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Desktop computer fan started making noise then stopped.

computer fan

We noticed it requires 12V and will run at 200mA. I wasn't sure about that at first. Does that mean it has an internal resistance of : 12V / 200mA = 60 Ohms?

If so that means I could supply it with 12V and it would consume 200mA and spin.

Question: Is that what I should assume from reading those details on the fan?

I decided to try it out with a bench power supply, but first I decided to measure how much resistance I could read.

Check Fan Resistance

When I hooked up my meter to read resistance I read 20 KOhms of resistance. Hmmm...?? Well, wasn't sure about that because if it were true then 12V would only supply it with only 0.6mA (600uA) and I knew that wouldn't work properly.

Applied 12V

I applied 12V to it figuring nothing lost here since it wasn't working. The the fan just barely moved then stopped.

I'm assuming the fan is dead, but I have a couple of questions.

What are the "normal" reasons these fans die? Dirt?

What generally makes these fans die? There really aren't any parts touching anything to wear out are there?

Is there any way to bring it back? (extreme cleaning of some sort? - blowing out dust or is it carbon build-up or something?)

Does the high resistance (20 KOhms) indicate that the fan is dead or would a working fan read that amount also?

One Last Interesting Point

After I was done testing it I put it back in the desktop case and hooked the computer's power to it again and it started spinning slowly 20-30 rpm or so (guessing). Why would it do that?

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  • \$\begingroup\$ FWIW, The voltage across the terminals of an ideal DC motor would be proportional to the shaft speed, and the current would be proportional to the torque. The constants of proportionality would work out such that the electrical power flowing in to (or out of!) of the motor terminals would be equal to the mechanical power that the shaft delivered to (or accepted from!) the mechanical system to which it was attached. Real motors are not ideal because of things like inertia, inductance, resistance, core losses, ... \$\endgroup\$ – Solomon Slow Jul 16 '19 at 19:19
  • \$\begingroup\$ Also note: These days, a fan like that could have a brushless, polyphase motor with a tiny-but-sophisticated electronic control circuit built right in. \$\endgroup\$ – Solomon Slow Jul 16 '19 at 19:20
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We noticed it requires 12 V and will run at 200 mA. I wasn't sure about that at first. Does that mean it has an internal resistance of : 12 V / 200 mA = 60 ohms?

No, but when running it behaves like one. A motor isn't a resistor.

If so that means I could supply it with 12 V and it would consume 200 mA and spin.

It may draw a higher current when starting but that depends on the internal electronics. They may have a "soft start" to prevent hogging the power supply on startup.

When I hooked up my meter to read resistance I read 20 k-ohms of resistance. Hmmm...?? Well, wasn't sure about that because if it were true then 12 V would only supply it with only 0.6 mA (600 uA) and I knew that wouldn't work properly.

Again, a fan motor is not a resistor. In this case there is an electronic circuit running a three-phase motor. The electronics will present a high resistance until powered up and operating properly.

I applied 12 V to it figuring nothing lost here since it wasn't working. The the fan just barely moved then stopped.

It could be a mechanical failure, an electronics failure or a coil failure.

Is there any way to bring it back? (extreme cleaning of some sort? - blowing out dust or is it carbon build-up or something?)

If you can find the fault and fix it then maybe.

Does the high resistance (20 KOhms) indicate that the fan is dead or would a working fan read that amount also?

No. See above.

After I was done testing it I put it back in the desktop case and hooked the computer's power to it again and it started spinning slowly 20-30 rpm or so (guessing). Why would it do that?

Maybe your bench PSU hadn't enough power to drive it when it is nearly stalled. Check if the voltage is collapsing when powered on the bench. The computer PSU may be more powerful.

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  • \$\begingroup\$ Very helpful. Thanks very much. \$\endgroup\$ – raddevus Jul 16 '19 at 18:12
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    \$\begingroup\$ The slow speed (if the fan is working) can be attributed to PWM on part of the motherboard temperature management, depending on the model and port that was plugged in. My high-end X470 board has PWM control of all case fans and automatically adjusts speed based on temperatures in the case. It spins them very slowly (but doesn't stop them) when the case is cool. \$\endgroup\$ – Ron Beyer Jul 16 '19 at 18:54
  • \$\begingroup\$ @RonBeyer I wondered about that very thing myself because I knew that at the time I added the fan back in the motherboard was cool. Thanks \$\endgroup\$ – raddevus Jul 16 '19 at 19:22
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    \$\begingroup\$ @RonBeyer, is that true with a two-wire fan? I thought computer motherboards could only adjust fan speed (via PWM) with three-wire or four-wire fans. \$\endgroup\$ – bitsmack Jul 16 '19 at 22:43
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This sounds like a bearing is seizing or a winding has gone open circuit.

It needs replacing.

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The fan is not a resistor, so you cannot use Ohm's law to find out its resistance. Furthermore, even if you do know the winding's resistance you cannot measure it on the wires. This is brushless motor, so there is switching electronics between the power wires and the motor windings.

One very common cause of the fan failure is dirt in the bushing, and excessive noise is usually associated with this.

If you peel off that green sticker, there should be an access to the rotor's shaft with either plastic or metal snap ring on it. Carefully remove the ring, pull out the rotor, clean everything, lubricate with a drop of machine oil and reassemble.

While you at it, the PCB inside should be visible with rotor off. Inspect it for any signs of burned-out parts. If that is the case, don't bother cleaning, it is most likely dead anyway.

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  • \$\begingroup\$ That's great info. Thanks very much. \$\endgroup\$ – raddevus Jul 16 '19 at 18:13

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