Why 4-20 mA Current Signal is Used Instead of Voltage Signal
In industrial control systems, signals are used to transfer information from sensors to controllers and from controllers to actuators. Two common types of signals are voltage and current signals. Both voltage and current signals can be used to transmit analog signals, but a 4-20 mA current signal is preferred over a voltage signal in many applications. In this blog post, we will discuss why a 4-20 mA current signal is used instead of a voltage signal.
Before we dive into the reasons why a 4-20 mA current signal is preferred over a voltage signal, let us first understand what is 4-20 mA current signal and voltage signal.
A voltage signal is an electrical signal that varies in voltage between two points. Voltage signals are typically used to represent analog values. For example, a temperature sensor might output a voltage signal that represents the temperature at the sensor.
On the other hand, a 4-20 mA current signal is an electrical signal that varies in current between two points. In a 4-20 mA current loop, 4 mA represents the minimum value of the signal, and 20 mA represents the maximum value of the signal. The current signal is proportional to the measured parameter. For example, a temperature sensor might output a 4-20 mA current signal that represents the temperature at the sensor.
Now that we understand the basics of voltage and current signals, let's explore why a 4-20 mA current signal is preferred over a voltage signal in industrial control systems.
Noise Immunity In industrial environments, noise is a common problem. Voltage signals are sensitive to noise, and the noise can cause inaccuracies in the signal. 4-20 mA current signals, on the other hand, are less sensitive to noise. This is because the current loop provides a closed circuit, which makes it less susceptible to electromagnetic interference.
Voltage Drop Another advantage of 4-20 mA current signals is that they are less affected by voltage drops. Voltage drops can occur when the signal is transmitted over long distances. Voltage signals are affected by voltage drops, which can cause inaccuracies in the signal. 4-20 mA current signals, however, are less affected by voltage drops because the current loop provides a constant current source. This makes 4-20 mA current signals ideal for long-distance transmission.
Loop-Powered A 4-20 mA current loop can be powered by the same power supply that is used to power the sensor. This is known as loop-powered or two-wire. This means that only two wires are needed to transmit the signal and power the sensor. Voltage signals, on the other hand, require a separate power supply to power the sensor, which adds complexity to the system.
Easy Calibration Calibration is the process of adjusting a sensor or instrument to ensure that it is measuring accurately. With 4-20 mA current signals, calibration is easy. This is because the signal can be calibrated at the controller end of the loop. This means that the signal can be adjusted without having to access the sensor itself. Voltage signals, on the other hand, require calibration at the sensor end of the loop, which can be difficult to access.
Safety In industrial environments, safety is a top priority. Voltage signals can be dangerous because they can deliver a high voltage that can cause electric shock or damage to equipment. 4-20 mA current signals, on the other hand, are safer because the current is limited to 20 mA. This means that even if the current loop is shorted, the current is still limited to a safe level.
In conclusion, a 4-20 mA current signal is preferred over a voltage signal in many industrial control systems because of its noise immunity, voltage drop immunity, loop-powered operation, easy calibration, and safety. While voltage signals have their own.
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