Why 4-20 mA Current Signal is used instead of Voltage Signal?

[Ajmal_ismail]

The electrical signals can be transmitted in two different ways. They are

Current signal and

Voltage signal.

When a voltage signal is transmitted over a long distance, the voltage in the wires drops.

There could be a significant voltage difference between the sending and receiving ends. As our process variable reading is in voltage form, this causes measurement errors.

As a result of the increased measurement error, voltage signals are rarely used.

Current Signal

The current signal, on the other hand, is unaffected by voltage drops in the loop wires. The measuring accuracy of a current signal transmission signal is far superior to that of a voltage transmission signal.

The transmitter designed for automation can output 0-20mA or 4-20mA signals.

Because a broken wire can be easily detected with a 4-20 mA signal, it is preferred over a 0-20 mA signal.

If the current observed in a 4-20 mA transmitter is zero, the transmitter will generate an error signal.

The current signal can be transmitted without error over a 1000-meter distance.

Why 4-20 mA Current Signal?

In most cases, the transmitter is loop-powered with a 24 volt supply. This type of transmitter is known as a two-wire transmitter.

By connecting an ampere meter in series with the 24 volt supply source, the processed signal can be measured.

Because the response of a current signal is more linear than that of a voltage signal, the current signal is preferred over the voltage signal.

It is desired that the signal be reproduced exactly as it was produced at the transmitter end so that both sides read and display the same values.

The signal must not be interfered with by external electromagnetic or electrostatic interference. When these external signals are superimposed on the measuring signal, it can result in a significant measurement error.

The noise immunity of the current and voltage signals is determined by the source’s impedance. In comparison to a voltage source, the current signal has a lower impedance to the sensor/transmitter and thus has better noise immunity.

Because of its low power, a 4-20 mA current signal is suitable for hazardous areas, and there is no risk of electric shock or fire.

[carl_e]

Yes, the lack of signal loss (voltage drop) in a current circuit and its robustness in comparison to proportional voltage signals are two main reasons for preferring current over voltage for proportional signals.

A third reason was not mentioned: the ability of 4-20mA to loop power many measurements using the power available on the 2-wire circuit. 2-wire, loop powered field instruments use 3.6-3.7mA, the current below the signal’s ’live zero’, to run their own electronics.

The 2-wire loop powered feature is and has been the major reason 4-20mA has not only survived but thrived for 60 years in industrial instrumentation. The simplicity of a 4-20mA loop speaks volumes; any reasonably astute technician can pick up the basics and work self sufficiently.

[Rabert]

Why is it 4mA?

There will never be a current signal below 4mA in normal operation. The loop current drops to zero when a malfunction causes the transmission line to be disconnected, therefore 0.2mA is frequently used as the disconnection alert value. Two reasons exist for this:

1. Initially, 4-20 mA operates on a two-wire system (signal and power lines), and 4 mA is used to supply the circuit’s static operational current to the sensor.
2. The second reason is to prevent interference.