Understanding 4-20 mA Current Loops
Current Loop History
Many people in the HVAC/R industry remember the days of pneumatic control; some buildings in fact still use pneumatic control systems. In these systems, ratio controllers, PID controllers, temperature sensors and actuators are powered by compressed air. Three to fifteen pounds per square inch is the modulation standard, 3 psi for a live zero and 15 psi for 100%. Any pressure below 3 psi is a dead zero and an alarm condition. In the 1950’s electric and electronic controls made their debut. The new 4-20 mA current signaling emulated the 3-15 psi pneumatic signal. Current signaling quickly became the preferred method because wires are easier to install and maintain than pneumatic pressure lines and energy requirements are a lot lower – you no longer needed a 20 to 50 horsepower compressor for instance. Also, electronics allowed for more complicated control algorithms.
4-20 mA Current Loop Basics
The 4-20 mA current loop is a very robust sensor signaling standard. Current loops are ideal for data transmission because of their inherent insensitivity to electrical noise. In a 4-20 mA current loop, all the signaling current flows through all components; the same current flows even if the wire terminations are less than perfect. All the components in the loop drop voltage due to the signaling current flowing through them. The signaling current is not affected by these voltage drops as long as the power supply voltage is greater than the sum of the voltage drops around the loop at the maximum signaling current of 20 mA. Figure 1 shows a schematic of the simplest 4-20 mA current loop. There are four components: 1. A DC power supply; 2. A 2-wire transmitter; 3. A receiver resistor that converts the current signal to a voltage; 4. The wire that interconnects it all. The two “Rwire” symbols represent the resistance of the wires running out to the sensors and back to the power...