The pre-electronic industry used pneumatic controls. Compressed air powered all ratio controllers, temperature sensors, PID controllers and actuators. The modulation standard was 3-15 pounds per square inch, with 3 psi standing for an active zero and 100% represented by 15 psi. If the pressure went below 3 psi, an alarm would sound.
Electronic controls made their debut in the 1950s. A new signaling method with 4-20 mA current emulated and replaced the 3-15 psi pneumatic signal. As wires were easier to handle, install and maintain, current signaling quickly gained popularity. In contrast, pneumatic pressure lines and energy requirements are much higher – you need a 20-50 HP compressor, for instance. Moreover, with electronics you can add more complicated control algorithms.
The 4-20 mA current loop is a sensor signaling standard and a very robust one. Current loops are the favored form of data transmission method because they are inherently insensitive to electrical noise. In the 4-20 mA current loop, the signaling current flows through all the components. Therefore, the same current flows even if the wire terminations are not perfect. All components in the loop drop some voltage because the signaling current flows through them. However, the signaling current is unaltered by these voltage drops as long as the power supply voltage remains greater than the sum of the individual voltage drops around the loop at the maximum signaling current of 20 mA.
The simplest form of the 4-20 mA current loop has only four components –
− A DC power supply
− A 2-wire transmitter
− A receiving resistor to convert the current signal to a voltage
− A wire to interconnect all the above
Most 4-20 mA loops use 2-wire transmitters, with standard power supplies of 12, 15, 24 and 36 VDC. There are also 3-wire transmitters with AC or DC power supplies.
The transmitter forms the heart of the 4-20 mA signaling system. The transmitter helps to convert physical properties such as pressure, humidity or temperature into an electrical signal, a current, proportional to the physical quantity being measured. In the 4-20 mA current loop system, 4mA represents the lowest limit of the measurement range, while the 20 mA represents the highest limit.
Since it is much easier and simpler to measure voltage than it is to measure current, typical current loop circuits incorporate a Receiver Resistor. This resistor helps to convert the current into a voltage, following Ohms Law (Voltage = Current x Resistance). Most commonly, the resistor used in a 4-20 mA current loop is 250Ω, although some engineers use resistances of 100Ω to 750Ω, depending upon the application. When using 250Ω, four mA of current will produce a voltage of one VDC across the resistor, and 20 mA will produce five VDC. Therefore, the analog input of a controller can very easily interpret the 4-20 mA current as a 1-5 VDC voltage range.
The wire connecting all the components of a 4-20 mA current loop has its own resistance expressed in Ohms per 1,000 feet. Some voltage is dropped across this resistance of the wires according to Ohm’s Law and has to be compensated by the power supply voltage.
The major advantages in using 4-20 mA current loops are their extreme immunity to noise and power supply voltage fluctuations.