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Electronics => Projects, Designs, and Technical Stuff => Topic started by: Christopher on January 27, 2016, 07:30:56 pm

Title: PWM Fan Controller TACH questions
Post by: Christopher on January 27, 2016, 07:30:56 pm

I've been looking at designing a FAN controller which will vary the speed of an 80mm computer fan.

This is fine, I can use a micro and a MOSFET to switch the return of the fan using a high-frequency PWM signal...

(http://i.imgur.com/YFRjE6g.gif)

Now my question is, I want to possibly look at the tach signal to make sure the fan is not stalled, unplugged or is going too slow and needs replacing.

It is my understanding these pins are an open collector which is active low normally and is released when the rotors go past the hall sensor. Fine.

Now surely when the Mosfet is off in my (crude) diagram, there is a current path thru. the tach pin to let some current flow through keeping the fan on?!




Also, as a seperate Q, how are modern optos in terms of speed so I can isolate the fan from the control circuitry?

Title: Re: PWM Fan Controller TACH questions
Post by: oPossum on January 27, 2016, 09:43:49 pm

Much easier to use a 4 wire fan if you want PWM control and good quality tach signal.

For a 3 wire fan the + supply should be adjusted with a linear circuit rather than PWM so the tach will work properly.