The electrical signals can be transmitted in two different ways.
- Current signal and
- Voltage signal.
The voltage signal when transmitted over a long distance it causes a voltage drop in the wires.
There might be large voltage difference between the sending end and receiving end. This leads to measurement errors as our process variable reading present in the voltage form.
Thus, voltage signal is generally not used because of more error in measurement.
On the other hand, the current signal does not get affected with voltage drop in the loop wires. The measuring accuracy of the current signal transmission is far better than voltage transmission signal.
The transmitter manufactured for automation purpose can output 0-20mA or 4-20 mA signal.
The 4-20 mA signal is preferred over 0-20 mA signal because the broken wire can be easily detected with use of 4-20 mA signal.
If current observed is zero in a 4-20 mA transmitter, the transmitter will output an error signal.
The current signal can be transmitted without error to a distance of 1000 meters.
Why 4-20 mA Current Signal?
The transmitter is generally loop powered with 24 volts supply. The transmitter with this arrangement is called 2 wire transmitter.
The process signal can be measured by connecting ampere meter in the series with the 24 volts supply source.
The response of current signal is more linear than the response of voltage signal, and therefore, current signal is preferred over voltage signal.
It is desired that the signal must be reproduced at the control system end exactly what was produced at the transmitter end so that both sides read and display the same values.
The signal should not interfere with the external electromagnetic interference or electrostatic interference. If these external signals superimposed on the measuring signal it can cause substantial error in the measurement.
The noise immunity of the current signal and voltage signal depends on the impedance of the source. The current signal provides low impedance to sensor/transmitter in comparison to voltage source, and the thus current signal has better immunity to noise.
4-20 mA current signal is safe for hazardous area because of low power, and there is no risk of electric shock or fire.
The 4-20 mA signal is fed in the 250 ohms resistance connected to the transmitter circuit.
The resistor converts the current signal into a voltage signal for onward conversion in digital signal through an analog to digital converter which is located in the analog input module of PLC or DCS system cabinets.
Author: Satyadeo Vyas
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