Hi,
We need the ckt diagram which is used to sense the rotation of DC Fan,our application is to stop the main o/p of the ckt when rotation of the DC fan is not working.

We have two 4 wire 12vdc fan.

 

hi,
Look here.
http://pcbheaven.com/wikipages/How_PC_Fans_Work/

 

Hello,

Thanks for this information, but actually i need to understand is it possible to design a circuit which is used to read these Tacho signals.

In our unit we don't have a motherboard. Presently we are using four wire fans.

 

I don't know the details, but I imagine you could build a missing pulse detector with a 555 that triggers if it doesn't see a pulse coming from the tachometer. The minimum rpm to trigger could be set by changing the timing details of the pulse detector.

 

Please Stop Double posting ...

 

The tach signal output for a 12V fan usually is either an open collector pulling to GND or an open collector pulled up to the fan voltage with a 5V zener to clamp the signal amplitude for logic circuits. You get 2 pulses per revolution, pretty close to a 50% square wave. Each fan channel takes a dual comparator. A typical circuit is included in many fan catalogs. AC couple the tach signal to the first comparator. It's output connects to the center of an RC pair, and it discharges te capacitor with every pulse. The pulses get far apart when the fan slows down, allowing the cap to charge up more and more as the fan gets slower. The second comparator has a voltage trip point that equates to the cap voltage when the fan is running at 50% of its rated speed. 50% is a typical value to detect imminent fan failure, but you can select something else. Note that when the fan is just below the trip speed, the output of the second comparator will pulse.

ak