measuring 12V or 24V fan RPM using pic16f84

how do i able to measure the rpm of a 12Vdc or 24Vdc fan using a pic16f84a displaying to a 2X16 LCD? what hardware parts do i have to use? does the pulse of a 12V fan is also 12V? do i need a 5V zener diode for this one?

i dont have oscilloscope here at home that's why i don't have an idea what does the fan's pulse(3 wire) to PC's CPU fan power connector look like. thanks

GraveYard_Killer said:

how do i able to measure the rpm of a 12Vdc or 24Vdc fan using a pic16f84a displaying to a 2X16 LCD? what hardware parts do i have to use? does the pulse of a 12V fan is also 12V? do i need a 5V zener diode for this one?

i dont have oscilloscope here at home that's why i don't have an idea what does the fan's pulse(3 wire) to PC's CPU fan power connector look like. thanks

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As long as you feed it into the PIC through a series resistor the protection diodes in the PIC will clamp it to 5V anyway, try a 10K series resistor.

thanks for the very quick reply.

will there be no any issues/concerns regarding the speed of the pulses being fed to the zener diodes? what are the other ways i could fed high speed 12Vdc pulses and up with PICs?

The 3rd wire on pc cpu fans is a tacho output, not a pulse output, check it first.

GraveYard_Killer said:

thanks for the very quick reply.

will there be no any issues/concerns regarding the speed of the pulses being fed to the zener diodes? what are the other ways i could fed high speed 12Vdc pulses and up with PICs?

Click to expand...

The pulses are only low speed, it's only a motor, and speed shouldn't affect the interface much (if at all) anyway - a simple resistor should be fine.

If you want the maximum possible speed you can input directly to one of the counter/timers, these work up to 50MHz - but a bit of overkill for a motor, unless you are running at 3,000,000,000 RPM.

Exo said:

The 3rd wire on pc cpu fans is a tacho output, not a pulse output, check it first.

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The output from the pc fan is basically the open collector output of a hall effect sensor. So its frequency varies with the speed of the fan. The problem is that you cant vary the speed too much because the sensor shares its power with the fan. I think when I was experimenting, once you get below 8V or so (I don't remember exactly), the output stops working.

On other motors it is wound like a generator so you get increasing voltage and frequency from the tach output of the motor. In this case, somewhere on these forums I put a circuit and traces from a 311 comparator/hysteresis circuit to "square-up" and clean the incoming tach signal.

Here is a picture of what the output of a pc fan captured on my scope.

The Fan on my compaq is a tacho for sure...

Could just be compaq doing it diffirent from the rest of the world ..

nm my post then.