Fan wiring question

Hi. I have a case-cooling fan wiring question. I know a little about electronics, but I've not worked on PCs. I'm getting ready to change the 80mm fan that's inside my power supply, so I popped the P/S cover and took a peek at the existing fan. I saw just two wires running into it, which is what I expected for a simple 12 volt DC motor.

The replacement fans that I've seen for sale (and the one I bought) have three wires. The third (yellow) wire is apparently for RPM regulation which I won't be using. I just want this thing to run flat out. Should I just ignore the yellow wire?...leave it unattached?

Also, I'm curious about how the fan speed is regulated. Is there some sort of countering voltage supplied on the yellow wire that reduces the 12 volts?

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  1. Fan speed in normally regulated by dropping the line voltage to the fan. For motherboards to do this is easy, they look at the CPU temp and raise the voltage up to full 12v for max cooling, and drop it down to around 7v for quiet cooling.

    Power supplies with "Smartfans" usually use logic chips to turn the fan speed up or down as the internal heatsink gets hotter or cooler.

    So nothing is actually done with that third wire, it's so YOU can see how fast the fan is turning.

    Of course there are a few boards that need an RPM signal in order to establish that the fan is actually working.

    <font color=blue>Watts mean squat if you don't have quality!</font color=blue>
  2. Thanks, Crashman. I appreciate your response.

  3. Just be careful messing around inside your PSU ok?

    <font color=blue>Many modern games <b>must</b> be played multiplayer. And those you encounter online are, almost as a rule, <b>complete and utter cockmongers.</b></font color=blue> :frown:
  4. I repair them and I've never been zapped...DOH!

    Like I said, I've repaired a lot of them and only ever been zapped once...DOH!

    Like I said, I repair them often and rarely get zapped...DOH!


    Really, in all seriousness, I've never been zapped. But then again, you might. I've actually tried to get a spark off a capacitor before, and they've all been discharged!

    <font color=blue>Watts mean squat if you don't have quality!</font color=blue>
  5. for the record, the extra pin is for rpm monitoring not regulating.

    <font color=red> black </font color=red> <font color=green> white </font color=green> <font color=blue> yellow </font color=blue> <font color=orange> purple </font color=orange> <font color=black> red </font color=black> <font color=yellow> green </font color=yellow> <font color=purple> blue </font color=purple>
  6. So touching my tongue to the terminals of a big electrolytic filter capacitor is definitely out.
    I will take care. Thanks.

  7. So if I connect the red and black wires to 12 VDC and leave the yellow wire unconnected, the fan will run at full speed, correct?

    I know this is very basic stuff, but I'm just not all that familiar with the wiring of PCs. I want to make sure I do (or don't) know what I'm doing before I do it.

    Thanks, again.

  8. Yes it will run at max, the only way to alter the speed the fan is running is by changing the voltage.

    My CPU fan spins so fast that it creates a wormhole :eek:
  9. Getting shocked isn't that bad......a 486 did it several times to me while I was screwing around with it when it was running (ahem, yes you may call me a genius for that). I turned out all right, didn't I? *starts ticking and slams into wall*

    Hilbert space is a big place.
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