4-20mA to 0-10VDC Conversion

When working with Analog Inputs, sometimes you may find yourself needing to work with a 4-20mA device. These devices can be powered from an external DC voltage source or the LX controller itself – please refer to the label on the controller for wiring diagram examples.

In order to use a 4-20mA device with CBAS, we would typically use a 250 Ohm or 500 Ohm resistor to convert the 4-20mA signal into a 1-5VDC or 2-10VDC signal.  If those specific resistors are not available, did you know you can use any resistor value less than 500 Ohms?

Below is an explanation of how the range of voltage changes based on the resistor used, courtesy of Bobby Rodriguez, our Director of Application Engineering.

Generally, we tell people to use 250 or 500-ohm resistors to convert 4-20mA signals into VDC signals that can be read by our boards.

This relies on Ohms law, which is V = IR (voltage = current * resistance) 

We tell people 250 or 500 because that works out to nice round numbers of 1 to 5 VDC (250ohm) or 2 to 10VDC (500ohm). However, if you don’t have access to a resistor of that value you can always use anything less than 500ohms (anything more would put you outside our 10VDC max on points). You just have to calculate the endpoints (and verify that your sensor is outputting a linear function)

If we use a 470ohm resistor as an example:

At bottom of the scale we are getting 4mA from the device, so using a 470ohm resistor would look like:

(.004A) [converted to amps because that is what the formula uses and so we don’t have to include any correction factors] * (470 Ohms) = 1.88 VDC 

At the top of the scale we are getting 20mA from the device, so using 470ohm resistor would look like:

(.02A) * (470 Ohms) = 9.4 VDC

We would then look at the output range of the sensor. Let’s pretend we have a temperature sensor with a range of 0 degrees to 100 degrees. We would set our point parameters in CBAS to be 

1.88 VDC = 0 Degrees

9.4 VDC = 100 Degrees