SPCR

How do you undervolt a case fan that attaches to a three-pin motherboard fan header?

Or, for that matter, one that attaches to a 4-pin Molex connector from the PSU?

Is there any advantage to undervolting, say, a 1200 RPM fan, rather than just using the 800 RPM version of the same model?

Sure. Two main ways to do it, you either lower the voltage or you keep the voltage the same but send it in pulses so that the voltage the fan "sees" is lower (a little bit like +12v, 0v, +12v, 0v really fast). First one is more easy if you want to create your own solution. Most of the time you can have the motherboard do this kind of thing for you. There are also stuff like the Zalman Fanmate that are cheap.

I haven't bought a motherboard in a number of years. Do the motherboards actually adjust the voltage or do they use PWM? I assume this is configured in the BIOS. Can they be configured to set fans at a fixed speed, because I thought PWM was used primarily for varying fan speed according to some onboard temperature sensor.

If it can't be done through the mobo, apart from fan controllers like the Zalman, does someone sell a (non-adustable) hardware device that you can just plug inline?

How the motherboard controls the fan depends on the motherboard and the BIOS settings. What are you aiming to do, just set the fan to a constant lower speed?
There are inline resistors that you can just plug in, but I would recommend a Fanmate since thats let you set the speed. A bit like a variable resistor.

Vicotnik wrote:How the motherboard controls the fan depends on the motherboard and the BIOS settings. What are you aiming to do, just set the fan to a constant lower speed?

I was thinking of that, but was mostly just wondering how it was done.

And again, wondering about the reason/advantage of undervolting rather than just using a lower RPM fan.

The obvious advantage is that the fan could spin faster if the need should arise.

JJ wrote:Is there any advantage to undervolting, say, a 1200 RPM fan, rather than just using the 800 RPM version of the same model?

No, other than the flexibility in the future referred to by Vicotnik.

You can pretty much do anything you want with this... and do it in a reproducible manner so you can test and retest different settings in an easily reproducible manner.

2x 12V 3PIN Connector
2x 10V 3PIN Connector
2x 3PIN 8V plug
2x 3PIN 6V plug
2x 3PIN 5V connector
2x 12V 4PIN Molex connectors or
2x 4PIN Molex 5V connector
2x 10V 4PIN Molex connector
2x Molex 4PIN connectors 8V

http://www.performance-pcs.com/catalog/ ... s_id=28783

ces wrote:You can pretty much do anything you want with this... and do it in a reproducible manner so you can test and retest different settings in an easily reproducible manner.

2x 12V 3PIN Connector
2x 10V 3PIN Connector
2x 3PIN 8V plug
2x 3PIN 6V plug
2x 3PIN 5V connector
2x 12V 4PIN Molex connectors or
2x 4PIN Molex 5V connector
2x 10V 4PIN Molex connector
2x Molex 4PIN connectors 8V

http://www.performance-pcs.com/catalog/ ... s_id=28783

That's quite interesting! I may get one eventually.