With regard to Fan Sensors, there are two very common methods for detecting whether the fan is doing what it's supposed to or not.

  1. Locked Rotor- Detects when the fan is working, or when it has stopped. (on or off voltage signal)

  2. Tachometer- Detects the speed of the fan, to determine whether it is performing optimally. (pulsed voltage signals)

My Question:

Q- What is the difference between using these two types of fan sensors in 'some' design?

Additional info:

  • I don't know what this 'design' is, but I'm asking for the practical differences if I picked either one.

  • I understand that both setups use different voltage signals, and require different circuitry to operate. That is beyond what I think would be the practical scope of this question.


It's a trade-off between the simplicity of the sensor and how comprehensive the reported data is. The locked rotor sensor only reports whether or not the motor is spinning, not how fast it's spinning or how much resistance is on the fan. It is a very simple signal though, it's either on or off.

On the other hand, the tachometer signal is a bit more complex: the signal will need to be converted or interpreted in some way to extract the fan RPMs, but, you can use this data for much more than just indicating whether the fan is running or not. You could compare the measured RPMs to a table of load vs. RPMs to estimate the load on the fan, for example.

The choice of which sensor to use should depend on what information you need of course.


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