[FAQ] What is a 4-20mA current transmitter?

4-20mA transmitters have the capability to transmit data over very long distances between control centers and sensors or actuators. This works because Kirchhoff’s laws state that current is constant at any point in a closed loop. Attempting a voltage transmission over the same distance would result in a significant voltage drop due to the long transmission wires. 4-20mA transmitters most commonly use two or three wires to transmit a 4-20mA signal representing a 0-100% sensor or control level. A simple receiver would translate this 4-20mA signal back to a voltage with a resistor. A typical value for this resistor is 250Ω, resulting in a final voltage of 1-5V.

The attached articles go into more detail on the basics of 4-20mA current loop transmitters:

The basics of 4-20mA current loop transmitters

2-wire 4-20 mA sensor transmitter measurement basics