Please help, need to change a 3000rpm fan signal to 3600rpm

[steveb72]

Hi,

I currently support some really old unix servers, now we have an issue where the fans have started to fail. Unfortunately i cannot source a replacement fan of the same spec. The issue i have is the server is looking for a 3600 tach output from the fan, the new fan has a 3000 tach output, is there an easy way to change this output.

The new fan actually has a higher CFM so the cooling aspect should not be a problem.

Thanks in advance
steve

 

[Reloadron]

This is just winging it but a typical tachometer output from a fan as used in a computer is two pulses per revolution to the system. Therefore a fan spinning at 3600 RPM would be 3600 / 60 = 60 pulses per second or 60 / 2 = 30 revolutions per second.

The system should be looking for a minimum frequency from the tachometer signal of 60 Hz. and with a 3,000 RPM fan is seeing about 50 Hz. and assumes the fan speed is low. So the trick becomes providing it with a 60 Hz. minimum frequency signal (a square wave should work fine). You could use a PIC chip or in the interest of keeping it simple a 555 oscillator circuit to provide a tachometer input to the system. Matters not how you get the frequency as long as you satisfy the system. I would use 5 volt pulses too as I forget if the system will like anything more in amplitude.

If the fan(s) are fed off the motherboard with a 3 pin fan header you would feed the simulated tachometer signal in on what is generally the white wire (that can vary but it should be pin 3 of the header).

If you can program a PIC chip or have someone who can it would be a quick solution and power it off the syatems 5 volt bus of the power supply.

There are other ways to do it. However, there is a big downside (risk) involved. If a fan fails the system won't know it failed and things could get toasty and ugly. Other circuitry could be added to prevent that problem including the use of the same PIC monitoring the actual tachometer speed from the fan(s).

I am sure others may have some helpful suggestions.

Ron

 

[smanches]

Can you change the tach threshold instead? That sounds like it would be WAY easier than trying to change the behavior of the fan or the sensor.

 

[Reloadron]

Can you change the tach threshold instead? That sounds like it would be WAY easier than trying to change the behavior of the fan or the sensor.

My guess is that changing the threshold would involve changing the sensor or to be more on target the chip designed to measure fan speed limits. The motherboard likely has a fan speed controller chip that has pre programmed limits. IF Fanspeed > whatever THEN whatever or something like that. Sort of like once a PIC is programmed then that is it unless it is reprogrammed.

Nice thought though if it could maybe be changed.

Ron

 

[smanches]

Most sensors that I've seen only report the fan speed. The thresholds are all done in software. That, obviously, is not always true, but I think it is worth investigating.

 

[jimlovell777]

What about a frequency doubler **broken link removed** If the fan comes to a stop you'd still know.