In a Process Plant Industrial Automation is carried out by transmitting Signals from Field Instruments and Sensors to a Controller like PLC.
Based on this Received Signal the Controller would issue a Command signal to a Final Control Element like a Valve, So that the Indented Process can be Controlled.
The Input or Output of a Controller like PLC can be an Analog Signal or a Digital Signal.
The Analog Signal is transmitted as either a Voltage Signal or as a Current Signal. Usually Analog Voltage Signal can be 0 to 10 volt or Sometimes 0 to 5 volt. Analog Current Signal can be typically 4 to 20 ma.
Since most Sensors naturally generate Output in the form of Voltage, 0 to 10 Volt and 0 to 5 Volt Outputs can be directly achieved.
In 4 to 20ma circuits, the Voltage Output of the Sensor is sent to a transmitter which will convert the Voltage Signal into a Current Signal.
This Current Signal is Converted back into a Voltage Signal for Processing by the Controller.
Now you might be thinking,
Why Should we usually use a Current Signal instead of Voltage Signal for transmitting signals?
It is because the Current Signals have specific Advantages over Voltage Signals.
Voltage Signal when transmitted over a long distance causes a Voltage Drop in the wires. On the other hand Current Signal does not get affected with Voltage Drop in the wires.
The Measuring Accuracy of the Current Signal transmission is far better than the Voltage Signal transmission. Voltage signals are highly Susceptible to Electrical Noise and Interference.
Let’s Consider that a Fluid Passing through a Pipeline Exerts a Pressure in the Range of 0 to 10 Bar. A Pressure Tran
Pressure Transmitter Calibrated to generate a Current Signal in the Range of 4 to 20 Ma is on the Pipe. The Output of the Pressure Transmitter is Proportional to the Pressure being Exerted by the fluid on the Pressure Sensor. Pressure transmitter is then Connected to the Input Card of a PLC.
When there is no Fluid Flowing through the Pipeline No Pressure is Exerted on the Sensor, But the Pressure Transmitter still generates 4MA current signal at the Output Terminal.
if the Pressure Exerted on the Sensor is at its maximum Calibrated Value of 10Bar, then the transmitter generates 20Ma Current Signal at the Output terminals.
So if No current Flows through the Wire between the transmitter Output terminals and the Input Card of the Controller, then it can be Concluded that there is a Break in the Wire Connected Between the transmitter Output terminal and Input Card of the Controller.
Now Consider the Other case that Fluid passing through the Pipeline Exerts Pressure in the range of 0 to 10Bar and the transmitter is Calibrated to generate Current Signal in the range of 0 to 20MA.
When there is No Fluid Flowing through the Pipeline the transmitter generates 0 Ma Current at the Output terminals.
In this case it would be Extremely difficult to identify that either 0 Ma Current is due to the Wire Break of the transmitter or it is due to NO Pressure of the Fluid.
Hence if the transmitter is Calibrated to generate a Current Signal in the range of 4 to 20 Ma, the faults like Wire Breaks can be easily detected but this reason is a Secondary reason for the widespread use of 4 to 20MA signal.
The Primary reason that 4 to 20MA is used instead of 0 to 20 Ma is because, it allows to Power the Field devices on the same two wires that is used for transmitting Signals.
while using 4 to 20 Ma Current Signal ,since there will be at least 4Ma flowing through the loop, the connected Instrument can be powered using the same two wires that transmit signals.
Process Control devices such as digital displays that take advantage of this Principle to Power itself using the 4 to 20MA loop is called Loop Powered Device. Loop Powered instruments Consume very little Power in order to be able to operate with the Power supplied to the loop.