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4-pin pwm fan.

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.

Reply to
ghosttwo
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The fan should run just fine at full power without significantly shortened life.

You can get small fan speed controllers that are standalone, they're cheap.

Reply to
James Sweet

It's only 0.5 mA so you'll probably be okay.

Non-hardware options: Call Dell and complain. Use SpeedFan (software control).

Reply to
mng

I had a noisey fan like that once and put a resistor in series with it to make it run at 9V which was much quieter. It ran for years and is still fine.

Reply to
TonyR

If it's newish, call Dell and have them send you a new motherboard, or a new fan, or both, under warrantee.

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Reply to
Ecnerwal

I've considered the speed fan route, but the machine is most likely going to spend most of it's time with a logon prompt, which very few aplications will continue to run on. It'll mainly be used for data entry and research. To machine itself is pseudo-departmentally owned, and this seems to be the most reliable technique. I considered using a resistor on the 12 volt line, but that would require either a three watt resistor, or 12 quarter watts in parallel (I can get .25W's for free, but ppl will ask questions if I take that many. It'd be a mess too). Putting a 2.6k resistor between pwm and ground is the most sensible (and fastest) way to go, especially since I need this up and running tommorow; plus at 1/588 watts, it is well within the tolereance of the available resistor(s). I have tested this approach thourougly enough to make it final, and all of the variables are well within range.

Reply to
ghosttwo

I agree with Rich. You don't have the knowledge or skills to repair this. Speak to your supervisor!

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              Baron.
Reply to
Baron

But mom! Already did. I did the hack (with the proverbial nod), and it works fine. Still a little bit on the loud side, but not nearly as bad as it was before.

Reply to
ghosttwo

I dunno about that. He did after all narrow down the HDD faults to improper cooling. That's pretty good.

That's always a good idea. If s/he is around, that is. Besides, I don't think having a noisy fan is that big of a problem. Better to have more cooling.

Michael

Reply to
mrdarrett

If you experiment with adding a diode in the line you cut with the cathode towards the motherboard, Your resistor to ground (on the fan side of the diode) should set the minimum fan speed but the motherboard could still increase it as required. If you've got a fan extension cable spare you could butcher, I'd splice neatly and heatshrink sleeve the cut wire and make up the kludge as a plug in unit, (avoid hassle if it ever needs to go to Dell).

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Reply to
Ian Malcolm

On Wed, 30 Jan 2008 16:29:44 -0800 (PST), snipped-for-privacy@gmail.com put finger to keyboard and composed:

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

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Reply to
Franc Zabkar

He's probably biassing the switching transistor in the fan into its linear region.

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Ian Malcolm.   London, ENGLAND.  (NEWSGROUP REPLY PREFERRED)
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Reply to
Ian Malcolm

On Wed, 30 Jan 2008 18:04:40 -0800 (PST), snipped-for-privacy@gmail.com put finger to keyboard and composed:

IMHO that's not a good idea because the fan's electronics expects a stable DC supply for the Hall sensor and possibly for the PWM circuit.

- Franc Zabkar

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Reply to
Franc Zabkar

These fans are almost always controlled by an external PWM circuit, the electronics in the motor are very simple and work fine with this arrangement. Internally temperature controlled fans are available, but relatively uncommon.

Reply to
James Sweet

Not to stir up the pot or anything.. But I've seen a couple of CPU 2 wire fans, designed to operate from 5 Volts (Low speed) up to 12 volts (full speed). the internal electronics is regulated for 5 volts always and uses the incoming voltage to govern speed for the controller internally. The last MB I played with used a PWM circuit into a local LC circuit that generated a rather smooth variable DC voltage for the fan supply. On this MB, a jumper had to be set to indicate you having one of these fans.

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Reply to
Jamie

On Thu, 31 Jan 2008 22:11:54 GMT, "James Sweet" put finger to keyboard and composed:

OK, I stand corrected. However, here is an interesting thread that shows that strange things can happen when you reduce the supply voltage of an internally temperature controlled fan:

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Specifically, the fan RPM may *increase* when you *reduce* the supply voltage. Of course the OP's fan is not one of these.

- Franc Zabkar

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Reply to
Franc Zabkar

He is talking about the case fan in a Dell machine which draws 0.2 amp @ full power. Dropping the whole 12 volts at that current would be 2.4W

If we assume that we use a resistor to loose 3v, that would only be 0.6W

This guy needs supervision and guidence !

Maybe he did ! But he his changing the manufacturers design to disguise a fault ! I would not accept a repair done in this manner. In simple words "its a bodge" find and cure the fault.

More cooling is always better.

If the machine is your own property you can do what you like with it ! In this case its not. If his supervisor takes that decision, then fine, it becomes the supervisors responsibility.

--
Regards:
              Baron.
Reply to
Baron

No it wouldn't, if you dropped 3V off the supply the fan would be drawing around 280mA and the series resistor dissipating 0.85W

A better way with case fans is a 1.3W zener in series with the 12V line. Plenty of values in the 3V-5V range to pick from.

But first check the BIOS. There should be an option, possibly on the ""PC Health" page, to give either PWM control or voltage control to the fan headers, "for fans that don't meet the Intel 4-wire fan spec". Or swap the fan for a quiet 3-pin, they still fit and work on most boards -- they've got to, 4-wire fans are still rare.

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Reply to
cpemma

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