RPM METER GOES CRAZY

riversidedan

H-M Supporter - Silver Member
H-M Supporter - Silver Member
Joined
Feb 17, 2021
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466
so I got one of those $20 digital RPM meters for the table mill hooked it up per instru. and it will not hold a steady reading and jump all over the place! >>>>>>>>>> have any experience with those??
 
More details are needed.

My guess is that it is a Hall sensor type of meter and you have it too close to a big motor.
 
Do you have the correct magnet orientation, as it will only work in one direction/polarity. Also try running the RPM meter using a small 9V battery to see if that makes a difference, often cheap power supplies and/or machines with VFD's will generate too much electrical noise, one can add a small line filter to the power supply. Gap between magnet and sensor should be around 0.050-0.1". Some of the Tachs are just crap no matter what you do, the one below is what I have currently been using and seems to work ok.
DIGITEN 4 Digital Red LED Tachometer RPM Speed Meter
 
The magnet is best mounted in aluminum or plastic so there is no interference or false signal at the hall sensor. The sensor should be wired with twisted-pair to the head unit, and those wires should be sleeved with a braided shield with a drain grounded at the head unit only, no contact at the sensor end. Power in should be looped through a ferrite ring nearest the head unit. That should be good for what ails ya.
 
Mark's suggestion above is a good one- try running temporarily off a 9V battery
Some switching type wall wart power supplies are very noisy- you want the transformer-based kind
-M
 
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