Voltage follower question / Fan tacho speed signal

[hobbyelectronics]

Hello,

I have a need for a simple circuit to measure the speed of a fan using a PIC MCU but this will be used in an industrial environment.

Currently, I have a fan with a open collector RPM output.
The output then goes through a 100R resistor, then through a BAT54-S (Tied to 5V and 0V and com on the RPM signal) then through a resistor and cap to ground as a low pass filter.
This signal is then tied to 5V through a 10K pull up before going into a PIC input pin. See basic circuit below.



This seems to work perfectly without any problems and has been running for sometime.

On the board I also have a op-amp as a voltage follower measuring the voltage from a 10K / NTC potential divider, just as an over temp reading for the PCB.

Now my question is really, would it be a bad idea to also have a voltage follower to buffer the RPM signal before it goes into the PIC input pin?
What about a Schmitt trigger instead? Would that be more suitable as its really a digital signal?

Really, this is just a general discussion as the best way to sort of isolate the RPM signal from a 4 wire fan that is maybe a foot away for the PCB but external to the case.


Thanks




 

[David Hess]

A voltage follower would not really add anything useful, but a Schmitt trigger (after the RC filter) would increase noise immunity.

[hobbyelectronics]

Hi David,

Thanks for your reply. I did think that may be the case but was not sure if it would add any benefit. But what you said makes perfect sense.
I now need to find a single channel low cost Schmitt trigger. I am sure I was looking at this option before and found a simple device somewhere.

Thanks

[hobbyelectronics]

Hi,

One other thought that comes to mind here is about protecting the FAN Tacho input from high voltage noise. Would it be advisable to maybe use a TVS diode instead of (Or as well as) the BAT54-S pair of diodes to protect against input noise. The fan cable is only about 200mm long but runs through a 400V ac part of the enclosure so may pick up noise. There are also relays switching at 400Vac (2A) (Need to look at FET replacements at some point) but the firmware stops checking the fan when these are being switched. Also as the fan is external to the enclosure, to protect the input form other outside noise would a TVS may also help. Maybe even an opto between the FAN after the filter and the MCU??

Any suggestions on this would also be helpful.

Thanks