How to fool a mobo

nobody · Joined: Mar 09, 2008 Posts: 1687 Location: Not here

This would go into an equipment or DIY thread except it's not a musical equipment. The computer I'm looking to do this on is my Linux music workstation, though.

I figured someone here might at least have an idea, because we've got a lot of people working with circuit boards. I need to somehow fool my computer's motherboard into thinking that its own internal fans are present when they are not (trust me, I have an ample, custom external cooling system that works wonderfully). At the moment, I need to trick the PSU and/or the mobo into believing the PSU fans are present when I disconnect them. Without the PSU fans, the machine will not even bother to start and will put up an amber LED warning. It will NOT be a danger to run the computer without the PSU fans (I know what I'm doing). I just need the computer to believe the fans are there so I can disconnect the last two internal fans - the PSU fans - and have a silent audio workstation.

If you want more info on what I'm doing: http://blog.audiodef.com/2010/01/adventures-in-creative-computing-vol-1.html

seraph · Posted: Tue Jan 26, 2010 9:17 am Post subject:

I thought a mobo was some kind of monkey Shocked

DrJustice · Posted: Tue Jan 26, 2010 11:02 am Post subject:

I thought it might be about fooling a molbo Confused

For what it's worth, they're said to be easy to fool. But molbos don't really have a lot of fans...

How about replacing the fan with a resistor, if they're two-wire ones? AFAIK the three-wire variety are monitored for actual rotation and may not be so easy to fool. Don't some BIOS'es have options to ignore temperature and fan alarms?

DJ
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nobody · Joined: Mar 09, 2008 Posts: 1687 Location: Not here

nobody · Joined: Mar 09, 2008 Posts: 1687 Location: Not here

dmosc · Joined: Jun 23, 2003 Posts: 298

Any fan monitoring is generally done through the middle wire, the yellow wire, which the motherboard will read a voltage from. It interpolates that voltage into an RPM. The actual appropriate RPM for a given fan varies greatly so generally they have settings in the motherboard. If your motherboard is unhappy with a 0RPM value no matter what you tell it, you can fake any given fan speed by supplying a small voltage at the pin in question. I suggest using the +3.3V line you already have from your power supply. I forget the conversion but it should be like 10k RPM's or somesuch.

nobody · Joined: Mar 09, 2008 Posts: 1687 Location: Not here

I got the main fans taken care of. Of six, one was left inside to tell the mobo that something is there. What I'm trying to do now is get the PSU fans out (since sufficient air flow is generated elsewhere). I tried them in series, but that was not acceptable to the PSU. I believe the PSU, when you push the power button, ramps its fans up to max speed so that it knows that capability is available, and in series, they won't reach max speed. So I will next try one of two things: either insert a 555 timer that tells the PSU what it wants to hear, or leave them in but insert a switch so that x seconds after boot, the PSU fans can simply be turned off.

Now that I think about it, I'm in favour of a 555 timer, since for all I know, the PSU continuously monitors its fans and will shut the system down if they aren't running at all. A 555 timer will always inform the PSU that an acceptable RPM exists.

dmosc · Joined: Jun 23, 2003 Posts: 298

It might help if you were more specific on your system components. I've never used a power supply that cared at all about fans.

nobody · Joined: Mar 09, 2008 Posts: 1687 Location: Not here

I don't know how to identify individual electronic components, but it's an HP ProLiant DL145 G2. I'm assuming that because this machine was factory-designed to be a high-load device, the parts were designed with fail-safes. While that's good for it's intended purpose, I'm hardly using it for that, so the fail-safes are just fails for me. Razz