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Scaling resistance to voltage

Deanne 2 months ago updated by Gustav Widén (System support) 2 months ago 7

I am trying to calibrate a level sensor that has a resistance output of 240 ohms (empty) to 33 ohms (full). The calibration below was what the setup is currently, but I am getting a high error on the VIN input. I am looking to fix the min and max, but I am unsure of how to scale this to voltage. I am getting a raw value of ~4999 at full. 

Image 4063

That error isn't related to what you have set your scaled min and max to. What resistor value did you calculate to add into the circuit?

We have a 120 ohm resistor in the circuit for that sensor

Hmm, you shouldn't see 4999mV if the circuit is correct. I can't remember if it will error out to that if you exceed your range. What I usually do is set the sensor range to 500mV-4500mV and do the scaling calculations in a math channel later. Regardless, try adjusting your min/max mV to give yourself some extra wiggle room.

Thanks, I will triple check the circuit and play around with some values. I also noticed that when I was moving the magnet, the values weren't changing very much. It sat around 4999 to 4950 when I was moving the magnet up and down. I am assuming it's from the calibration?

That doesn't sound right. This is how I connect them in similar setup.

Image 4064

Sorry had to do this up quickly but this is our circuit

Image 4067

+1

That circuit won't work. Should be similar to this. 

Vin=+5Vref

Vout=IQAN VIN

GND=-5Vref

Z1=120 Ohm

Z2=Sensor

Image 4068