Good Day All.

I tought I would start a new topic as I am starting from a fresh sheet of paper.

I know the best route to go down, would be to buy new 4-wire PWM Fan.
But I am yet to find any 40x40x28mm 4-Wire PWM Fans here in the UK.

I have been playing around with the idea of using PWM to control the fan speeds for my U1 Cobalt RaQ 550 Servers.

I used low-side siwtch at 20kHz and the fans run fine, but the tach signal is a mess and the servers can not read the right fan speed, and shuts down after 5 sec.

The server fan monitoring can not be disabled.

Here is the fan running at 20kHz PWM @ 95% Duty Cycle.

And again at 100% Duty Cycle or full power, how ever you want to read it.

Blue trace is the PWM output from a PIC.
Red tracie is the Fan Tach output.


I am now looking at some way of getting PWM to work and have a clean tach output.
Or some other way of driving the fan with adjustable power, like an adjustable Voltage Regulator, but if this is possible I don't know how I would interface it with a PIC.

Can you please advise.

Thanks for your time.

Best Regards.

 

Simple RC low pass filter on the TACH line should clear that up. Something like this:

Fingers crossed it goes into a Schmidt type input on the motherboard. If not, a comparator/buffer may be required to ‘square’ up the pulse edges (Not very likely though).

Your messy TACH signal should look something like this after it’s been through the filter:

http://www.zen86415.zen.co.uk/wave1.gif

 

boriz: Thank you for your reply.

In and ideal world the tach output filter should look like this then.

http://www.mad-professor.co.uk/Misc/RC Lowpass Filter (1).jpg

The problem I have is that the 5volt pull up is on the motherboard so my rc filter would look somthing like.

http://www.mad-professor.co.uk/Misc/RC Lowpass Filter (2).jpg


I would guess that the outputs would not be the same on both rc filter types.