picture below shows how the various voltages can be obtained:
Note that the pins on the IDE power connector are not colored; the wires that lead to them are. The red and black lines represent the wires from the fan. Also note that sometimes the ground ( - ) wire from the fan is not black, but blue. The positive lead (+) is almost always red for all electrical or electronic components.
- To obtain 12V, the red fan wire goes to pin #1 (yellow), and the black fan wire goes to pin #2 or #3 (black).
- To obtain 5V, the red fan wire goes to pin #4 (red), and the black fan wire goes to pin #2 or 3 (black).
- To obtain 7V, the red fan wire goes to pin #1 (yellow), and the black fan wire goes to pin #4 (red).
Another image in case this is not clear:
|Caution! There are people, who will say "You can't feed current back into your power-supply" and they are right!
If you plug something between the 12V and 5V lead, you have to be absolutely sure, that no current is fed back into the PSU.
Which means, that you need to have other loads on that 5V lead, which suck out more current than you feed to that lead from the 12V lead.
For example: Your fan uses 100mA at 7V. You take 100mA from the 12V lead and feed them into the 5V lead. If you have a different fan (or whatsoever), that sucks out 200mA from the 5V lead and feeds them to ground, you're fine.
100mA are sucked out of the 5V lead, 100mA come from the 12V lead and 200mA go to ground.
Since usual fans only use 1...3W, you are usually perfectly safe with using the 7V trick there. There a lots of devices in typical computers, which draw much more power out.
Peltiers or things with power consumptions running up to dozens of watts, require careful calculation of what goes out of the 5V lead and what's going in there from the 12V lead.
for a 12V/5V switch
1 - double-pole double-throw (DPDT) ON-ON
1 - "male" IDE connector with prewired pins
You will also need a soldering iron or gun and wire strippers or cutters. A clamp or vise to hold the switch will be handy; a pair of pliers can also be used.
A DPDT switch has three positions (2 polarities), and can switch two independent circuits at one time. The images below explain more clearly.
The image on the left is a typical switch; the image on the right is the wiring diagram for this 12V / 5V switch. The IDE pin numbers referenced are as shown in the previous IDE connector drawing.
The photo below shows the switch with IDE "male" connector and fan, all soldered together and ready to go.
Just plug the male IDE connector to a spare from the power supply. The switch will toggle the fan between 12VDC and 5VDC. There are usually many small holes on the back panel that the switch can be mounted in. If you have to drill, take care not to let the metal shavings get into your components. It's probably wise to remove all the components from the case before drilling -- or drill before installing any components. You can wire up several fans in parallel to the same switch -- but watch that the ampere rating of the switch is not exceeded. (Add up the amperage of the fans and ensure that the total is lower than the switch rating.)
Here's a photo of my setup, inside and out.
That's it. Simple as can be. Enjoy.
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