A 4-20 mA (milliampere) loop is a common current signal standard in process automation and instrumentation. It is utilized to send measurement data from sensors (or) transmitters to the control devices like
• PLCs (Programmable Logic Controllers) and
• DCSs (Distributed Control Systems).
Here’s a summary of the main features of a 4 to 20 mA loop:
Current Loop Principle
In a 4 to 20 mA loop, the variable being measured is expressed by a current signal ranging from 4 to 20 mA.
4 mA is normally the least measurement value, whereas 20 mA is the maximum measurement value.
The current value within this range corresponds to the measured variable. For example, 4 mA might be the lowest measurement value, whilst 20 mA could be the maximum.
Loop Components
Sensor/Transmitter: The sensor or transmitter converts physical measurements like temperature, pressure, and level into an electrical signal.
Power Supply: A power supply source, often found in a control room (or) equipment panel, that provides electrical power for the loop.
Wiring: A loop connects the sensor (or) transmitter, power supply, & control device in the series circuit. It is normally made up of two wires, with positive (+) wire carrying current signal & the negative (-) wire acting as a common reference or ground.
Control Device: A PLC or DCS evaluates the current signal in order to find the variable’s value.
Load Resistor: A load resistor, which is commonly found within the control device, completes the loop circuit by functioning as a conduit for current to flow.
Signal Interpretation
The control device analyzes the current signal between 4 and 20 mA to determine the measured variable’s value. In general, a linear relationship is taken into account, with 4 mA representing the lowest value and 20 mA representing the highest.
The control device may use scaling factors (or) calibration to convert the current signal to engineering units for display or processing.
Advantages of 4-20 mA Loops
- Current signals are less prone to electrical noise interference than voltage signals, resulting in more dependable and precise data transfer.
- The 4 to 20 mA current signal can be carried over great distances without severe signal loss.
- Many sensors, transmitters, & control devices are intended to operate on the 4 to 20 mA standard, allowing for easy integration and interoperability across diverse components.
- The control device detects faults in the loop, including open circuit (no current flow) (or) short circuit (constant maximum current).