The Basics of 4-20 mA Current Loops

Current loop history

Some of us still use pneumatic control systems, and many of us still recall the era of pneumatic control. Compressed air is used to power actuators, temperature sensors, PID controllers, ratio controllers, and PID controllers. The modulation standard ranges from three to fifteen pounds per square inch (3 psi for an alive zero and 15 psi for 100%). A dead zero and alarm situation was any pressure lower than 3 psi.

Electric and electronic controllers first appeared in the 1950s. The 3 to 15 psi pneumatic signal was simulated by four to twenty mA signaling. Wires use much less energy than pneumatic pressure lines, don't require a twenty to fifty horsepower compressor, and allow for more complex control algorithms than pneumatic pressure lines.

4-20 mA Current Loop Components

1. Sensor

First, there must be a sensor capable of measuring a process variable. Typically, a sensor measures temperature, humidity, flow, or pressure. Depending on what the sensor is designed to monitor, the sensor's underlying technology will differ significantly, but that is irrelevant to this subject.

2. Transmitter

Second, there must be a means to convert the sensor's measurement into a current signal between four and twenty milliamps, regardless of what the sensor is detecting. In this case, a transmitter will be necessary. If, for example, a sensor was monitoring the height of a fifty-foot tank, the transmitter would need to interpret 0 feet as the tank being empty before transmitting a four-milliamp signal. In the opposite direction, it would interpret fifty feet as the tank being full and then transmit a twenty milliamp signal. If the tank were half filled, the transmitter would signal at twelve milliamps, the midway mark.

3. Power Source

In order to produce a signal, there must be a source of power, just as there was a source of water pressure in the water system analogy. Keep in mind that the power supply must produce DC current (meaning that the current is only flowing in one direction)

9 volts, 12 volts, 24 volts, etc., are frequently utilized with 4-20 mA current loops, depending on the arrangement. Consider that the voltage of the power supply must be at least 10% higher than the total voltage drop of the associated components when determining the voltage of the power supply for your particular setup (the transmitter, receiver and even wire). Inappropriate use of power supply can result in device failure.

4. Loop

In addition to a sufficient VDC supply, a loop, which is the real wire connecting the sensor to the device receiving the 4-20 mA signal and then back to the transmitter, is required. The transmitter regulates the current signal on the loop based on the sensor's measurement. Due to the pervasiveness of wire in modern electronic systems, this component is frequently disregarded in a current loop arrangement, but it should be included in our investigation of the fundamentals. While the wire itself is a resistance source that creates a voltage drop in the system, this is typically not a reason for concern because the voltage drop of a wire segment is negligible. Depending on the wire's thickness (gauge), this can add up to a significant amount over long lengths (more than 1,000 ft).

5. Receiver

Eventually, the loop will contain a device that can receive and interpret the current signal. This current signal must be converted into units that are easily understood by operators, such as feet of liquid in a tank or degrees Celsius. This gadget must also either show the received information (for monitoring purposes) or automatically perform some action in response to it. Incorporating digital displays, controls, actuators, and valves into the loop is common.

These are the only components required to complete a 4-20 mA current loop. The sensor measures a process variable, the transmitter converts this measurement into an electric signal, the signal travels through a wire loop to a receiver, and the receiver displays or takes action based on this signal.

Advantages of 4-20 ma

• The 4-20 mA current loop is the prevalent standard in a number of sectors.
• It is the most straightforward method to connect and configure.
• It requires fewer wires and connections than other signals, resulting in a substantial reduction in setup expenses.
• Current does not diminish over lengthy connections, unlike voltage. This makes it superior for long-distance transport.
• It is less susceptible to electrical noise in the background.
• Due to the fact that 4 mA corresponds to 0% output, it is quite easy to notice a system malfunction.

Disadvantages of 4-20 ma

• Only one process signal can be sent through a current loop.
• When there are a lot of process variables that need to be sent, you need to make more than one loop. When you run so much wire, you could have problems with ground loops if the separate loops are not properly separated.
• As the number of loops goes up, these isolation requirements get a lot harder to meet.