The 4-20 mA Current Loop

The 4 to 20 milliamp current loop is widely recognized as the standard method for industrial automation analog measurement and control. This method is highly favored for transmitting process information due to the constant current it maintains from the transmitter to the receiver. It offers simplicity and cost-effectiveness, making it an ideal choice for many industries. However, it is essential to be aware of certain considerations, such as the impact of voltage drops, the number of process variables to be monitored, and the associated cost and complexity.

By understanding the fundamental principles of the 4-20 mA current loop, informed decisions can be made regarding process control, ultimately affecting the efficiency and cost-effectiveness of operations.

Pros:

• The 4-20 mA current loop is widely considered superior and standard in various industries.
• It provides a straightforward and easy-to-configure connection.
• It is cost-effective as it requires fewer wirings and connections.
• Suitable for long-distance transmission without current reduction over extended connections.
• Less sensitive to background electrical noise.
• In case of system faults, it is easy to trace as 4 mA represents 0% output.

Cons:

• Current loops can only transmit one specific process signal.
• When multiple process variables are required to be transmitted, multiple loops are necessary, potentially leading to complexity and isolation challenges.

Components of a 4-20 mA Current Loop: The 4-20 mA current loop comprises several essential components, including:

• Sensor: Measures a specific process variable such as temperature, humidity, flow, level, or pressure.
• Transmitter: Converts the sensor's measurement into a current signal.
• Power Source: Supplies the required voltage for the loop, with consideration for voltage drop of connected components.
• Loop: The actual wire connecting the sensor to the device receiving the 4-20 mA signal, typically using shielded cables to resist electrical noise.
• Receiver: Interprets the current signal and converts it into easily understandable units for operators.

Working Principle of the 4-20 mA Current Loop: The Ohm's Law, V = I x R, plays a crucial role in understanding the process. Voltage (V) equals the current (I) multiplied by the resistance (R). While every element in the loop contributes to the voltage or experiences a voltage drop, the current remains constant throughout the loop. This principle ensures that the current is identical at all points within the loop, much like the flow of water in a pipe. The flow of electrons, or current, is akin to the flow of water in the pipe. Just as the pressure pushes water through bends and restrictions in the pipe, resistors limit the current flow in the circuit. Despite voltage drops at various points, the current through the wire remains constant, making the use of current for conveying process information reliable and well-founded.