Measuring a 4-20ma Input With a Voltage Input Device

we will assume that a 0-10Vdc input will be used to measure 4-20ma signal.

Ohms law states: R=V/I where V is the Voltage, I is the current and R is the resistance

R=10V/.020A = 500 Ohms

When 20ma flows through a 500 Ohm resistor, it will drop 10 volts

When 4ma flows through a 500 Ohm resistor, it will drop 2 volts

Therefore, 4-20ma signal through a 500 ohm resister will drop 2 to 10 volts

PLC
if you’re using instrument with 4-20mA and connect it to:

1. range 0-20mA – your value of the instrument will be from that 5529 to 27648, value from 0 to cca 5500 will be not showed, but it depends on instrument.

2. range 4-20mA – your value of instrument will be from 0┬áto 27648.

References
https://www.omega.co.uk/techref/das/4-20ma.html
https://support.industry.siemens.com/tf/WW/en/posts/analog-input-evaluation/146749?page=0&pageSize=10
https://www.omega.com/en-us/resources/data-acquisition-measuring-420-input-with-voltage-device

Optocoupler

An Optocoupler, also known as an Opto-isolator or Photo-coupler, is an electronic components that interconnects two separate electrical circuits by means of a light sensitive optical interface.

Optocoupler Types
The photo-transistor and photo-darlington devices are mainly for use in DC circuits while the photo-SCR and photo-triac allow AC powered circuits to be controlled. There are many other kinds of source-sensor combinations, such as LED-photodiode, LED-LASER, lamp-photoresistor pairs, reflective and slotted optocouplers.

References :
http://www.electronics-tutorials.ws/blog/optocoupler.html