4–20 mA Basics – The Foundation of Analog Instrumentation
The 4–20 mA current loop is the most widely used analog signal in industrial instrumentation. It is simple, reliable, noise-resistant, and ideal for harsh plant environments.
What is a 4–20 mA Signal?
A 4–20 mA signal represents a process variable by varying the current flowing in a closed electrical loop.
- 4 mA → Minimum process value (LRV)
- 20 mA → Maximum process value (URV)
- Same current flows through entire loop
Unlike voltage signals, current remains constant even if cable resistance changes.
Typical 4–20 mA Loop Diagram
The loop must remain closed. Any break in the loop causes current to drop to 0 mA.
Why Current Instead of Voltage?
- Cable resistance does not affect signal accuracy
- High noise immunity
- Reliable over long distances (hundreds of meters)
- Simple fault detection
Why 4 mA and Not 0 mA?
4 mA is called Live Zero.
- 4 mA → Valid minimum measurement
- 0 mA → Cable break / power loss / transmitter fault
Live zero allows technicians to differentiate between a real zero value and a wiring problem.
Typical Loop Fault Indications
- 0 mA → Open circuit
- Below 3.8 mA → Underrange or fault
- Above 20.5 mA → Overrange or fault
- Constant 20 mA → Signal saturation
Where 4–20 mA Is Used
- Pressure transmitters
- Temperature transmitters
- Flow and level instruments
- Gas and liquid analyzers
Golden Field Rule
Always measure loop current with a multimeter before adjusting scaling, replacing transmitters, or blaming DCS configuration.