Mini Article - The 7-Volt Mod

Many people have fans they want to quieten, stock CPU fans that sound like a tornado, generic case fans that are worth 99p (and are as noisy as the ice cream van which sells the treat by the same name).

Yes, fan noise can be a great distraction to your computing experience, and while many live with it, others choose to do something about it, either by buying specially designed 'quiet' fans, such as those made by SilenX or Nexus, or by 'modding' their existing fans to make them quieter.

One very popular solution is to buy a fan controller, this is a unit which occupies a drive bay on the front of the computer, which in turn allows the user to reduce the speed of several different fans, usually while looking quite cool in the process!

Alas, some PC users cannot afford such luxuries as silent fans or fan controllers, but should they still have to suffer? The answer is gladly "no"!

To reduce the noise made by a fan, you need to reduce its operating voltage, this in turn lowers the speed at which it spins (reducing the amount of air it moves). Most fans in home computers run at the default voltage of 12V, but most will still quite happily work at voltages as low as 4 or 5V. Therefore, the cheapest way to slow a fan down is to just run it from a different voltage rail than normal.

*note* when slowing down a fan, keep a close eye on system temperatures, since the amount of air being pushed will be reduced as the fan slows down.

Most people know that the average computer has +/-12, +/-5 and 3.3V rails, but many people don't realise that there are also several 'virtual' rails, 7V, 6.7V, 15.3V, 17V and 24V. Theses aren't true voltages produced by the power supply unit (PSU), they are instead produced by substituting the ground wire for a different voltage rail. Now normally, voltages are produced like this:

12V = 12V rail + Ground
5V = 5V rail + Ground
3.3V = 3.3V rail + Ground

The popular '7 volt fan mod' is produced by replacing the ground wire with the 5V rail like so:

7V = 12V + 5V

The 5V 'pushes back' at the 12V, giving an effective voltage of 7V (12-5).

The 7 volt fan mod is popular because the common 4-pin Molex connectors in computers (which are used to power hard disks and optical drives) have a 12V line, two grounds, and a 5V line. Also, some fans refuse to turn at something as low as 5V, but 7V is pretty much guaranteed to work fine.

The other voltages I mentioned are obtained by bringing a wire from the ATX connector which plugs into the motherboard, but this isn't recommended due to safety reasons. This also reminds me of another safety issue; only use this mod with a decent quality PSU as it may damage cheaper, lower quality PSUs. Also, if starting your PSU with the wire trick, do not use this fan mod, as there will be no additional load on the PSU (this could result in physical damage to your PSU).

So to sum up, here are three images which should be self explanatory. Happy modding!

A fan running at 12V:

A fan running at 5V:

A fan running at 7V: