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PC no longer boots with new fan. Need to add more current to CPU_FAN connection.

  1. Feb 10, 2012 #1
    I don't know if this is the right place to ask this, but I'll give it a try.

    I have a PC with a rather noisy fan which I'd like to replace so I got a new one with lower RPM and of better quality. However, there is one little problem: The PC won't boot with the new fan. I contacted the manufacturer of the motherboard and they told me that there is a "safety check" done on cold boot on the CPU fan connector to determine whether there is a fan connected or not.

    Apperantly the new fan doesn't use enough current (on boot) so I need to increase the load.
    (The fan works perfectly fine when connected while the PC is running. The only problem is a cold boot)

    The original fan is: 12v 0.5A
    The new fan is: 12v 0.18A

    If I connect 2 of the new fans in parallel the PC boots just fine. So I believe I need to add something that increases the load from 0.18A to about the double. (0.18 x 2 = 0.36A)

    I would assume adding a resistor in parallel to the fan is all that's needed. I've been looking at Ohm's law but I haven't figured out how it's done yet. Once I know what resistor to use, does the direction matter when connecting it?

    I'd be very thankful if anyone could help me.
  2. jcsd
  3. Feb 10, 2012 #2


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    12v ÷ 0.32A looks like about 33 Ohms

    It would be ideal to have a circuit that disconnected it after half a minute. No point in wasting power for no reason. Are you sure you don't need another little fan somewhere? :smile:

    Resistors don't have a preferred direction. But you should buy a 10 watt resistor as it will get quite warm. If you can't find a 10 watter, buy two 5 watt resistors, each of 18 Ohms and join them in series.

    It's a pity, this waste heat just adds to what the new fan is being asked to remove from the case. A timing circuit to switch the resistor current off is sounding more attractive.
    Last edited: Feb 10, 2012
  4. Feb 10, 2012 #3
    Thank you.

    Maybe 12v ÷ 0.18A = 66,7 is more accurate as I already have 0.18A on the fan that's connected?

    I'd rather not have to disconnect it as I would prefer to just mount the fan, close the case and forget about it.
    Do you have any other suggestion? Maybe I could add a 12v lamp (led diode?) to the same connector and add draw some current that way?
    Last edited: Feb 10, 2012
  5. Feb 10, 2012 #4


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    Okay. A 68 Ohm resistor will do. If you need lights in the case you can add some LEDs, but check on their current. I think 0.2 A is way too much for indicator LEDs. You'll need a separate resistor to limit the current to what the LED needs.
  6. Feb 10, 2012 #5
    Well, look at one step back, something is sensing the current, it might be a low ohm resistor and the processor is sensing the voltage drop across the resistor. Change the sensing resistor to a little higher value so more voltage drop across. This might fool the processor to think you draw enough current......Just a thought. Or else, using extra current is small thing, you parallel resistor is going to get really hot and burn something. That is going to be your problem if you go with the parallel resistor. 121V across 68 ohm is over 2W, that is going to cook.
  7. Feb 10, 2012 #6


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    That is far too much current and voltage for a single LED, but you can get LED lighting strips like this one http://www.amazon.co.uk/strip-rigid-LEDs-200mA-37-5cm/dp/B003C2B5E8

    There are lots of similar products - that was just the first one I found with Google.
  8. Feb 10, 2012 #7


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    If you don't want to over heat your computer, your replacement fan should have similar airflow and pressure specifications as the original.

  9. Feb 11, 2012 #8
    Thank you for your replies.
    I'm starting to think a resistor may not be the best solution and that probably should try to find some other "device" to connect as well. I just don't know what yet.

    The airflow shouldn't be a problem. This is a custom build computer and I have chosen energy efficient parts just to make sure there wouldn't be so much heat generated. (65W CPU instead of the regular 95/125W versions, low end graphics card..)

    The system is a Shuttle SH67H3 barebone. I got this system just because I wanted a very quiet PC and was somewhat disappointed with the original fan.
  10. Feb 11, 2012 #9


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    The suggestion by yungman is a good one, providing you don't mind chopping a resistor lead on your motherboard. If you can make the motherboard's current sensing resistor 3 times greater, the software will think a current of three times the value is flowing, with virtually no increase in power dissipated in the tower.

    The alternative is to construct a small electronic circuit on a scrap of board, the circuit being designed to switch that 68 Ohm resistor out of operation about 20 secs after power is applied to the CPU fan or some handy associated connector.
  11. Feb 11, 2012 #10


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    Hmm... the skills required to swap SMD parts on a motherboard are not the same as those required to "design and build" a custom PC.

    Life was simpler back in the 1970s when resistors were big stripey things that actually had leads you could chop...
  12. Feb 11, 2012 #11


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    No one said "swap". A resistor designed to take 0.5A may be big enough to get to.
  13. Feb 11, 2012 #12
    This sounds like a good solution. I don't have the knowledge to come up with something without doing extensive amount of research though. :smile:
    I can solder alright, but I don't know what parts to get to get this to work. Any ideas?

    Replacing a resistor on the MB sounds a little scary. I just had a look and the new shuttle MB have a lot less resistors in general compared to my older barebones. I wouldn't know where to look, and on top of that, everything is really tight.

    EDIT: Maybe this is what I'm looking for?
    Last edited: Feb 11, 2012
  14. Feb 11, 2012 #13

    jim hardy

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    mount that second fan where it'll help move air out of the case?
  15. Feb 11, 2012 #14
    The box itself is so tight that there aren't any places to put it at.
    I added a link to the barebone system above. The fan I'm trying to exchange is the one in the back. (the "smart fan")

    Another though:
    Would it be completely mad to hook up the DVD drive to the CPU FAN connector with something like this? It may not use enough current though.
    Last edited: Feb 11, 2012
  16. Feb 11, 2012 #15
    The easiest way then is to buy another fan that draw the required current!!! Why spend so much time monkey with this? Whatever you do, it's not going to be easy as you are trying to dissipate quite a bit of power.
  17. Feb 11, 2012 #16
    Since you know it works with two fans, why don't you merely modify the case to satisfy two fans?
  18. Feb 12, 2012 #17
    Because I want it to be silent.

    I did solve it today though. I got one of these and it was enough to boot the PC.
    Thanks everyone!
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