Why 4-20 mA signal is used for transmission?

4 to 20 mA Current Transmission

In many process control applications, signals are transmitted in the form of current in the range of 4 to 20 mA. Current loops are used not only for receiving information from sensors and field instrumentation, they are also used for transmitting control signals to actuators or other devices to regulate a controlled action. For long distance signal transmission, current signal is preferred because of

(1) Compatibility: The 4-20 mA signal is widely used in industrial process control because it is a widely accepted standard. This means that a wide range of instrumentation and control equipment is available that can use this signal, making it easy to integrate into many different types of control systems.

(2) Noise immunity from EMI:  Industrial environments can be noisy places, with electrical interference from other equipment and power sources. The 4-20 mA signal is designed to be immuned to this type of noise, which helps to ensure that the signal remains accurate and reliable even in noisy environments.

(3) Unaffected by voltage drop along the line.

(4) No stray Thermocouples at joints, hence no error due to thermoelectric emf.

(5) Current signal can be transmitted over long distance till the compliance voltage requirement is met.

(6) Self monitoring ability. currents less than 4 mA and higher than 20 mA can indicate a fault in the circuit.

(7) Power Transmission: The 4-20 mA signal is self-powered, meaning that it can be transmitted over long distances without the need for an external power source.

4 mA lower limit known as "Live zero" provides ability to detect cable or connection fault. The current upto 3.6-3.8mA is used to power the loop instruments in loop-powered mode.

How current transmission provide noise immunity?

(1) Loop wiring: The 4-20 mA signal is typically transmitted in a loop configuration, where the signal is transmitted from the field device back to the control system. This creates a closed loop circuit that helps to reduce the impact of noise and interference on the signal.

(2) Low frequency: The 4-20 mA signal operates at a relatively low frequency, typically in the range of a few kilohertz. This low frequency reduces the impact of high-frequency noise and interference that can be present in industrial environments.

(3) Signal strength: The 4-20 mA signal is transmitted at a relatively high current level, typically between 4 and 20 milliamperes. This high current level helps to ensure that the signal remains strong and robust, even in the presence of noise and interference.

(4) Common mode rejection: Many industrial control systems are designed to reject common mode noise, which is a type of noise that affects both the positive and negative parts of the signal in a similar way. The 4-20 mA signal is typically transmitted in a differential configuration, which helps to reduce the impact of common mode noise.