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