Silence Your Fans With The ’7 Volt Trick’

by Jason Faulkner on Jul 26, 2008 | 12 comments

For the dedicated system tinkerers out there looking to make their case fans run a bit quieter, the 7 volt trick may be for you.

The idea here is (as I understand it) the fan only uses 7 volts of power instead of the usual 12, which in turn causes fewer RPMs which causes the fan to run quieter. Of course the lower RPMs will lead to slightly less air flow.

From what I have read about this, it appears to be safe for your power supply, but you must make sure you do not plug any other components into this modified plug! The only ‘side effect’ I have read about is either the fan does not work on only 7 volts or the fan does kind of an off and on spurt.

Has anyone tried this? If so, post your experience.

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Steve Stone
Doesn’t running a motor at a lower voltage cause that motor to run hotter and fail sooner? Maybe it is only an AC thing and not DC ?

http://kkomp.com Sharron (kkomp)
AC motors are generally much simpler than DC motors: I won’t go into the technical details, save to say that on both the resistance of the inductor coil(s) varies according to Ohm’s Law with a decreasing voltage causing an increase in the current used; therefore causing a greater heat output logarithmically proportional and relative to the increase in joules used by the unit at any given voltage. – Clear?
In short then; there is something in the point you query. All-in-all, however, with the high tolerances built in to the motor unit in question, running at limited wattages and using only milliamperes, any such proportional heat increase should be so negligible overall as to be academic – unless one gets down into the 1.* – Volt scale; at which point the motor wouldn’t function anyway until the voltage was increased substantially.
So yes the theory is correct; but for all practical intents and purposes in this particular application it can be discounted as virtually irrrelevant.

http://kkomp.com Sharron (kkomp)
I once tried a 10 watt resistor in series with the exhaust fan – I forget the value in Ohms that I used – which seemed to do the trick: The only problem being that a 10W resistor is fairly large, (I think the one I used was a wirewound ceramic out of the EHT circuit of an old TV.) not really cost-friendly if bought new, and can also generate more heat too depending upon its resistance.
I’ve also tried two fans in series but that was virtually a non-starter.
An idea I’ve just thought up this second, is to have a separate transformer with a 15 Volt or higher: 15 to 20 Volts should do the trick, secondary winding – It only need provide 500mA of current Min. – and use the AC half-wave potential to drive 2 fans at a lesser speed thus:
Connect a rectifier diode (I suggest a 1N4002) in parallel with small 10 nF capacitor across each fan opposing the polarity of the fan’s motor. e.g. connect the anode of the diode to the fan’s negative connection and vice-versa in each case.
Now connect both fans, including the two components that you’ve just connected in parallel with the fans as described above, in series so that the two negative leads from the fans are connected together/the two diodes’ anodes are effectively connected to one another via the fans’ series connection.
Connect both positive leads from the series-connected fans with the extra components added across the secondary winding of the transformer; it doesn’t matter which way round.
You should see the on power-up that both fans spin slowly and quietly. 15 Volts may not be quite enough voltage depending upon the type of fan used; so I would use an 18V secondary if possible.
How it works:
What you have flowing directly out of the transformer’s secondary winding is AC or Alternating Current. Alternating current changes polarity a given number of times per second depending upon its frequency – In this case 50 or 60 Hz.
When the polarity is one way round current flows through one of the diodes thus bypassing the fan that the diode is connected across. However since current can’t flow through the diode in series with it, due to it being against its polarity, it flows through the other fan and causes it to start to turn. When the polarity is the other way round on the following AC half-cycle the opposite happens: The diode that was conducting no longer conducts, causing the electricity to flow through the fan; however the diode across the other fan that was previously conducting and bypassing that fan is no longer able to conduct and therefore the half-cycle in question flows through the other fan.
The capacitors are just for interference-limiting, decoupling, and smoothing in a small way.
So basically the circuit that has just been constructed is sharing the entire available power between the two fans; thus causing each to run at half-power, effectively sharing the 18V between them at roughly 9V each.
As I say I’ve only just thought this up just before I wrote all this, so there are no guarantees: If you’d like to build the circuit and power up then please do: I’d be interested to hear your experiences.
I’ve basically written an entire post as a comment; so I’ll be putting all this in my blog at http://kkomp.com.

http://kkomp.com Sharron (kkomp)
Actually there is a flaw in the way that the comment is written, in the “How it Works” section. The circuit itself will work; but I feel I was too hasty in publishing and have a glaring error in there somewhere.

hitchface
I use a fan controller on my PC, but it essentially does the same thing for more money. Most every fan should be able to run on 7 volts. The only fans that I DO NOT recommend doing this to are ones that have controllers built in, like the Antec Tri-Cool units. Modifying a plug to give it a certain voltage and then using the switch to try and draw more or less than that might cause problems.
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ALaneBro82
This would theoretically work. However, you must be very careful to NEVER touch the 12V rail to the 5V rail. You would most like brick your power supply if you did touch the two together.
In all honesty, I wouldn’t recommend anyone actually try this. That 5V rail is meant to “source” current… in this configuration, it is “sinking” current instead.
A much simpler mechanism is called a potentiometer (think dimmer switch for your dining room light). It varies resistance via a knob and allows the user to directly control the speed of the fan.
You might think about removing this article. I feel it could do much more damage to people than it could possibly help them.

Jason Faulkner
I don’t think removing this in necessary. Lots of people on this site get help overclocking their CPU which could potentially fry your processor/MB etc.
If people want to try this, I’m just passing the information along. Personally, I am not comfortable doing this on my machine, but I’m also not comfortable OC’ing my CPU either.
Different strokes.

http://www.redfruitoil.com/ Redfruitoils
Would not really recommend this for high power computers.. since you need to take in all the cooling you can get.
An attempt to silence your fans could as well lead to your system being overheated.
hitchface
I’m with ya Jason. Lots of people do this trick successfully. If you are gonna screw with computer innards, know what you are getting yourself into and the potential risks. Goes with anything else.
Mark Curts
Jason,
It would be great if you could give first hand experience and say either, “Hey, I’ve actually tried it and it worked for me…” or “My experience was not so good…”
Many of us who are less hardware than software oriented want to hear from someone who’s willing to take a risk himself before we’re ready to commit to something we’ve never tried before.

Jason Faulkner
Mark,
I understand where you are coming from.
I was just posting on this site a well known ‘tried and true’ trick done by many. Do a Google search on 7 volts and you will find lots (and lots) of success stories.
I wouldn’t have posted it here if it is something which I knew (or had a feeling) was sketchy.
Personally, I don’t do any modification of hardware, but that shouldn’t mean I can’t post links to information which some people might want to try.