On Wed, 30 Jan 2008 16:29:44 -0800 (PST), snipped-for-privacy@gmail.com put finger to keyboard and composed:
I work as a coputer tech (fixing pcs) in a major university as part of
>a work study program. Im also a junior in computer engineering, so I
>know my way around a breadboard.
>
>One of the machines is a newish dell btx, that's having major cooling
>problems (random page faults, hd temp goes up to about 130 F, etc). I
>narrowed the problem down to the main case fan, which is a four pin
>PWM (pulse-width modulation). Apparently, the bios (happens without
>hd/os) forces the fan to spin at a ridiculously low speed,
>approximately 10% of capacity. After tinkering a bit, I found that
>cutting the blue (PWM) wire was adequate enough to disable pwm, and
>forced it to 100%. But now it sound's like one of those handheld
>vacuum cleaners, and is likely to have a very low MFT (mean failure
>time). I've tinkered a bit more, and found that by grounding the pwm
>with a suitable resistor, it slows it down to an acceptible level.
>I've run a few tests, and I've come up with the following data:
>
>Fan Power draw at 100%: ~375 mA
>Ideal Power draw (flow vs sound): ~210 mA
>
>Pwm voltage (fan to ground): ~3.266 V
>Pwm Current (directly grounded) ~0.52 mA
>Ideal Pwm>Resistor>Ground: ~3.2 kOhm
>
>I was wondering if anybody had any additional input about this before
>I screw something up royally. With these Ideal values, everything
>seems to work fine, and I don't notice any risky voltages or
>currents. I'm on a tight schedule, so I'm likely to begin soldering
>everything into place soon.
>
>Cheers.
The PWM input is a logic level pulse train, either on or off. Therefore it makes no sense to add a resistor between the PWM pin and ground.
See
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- Franc Zabkar