The 4-20 mA Current Loop: Precision in Industrial Automation
The 4-20 mA current loop is a cornerstone of industrial instrumentation, combining simplicity and reliability for process control.
Key Components:
- Transmitter: Converts physical parameters into a 4-20 mA signal.
- Load Resistor:Converts current to voltage (e.g., 250 Ω for 1-5V, 500 Ω for 2-10V). This conversion is critical for control systems to interpret the signal accurately.
- Power Supply: Drives the loop over long distances.
Why It Works:
- Live Zero:4 mA indicates operation; 0 mA signals a fault.
- Noise Immunity:Current signals resist interference.
- Scalability: Resistor values tailor the output for control systems.
This image highlights a fundamental concept every instrumentation engineer must master. Precision in design ensures reliability in operation.
As we already know, a current value of 4mA represents 0% of the scale, a current value of 20mA represents 100% of the scale and any current value between 4 and 20mA represents a corresponding percentage between 0% and 100%.
1. But why set 0% percent scale to 4mA and not OmA?
This is because it is easier for us to detect if there is an open circuit (broken wire/fault) on the instrument itself. We use 4mA as a “live zero”. That means if we measure the current at OmA, the instrument is probably faulty or our circuit is in an open loop.
2. Why use 20mA for 100% percent?
Why not use 50mA or more, since the higher the current range, the more precise our result will be?
This is for safety reasons. Because the human heart can mostly only withstand less than 30mA of current, if more than that risks our life
3. Why do we use current (mA) and not voltage (V)?
because it is more robust against electromagnetic interference.Very long cables have resistance which causes a voltage drop. The voltage drop affects the measured value. Current signals do not have this problem.