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

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Discussion Overview

The discussion revolves around a user experiencing boot issues with a new CPU fan that draws less current than the original fan. Participants explore potential solutions to increase the load on the CPU fan connector to allow the PC to boot properly, discussing various methods including resistors, additional fans, and alternative devices.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Exploratory

Main Points Raised

  • One participant notes that the new fan draws 0.18A compared to the original fan's 0.5A, suggesting the need to increase the load for booting.
  • Another participant proposes using a resistor in parallel to the fan to increase the current draw, recommending a 68 Ohm resistor and discussing the heat generated by it.
  • Some participants suggest using additional components like a 12V lamp or LED strips to draw current, while cautioning about the current limits for LEDs.
  • A participant raises the idea of modifying the motherboard's current sensing resistor to trick the system into thinking more current is being drawn.
  • Concerns are expressed about the potential overheating of components if a resistor is used, with suggestions for creating a timing circuit to disconnect the resistor after a short period.
  • There is a discussion about the feasibility of modifying the case to accommodate additional fans or using existing components like a DVD drive to increase current draw.
  • One participant emphasizes the importance of airflow specifications and suggests that the replacement fan should match the original's airflow and pressure capabilities.
  • Ultimately, a participant mentions successfully resolving the issue by finding a suitable solution that allowed the PC to boot.

Areas of Agreement / Disagreement

Participants express a range of ideas and suggestions, with no clear consensus on the best approach to resolve the issue. Multiple competing views remain regarding the use of resistors, additional fans, or modifications to the motherboard.

Contextual Notes

Participants highlight various assumptions about current draw and the implications of using different components, but do not resolve the technical details of the proposed solutions.

Who May Find This Useful

This discussion may be useful for PC builders or enthusiasts facing similar issues with fan compatibility and current draw on motherboard connectors.

danielkun
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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.
 
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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.
 
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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?
 
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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.
 
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.
 
danielkun said:
Maybe I could add a 12v lamp (led diode?) to the same connector and add draw some current that way?

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.com/dp/B003C2B5E8/?tag=pfamazon01-20

There are lots of similar products - that was just the first one I found with Google.
 
danielkun said:
...

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

...

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

Two important functional specifications are the airflow that can be moved, typically stated in cubic feet per minute (CFM), and static pressure.
...
The speed of rotation (specified in revolutions per minute, RPM) together with the static pressure determine the airflow for a given fan.

http://en.wikipedia.org/wiki/Computer_fan
 
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 http://img.ncix.com/images/60089_1.jpg barebone. I got this system just because I wanted a very quiet PC and was somewhat disappointed with the original fan.
 
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.
 
  • #10
NascentOxygen said:
The suggestion by yungman is a good one, providing you don't mind chopping a resistor lead on your motherboard.

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...
 
  • #11
No one said "swap". A resistor designed to take 0.5A may be big enough to get to.
 
  • #12
NascentOxygen said:
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.

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?
 
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  • #13
mount that second fan where it'll help move air out of the case?
 
  • #14
jim hardy said:
mount that second fan where it'll help move air out of the case?

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 http://www.logicsupply.com/images/photos/cables/dig-31521_big.jpg? It may not use enough current though.
 
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  • #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.
 
  • #16
Since you know it works with two fans, why don't you merely modify the case to satisfy two fans?
 
  • #17
yungman said:
Why spend so much time monkey with this?

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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