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3-Wire Cooling Fan Monitor


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Cooling fan is an essential part in almost all power electronics systems. Here is a simple circuit for monitoring the instrument cooling fan activity to raise an audio-visual alert incase of the fan failure. This is a proven solution to prevent costly disasters caused by over temperature inside the power electronics instrument. This little adapter circuit can be used with any 3-Wire instrument cooling/exhaust/blower fan.

In a 3-wire fan, first two wires are the power supply wires of the fan. The third wire, comes directly from the output of the built-in ‘Hall Sensor’ chip generates output pulses during the fan rotation. With the help of this third wire, a monitoring circuit can ‘see’ the rotation. If the monitoring circuit sees no pulses (fan off) or weak pulses (very low rpm), then it raises an alarm to inform the operator/user about the mishap!

Schematic of the cooling fan monitor circuit


Prototype was tested with a 80mm/12VDC/3-Wire cooling fan, using a 5VDC lab power supply. Even at 5VDC, the fan performance is good, and the operation is very silent because of the low rpm rate. Besides, total power consumption is comparatively low!
You can use almost all standard 3-wire cooling fans with this adapter circuit. However, for optimum performance it may become necessary to slightly change the values of components C1, R4 and C2. Finally, note that the connector of a common 2-wire fan has a red and a black cable. The red cable goes to the positive of the power supply (Vcc) and the black to the negative (Gnd). However two fans with 3-wire connectors may not have the same wire colors. Thus, instead of using the colors to distinguish the function, better try to study the internal wiring connection. Usually, yellow (or green) color wire is used in a 3-wire fan for the pulse output (tacho) connection.
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