Current Loop

Definition of Current Loop

Current loop is a communication method used in industrial and telecommunication systems, where a continuous flow of electrical current serves as the information carrier. It typically operates at two levels: 20 milliamps for high (digital 1 or “on”) and 4 milliamps for low (digital 0 or “off”). This signaling technique is known for its simplicity, noise immunity, and ability to transmit data over long distances with minimal signal degradation.


The phonetic pronunciation of the keyword “Current Loop” is:- Current: /ˈkʌrənt/- Loop: /luːp/

Key Takeaways

  1. Current Loop is a reliable communication method that transmits analog or digital signals using a continuous current flow, rather than voltage levels.
  2. It is highly resistant to electrical noise and interference, allowing it to maintain signal integrity over long distances and in harsh environments.
  3. Commonly used in industrial control systems, 4-20mA current loops are a standard means of transmitting sensor data to a central monitoring system, with 4mA representing the lowest value and 20mA representing the highest value of the variable being measured.

Importance of Current Loop

The technology term “Current Loop” is important due to its historical significance and efficacy in electronic communication systems, particularly in data transmission and industrial control.

Current loops provided a simple yet robust method by which signals were transmitted across long distances and through noisy environments, using a constant current flow to represent binary data.

This approach helped maintain signal integrity and enabled easy error detection, as electrical components could easily detect a change or break in the circuit.

Widely used during the early days of telecommunication and computer systems, the current loop technology laid the groundwork for modern protocols and contributed to the development of advanced communication systems that we rely on today.


Current loop technology plays a vital role in communication and control systems throughout various industries. It’s designed to transmit data over long distances, often in electrically noisy environments, while maintaining signal integrity and minimizing data loss. The purpose of this technology is to facilitate the accurate and secure transmission of analog or discrete signals, particularly in industrial settings where electrical interference is common, such as manufacturing plants, chemical processing facilities, and oil refineries.

By employing a closed-loop system, current loop technology ensures consistent signal quality and minimizes the potential for disrupted communication, which could lead to equipment damage or compromised safety. In a current loop system, a continuous flow of electrical current represents a specific value of the measured variable, such as temperature, pressure, or flow rate. The most common current loop configuration uses a 4-20 mA range, with 4 mA signifying the lowest possible measurement, while 20 mA indicates an upper limit.

This constant flow simplifies the conversion of analog measurements into digital data, allowing for seamless integration with modern control devices and monitoring systems. Furthermore, current loop technology offers remarkable resistance to electrical noise and can transmit a signal over considerable distances without degrading its quality. As a result, current loop communication systems have become indispensable in many critical industrial applications, ensuring accurate data transmission and, ultimately, contributing to the overall efficiency and safety of these facilities.

Examples of Current Loop

Industrial Automation and Control Systems: In industrial settings, current loop technology is used to transmit signals from sensors and transmitters to controllers and other devices within the system. For example, the 4-20 mA current loop is a widely used standard in process control industries, where instruments like pressure transducers, temperature sensors, and flow meters send signals to PLCs (programmable logic controllers) or DCS (distributed control systems). The 4-20 mA range corresponds to 0-100% of the measured variable, allowing for accurate signal transmission and reduced electrical interference compared to voltage-based systems.

Data Communication: Current loop communication has been utilized in the past for data communication between devices, particularly in telecommunication systems. For example, teletype machines, modems, and early computer terminals often used a 20 mA current loop to transmit and receive information via long cables. Transmitting data as current, rather than voltage, enabled increased resistance to electrical noise and more reliable communication over long distances. However, current loop technology for data communication has largely been replaced by more advanced, digital-based protocols.

Remote Environmental Monitoring: Current loop technology can be applied to monitor environmental parameters such as temperature, humidity, and pressure in remote locations. For instance, in a weather station situated in a remote location, various sensors could be connected to a data logger using a current loop interface. The sensors convert the measured parameters into a proportional current signal, which is then transmitted through the loop to the data logger. This method allows for long distance transmission of data with minimal signal loss or distortion, providing accurate and reliable results for analysis and decision-making.

Current Loop FAQ

What is a current loop?

A current loop is a communication method used in industrial automation systems to transmit signals between devices. It typically consists of a loop where a constant current flows, representing the analog value of a process variable or the digital state of a device. The most common current loop standard is 4-20 mA, where 4 mA represents the minimum value and 20 mA represents the maximum value.

What are the advantages of using a current loop?

Current loops offer several advantages, including inherent noise immunity, the ability to transmit signals over long distances without significant signal loss, and low susceptibility to electromagnetic interference. Additionally, current loops can provide power to field devices, simplifying the overall system design and reducing installation costs.

How does a 4-20 mA current loop work?

A 4-20 mA current loop works by modulating the current flowing through the loop to represent the signal being transmitted. For example, a transmitter measures a process variable, such as temperature or pressure, and converts the value into a corresponding current ranging from 4 mA to 20 mA. This current is then sent to a receiver device, such as a PLC or control system, which interprets the current and converts it back into the original process variable value.

What are the different types of current loops?

There are two primary types of current loops: active and passive. An active current loop is one where the transmitter sources the current and provides power for the loop. A passive current loop is one where the transmitter controls the current but does not provide power, relying on an external power source for operation. Both types offer their own advantages, with active loops typically being simpler in design and passive loops being more flexible and able to accommodate a wider range of devices.

What are some common applications for current loops?

Current loops are widely used in industrial automation systems, process control, and instrumentation. Some common applications include transmitting signals from sensors, transducers, and transmitters to control systems, data acquisition systems, and display devices. Current loops can also be used for remote monitoring and control of field devices, such as valves, actuators, and pumps.

Related Technology Terms

  • Analog Signal Transmission
  • Load Impedance
  • 4-20 mA
  • Loop Powered Devices
  • Two-Wire Transmitter

Sources for More Information


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