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3 pin fan with temp sensor

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  • Fan
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Last response: in Components
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July 14, 2012 3:46:23 PM

I am using a fan from an old cpu cooler as a case fan. It has a 3 pin connector and a built in temp sensor.
My question is if the fan speed is based on that sensor when the fan is pluged into the mobo as a case fan.

I use speedfan to monitor, and even though the fan is right next to the gpu, which is currently the hottest component when the system is under load, the fan seems slow to respond to increases in temp.

More about : pin fan temp sensor

a b ) Power supply
July 15, 2012 2:00:40 AM

I think you misunderstand fans, temp sensors and control systems. I've never seen a fan with its own temp sensor, and never seen a mobo that would use such a signal.

3-pin fans have 3 wires to them. 2 are the power supply for the fan motor (Black Ground, and Red +VDC), and the third, Yellow, is a line to take a pulse signal train generated in the fan motor back to the mobo. This signal is the fan speed (2 pulses per revolution), NOT a temperature sensor.

For control by the mobo, there IS a temperature sensor involved in each controlled fan port. The sensor for the CPU is actually built into the CPU chip itself and its signal fed out to the mobo on one of the CPU's pins. The CPU_FAN port uses this signal to guide its fan speed control to maintain proper internal CPU temp. The mobo also has its own temp sensor built into the mobo at some specific point, intended to measure the temperature inside the case generally, and it uses this signal to control the speed of a fan connected of the SYS_FAN port. (Often, this sensor will be mounted near the Northbridge and Southbridge chips that tend to run pretty hot themselves.) So, although you have mounted a fan (originally from a CPU cooler) near the GPU and plugged it into a SYS_FAN port of the mobo, the only temp sensor involved is the one the mobo has somewhere - not necessarily near the GPU. Thus your case fan will not respond to GPU temp and load.

Now, most graphics cards have their own fans on board to cool their GPU's, and the do their own control of that fan's speed to keep the GPU chip at a proper temp. About the only link between that and the case fan is that, if the case ventilation system is not good enough to keep the internal case temp low, the graphics card's fan will have to run faster to maintain its GPU chip under control. That is because the graphics cards fan's air source is the inside of the case.
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July 18, 2012 3:21:48 PM

Paperdoc said:
I think you misunderstand fans, temp sensors and control systems. I've never seen a fan with its own temp sensor, and never seen a mobo that would use such a signal....


Maybe I did't say it right, this is the exact fan I'm talking about. I was referring to the little green thing that pokes out where the wires go into the fan body. I know it's not technically a temp sensor, but it's supposed to function as one.


I guess my question should have been:
If the system is controlling the fan through the 3 pin plug, and then that thing is doing whatever it does, is it causing the fan to run slower than it should?
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a b ) Power supply
July 18, 2012 3:48:38 PM

I don't recognize that little green item - it looks like a small capacitor, but it certainly could be something else like a thermistor. A quick scan of the Thermaltake website did not show me one that looks exactly like that. If you have a model number, that would help get more info.

Your main question, though, is whether that item slows the fan down. I really doubt it. As I said in my first post, the main factor governing the fan speed, and its response to temperature changes, is that it is guided totally by a temp sensor built into the mobo, and NOT by anything sensing the GPU temperatures.

By the way, I'm assuming that, in adapting this fan to case ventillation, you have removed it entirely from the heatsink (which substantially reduces fan air flow) and mounted the fan only in your case.
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