RPM Wire on fan

Xenocide · Apr 1, 2008

Hey guys I have a mobo that won't boot unless the system fan is sensed at a certain RPM (or you hit F2) It is a server, so I need it to boot back up incase of power failure.

How do these wires work? I don't have a fan to test it with? Do just need to put an adjustable pot on the sense line and play with it? What value pot should i grab?

Thanks.

nd4spdbh2 · Apr 1, 2008

Xenocide said:
Hey guys I have a mobo that won't boot unless the system fan is sensed at a certain RPM (or you hit F2) It is a server, so I need it to boot back up incase of power failure.

How do these wires work? I don't have a fan to test it with? Do just need to put an adjustable pot on the sense line and play with it? What value pot should i grab?

Thanks.

pretty sure it sends out electrical pulses but dont quote me on that.... can you not set in the bios to just disgard all errors?

bchur83 · Apr 1, 2008

Many times in the bios you can tell it to ignore the sensor.

Xenocide · Apr 1, 2008

HP proprietary bios

nd4spdbh2 said:
pretty sure it sends out electrical pulses but dont quote me on that.... can you not set in the bios to just disgard all errors?

Ill see if i can find one and hook it up to an O-scope i guess...I guess i can make some kinda pulse generator.

Xenocide · Apr 1, 2008

http://www.overclockers.com/articles624/

states it just send a 12v pulse. I think I'll just o-scope it to see if i can find out what the rpm to pulse freq relation is and see if i can't rig a 555 timer to send out 12v to my fan header.

Cjwinnit · Apr 1, 2008

Xenocide said:
How do these wires work?

Every half-rotation of the fan, the RPM wire pulses. On a normally-wired fan, it pulses at 12v. On a fan running at 5v, it pulses at 5v, etc.

Its natural voltage is that of the ground wire. Hence a fan set with the 7v fan mod (i,e, 12v line at 12v, "ground" line at 5v) will pulse at 12v and sit at 5v. Connecting it to an RPM sensor (like on a motherboard) will give it permanent 5v with 12v pulse, rather than 0v ground with 12v pulse. So don't do that

I'm tempted to say it's a very very fast momentary switch but don't quote me on that, there might be a certain in-built resistance/capacitance...

=ACID RAIN= · Apr 1, 2008

Have you tried grounding the tach lead just to see what it'll do?

edit: At the header, that is. Dell used to do this for mobos that have tach requirements, so it might work here too.

Xenocide · Apr 1, 2008

Cjwinnit said:
Every half-rotation of the fan, the RPM wire pulses. On a normally-wired fan, it pulses at 12v. On a fan running at 5v, it pulses at 5v, etc.

Its natural voltage is that of the ground wire. Hence a fan set with the 7v fan mod (i,e, 12v line at 12v, "ground" line at 5v) will pulse at 12v and sit at 5v. Connecting it to an RPM sensor (like on a motherboard) will give it permanent 5v with 12v pulse, rather than 0v ground with 12v pulse. So don't do that

I'm tempted to say it's a very very fast momentary switch but don't quote me on that, there might be a certain in-built resistance/capacitance...

so why can't i just pulse from 5 to 12...???

nd4spdbh2 · Apr 1, 2008

Xenocide said:
so why can't i just pulse from 5 to 12...???

i think it would just be much simpler to make a simple PWM circuit or use a small precision switch IC.

Xenocide · Apr 1, 2008

got a datasheet for the chip i need?

Cjwinnit · Apr 2, 2008

Xenocide said:
so why can't i just pulse from 5 to 12...???

Rather than getting a pulse of current, it would be getting a continous current varying between 12v and 5v. You'd blow it out.

RPM Wire on fan

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