Basic Question: Undervolt vs. Reducing the Fan Speed

wpb · Post by **wpb** » Mon Nov 06, 2006 12:41 pm

Sorry for such a basic question, but I did a search and couldn't find an answer.

What (if any) is the difference between undervolting a fan vs. reducing its fan speed?

I have 92mm Nexus fan in the front of my case. I'm using a D975XBX MB and I can slow the fan down using the Intel Desktop Utilities software. But I'd like to know if it's more efficient to reduce its voltage. Regardless, I'd still like to understand how undervolting a fan is accomplished.

I don't think I'm brave enough to try undervolting my CPU, but if you could help me understand how that is done, I would appreciate it.

fyi - I looked in the BIOS but couldn't find any option to reduce fan or CPU voltage.

Thanks,
Bill

kesv · Post by **kesv** » Mon Nov 06, 2006 1:15 pm

There are two ways of lowering a fan's speed. Voltage regulation and pulse width modulation. Voltage regulation just drops the voltage fed to the motor. Pulse width modulation (PWM) uses constant voltage, but turns the power on and off in rapid 'pulses' effectively reducing the speed.

You can purchase fancontrollers using both methods, but PWM is more commonly found integrated on motherboards. If the PWM controller works there is no reason to change, but it does have the disadvantage of causing annoying buzz on some fans.

wpb · Post by **wpb** » Mon Nov 06, 2006 7:57 pm

Thanks for the reply.

So if I want to adjust the voltage, then I will need to purchase a fan controller like the Zalman Multi Fan Controller (ZM-MFC1).

And I assume PWM is already integrated on my motherboard since I can adjust the fan speed using the Intel Desktop Utility application.

Correct?

disphenoidal · Post by **disphenoidal** » Mon Nov 06, 2006 10:42 pm

If the motherboard allows you to reduce the fan speed and you're satisfied with that, there's no need for anything else. Like kesv said, the motherboard most likely provides fan control by pwm and if that works, you don't need anything else.

Undervolting, as in providing a continuous voltage between 0 and 12, usually requires extra hardware, as motherboards don't really do analog control as far as I know. Many aftermarket parts allow you to do this. Undervolting can also simply be accomplished by plugging a fan into 5V or 7V power connectors, or by placing resistors or diodes (diodes are preferred) in series with the fan.

PWM in theory allows you to run slower speeds than analog control, because the fan gets 12V impulses that help it start. I imagine it would be cheaper to implement on a motherboard because it can be all digital. Analog control prevents the buzzing noises of pwm because the voltage is continuous. It would be cheaper to implement off a motherboard, as it requires few parts.

Note that you wouldn't want to combine these two methods, one or the other will work for you.

http://www.cpemma.co.uk/ Homebrew fan controller page, tells how many analog fan controllers work, and how to build your own pwm controller.

wpb · Post by **wpb** » Tue Nov 07, 2006 1:20 pm

Excellent explanations. Thanks!

bushum · Post by **bushum** » Mon Nov 13, 2006 2:36 pm

disphenoidal wrote:
Note that you wouldn't want to combine these two methods, one or the other will work for you.

Why is this?

I have a 7V "Y" connector on two 120mm fan in order to get them both running off of the CPU fan on the MOBO. I also use SpeedFan to lower the speed. Should I find a standard "Y" connector or is this even a problem?

disphenoidal · Post by **disphenoidal** » Mon Nov 13, 2006 6:09 pm

Not necessarily. The only reasons not to use both methods are either your fans don't get enough voltage to start, or it's sufficient to use one method, and it would just be extra effort/hardware to use both.

Since you already have the hardware, as long as your fans get enough voltage to start, you can keep it the way it is. I was just trying to advise the OP that he wouldn't need extra hardware.

wpb · Post by **wpb** » Mon Nov 13, 2006 7:55 pm

As an update to my original post, I decided to just use the Intel utility software to adjust my fan speeds (front and rear case fans and CPU fan.)

However last weekend I replaced the stock Intel HSF with a XP-90 and a Nexus fan on the heatsink. The Nexus has a 3-pin connector to the CPU. I can no longer control the CPU fan with the Intel utility software. I assume it's because the fan is 3-pin and not 4-pin like the stock Intel HSF.

So if I understand correctly, in order to reduce the CPU fan speed, I will need to purchase a fan controller to reduce the voltage to the CPU fan.

Is that correct?

bushum · Post by **bushum** » Mon Nov 13, 2006 8:32 pm

Ok thanks disphenoidal.

dfrost · Post by **dfrost** » Tue Nov 14, 2006 1:51 pm

disphenoidal wrote:Undervolting, as in providing a continuous voltage between 0 and 12, usually requires extra hardware, as motherboards don't really do analog control as far as I know.

What I wrote about Abit motherboards in another post:

dfrost wrote:I'm quite pleased my Abit AI7 (socket 478 motherboard), and particularly like the FanEQ function that supplies variable DC voltages (not PWM-variable) to two fan headers independently, depending on user-defined temperatures. Just wish that the voltage range went below 8V. I believe that the newer Abit boards typically control more headers, and some headers allow voltages down to 6V. Not sure if this function is only available on boards with "uGuru."

Anybody know if newer Abit motherboards still do it this way with FanEQ?

levander · Post by **levander** » Wed Nov 15, 2006 12:09 am

wpb wrote:So if I understand correctly, in order to reduce the CPU fan speed, I will need to purchase a fan controller to reduce the voltage to the CPU fan.

Is that correct?

I've never used on of those fan controllers, so I'm not 100% sure what they do. But, you can also just use an adapter that will downvolt the fan.

Downvolt 12v to 7v
Downvolt 12v to 9v

It's interesting that adapter that brings down to a 9v seems to still plug into a fan connector. I guess that's so that you can still get an RPM reading off the fan. But, from reading. When most people downvolt, they don't stop at 9v, they stop at 7v. I'm guessing the down to 7v adapter has to plug into the power supply is not just because the cable designers were lazy, but because there's something electrical that requires them to do it like that.

But, if you don't guess you're desperately trying to reduce noise, but do want some improvement, and would like to see the fan's RPMs show up in software that tries to read it from the motherboard, you could do the down to 9v cable.

Jab-tech is a good store to buy stuff from, especially fans and accessories.