Anyone volt modded a PWM fan?

[Skree]

Sounds silly i know but it would be useful for people with limited PWM range or fixed pwm numbers on graphics cards for idle - like the EVGA GTX750ti i'm looking at is restricted to 42 or 43% in its lowest speed

It's my understanding of PWM that it works by switching the supply voltage on and off, varying the length of the ON pulse to achieve average voltage, ergo, i theorise if the +12v line to a PWM fan is reduced, say by putting a couple of volts worth of diode in the line, it should allow slower speeds by simply reducing the voltage the PWM controller works with

but i've no idea what controllers if any are in PWM PC fans, whether they would like it or not.. how they interface with motherboard controllers, etc. I can't test it because the only pwm fans i have are in my main pc, just wondering if anyone here has tried something like that.. i get such ideas from time to time and can never tell whether it's genius or stupidity

[lodestar]

Scythe had a PWM fan that used a rotary variable resistor in the +12v wire. It was called the SY1225SL12HPVC and came with a PCI slot controller allowing the PWM speed range to be set to Low or High or any point in between. These adjustable PWM ranges varied from about 400 to 1300 rpm on Low to around 740 to 1900 on High. Scythe fitted this fan to at least one of their CPU coolers, the Yasha. The resistor used by Scythe was marked B20K, so presumably was a 20K Ohm part(?). I don't know if Scythe made any changes to the fan to accommodate this control arrangement. But in theory it could be applied to a PWM graphics card fan.

This picture from the SPCR review of the Scythe Yasha and Ninja 3 CPU coolers shows at bottom left both the fan and PCI slot controller (with the Yasha cooler).

[quest_for_silence]

can never tell whether it's genius or stupidity

AFAIK Noctua sells reducing-voltage PWM cables working that way (or sort of). The actual problem with any VGA card is the female connector, which quite often is different and not so easy to find/swap (so that it's often preferable to mod the stock fan cable).

Anyway, with the GTX 750Ti that's not an issue: either slap a passive Arctic Cooling Accelero S1 Plus on it, or pick a much quieter card (MSI, ASUS).

[Skree]

i need the displayport the evgas have, even the SC version has noise complaints

was just trying to find out if someone has figured out the pinout of the fan header on a modern vga card.. not having anything more recent than a GeForce4 myself, i can't test it.. if it's the same as motherboard headers, +12v should be pin 2.. if so i can't see what woiuld stop it working unless there is some components dependant on a 12v supply within the fan

[Abula]

i need the displayport the evgas have, even the SC version has noise complaints

was just trying to find out if someone has figured out the pinout of the fan header on a modern vga card.. not having anything more recent than a GeForce4 myself, i can't test it

You can run certain PWM fans undervolted, proof is the fans on the ACCELERO IV can be ran with the pwm of the gpu or you can have a 7V molex to GPU adapter, that said there is no guarantee the fans will run lower than what you are already achieving.

Btw the Asus GTX750Ti strix does have display port.

[Vicotnik]

i need the displayport the evgas have

Same here. Luckily I had an old Accelero S1 I could use, with a quiet slim 120mm fan zip tied to it just in case case airflow alone wouldn't be enough.

[Irrelevant]

Yes, by decreasing the supply voltage you will also decrease the fan's speed for the same PWM signal. However, I've heard that CPU and GPU fan headers usually establish lower limits by measuring RPM (can't remember where, though, so take that with a heap of salt). If that's true, then decreasing the supply voltage won't have any positive impact. The fan header will simply adjust the minimum PWM signal higher to compensate.

If you try it, let me know how it goes. I've got some PWM fans I wouldn't mind adjusting lower myself.

[Skree]

Yeah. I was looking at zener diodes but i think the power dissipation might be a bit high for them, can't find any current stats for the fans on the EVGA FTW edition 750ti
I think 3 rectifier diodes in series for ~2.1v drop should do it

failing that, a linear voltage regulator.. got the bits for the diode solution here though

[Mr Evil]

I tried undervolting a 140mm "be quiet!" PWM fan and it didn't really work, because the speed it runs at 0% and 12V was already only slightly above its stall speed. Decreasing the voltage therefore leads to it stalling, then spinning fast for a second as its on-board controller attempts to restart, then stalling again. Not a pleasant sound.

I also tried with an 80mm Akasa fan. It also runs very close to the minimum possible speed already, and so can't be unervolted to any significant degree, but its controller doesn't try to restart it when it stalls.

Other fans might behave differently of course.