The Basics of Current Loop
The current loop from 4 to 20 mA is a very robust sensor signalling standard. Current loops are ideal for data transmission. All the signalling current flows through all the components; the same current flows even if the cable terminations are less than perfect. All components of the loop lower the voltage due to the signalling current flowing through them.
The 4-20 mA Idea
Kirchhoff says: Current is constant along any wired loop
Even if there is significant electrical resistance in the line, the current loop transmitter will maintain the proper current, up to its maximum voltage capability.
• Only two wires are required to send the signal and also supply power to the sensor
• Signal may be analog or digital
• The live-zero represented by 4 mA allows the receiving instrument to detect some failures of the loop
• Also allows transmitter devices to be powered by the same current loop (called two-wire transmitters).
• Such instruments are used to measure pressure, temperature, flow, pH …
• A current loop can also be used to control a valve positioner or other output actuator.
• An analog current loop can be converted to a voltage input with a precision resistor.
There are 2 types of current loop circuits
2 The Simplex 20 mA Circuit
1.Full-duplex 20 mA Circuit
Figure is a full-duplex 20 mA current loop circuit.Simultaneous two-way communications is possible with this circuit.Two 20 mA current generators are necessary with this circuit. It is possible to have one of the two current generators in one current loop interface and the other current generator in the other interface.For example, the original IBM PC serial adaptor card had a current loop interface that contained only one current generator. When you made a correct connection to this current loop interface, the second current loop device would need to provide one current loop generator
2.The Simplex 20 mA Circuit
Figure is a diagram of a simplex 20 mA current loop circuit. The fundamental elements of a 20 mA current loop are a current source, a current switch, and a current detector. The transmitter is the current switch and the receiver is the current detector. The interface that contains the current source is called the active unit and all other units are referred to as passive units. Figure is a diagram of the levels in an RS-232 interface and how they relate to the presence and absence of current in a 20 mA current loop circuit. In a 20 mA loop the current flows when the loop is idle(no data being transmitted). In a simplex type circuit a number of transmitters and receivers are put in series in a current loop. As long as only one transmitter sends data, all receivers receive the data
4 to 20 mA Analog Current Loop
The diagram shown in Figure is an analog 4 to 20 ma current loop. This circuit is mentioned here because it is sometimes confused with 20 mA digital current loop. The purpose of 4 to 20 mA analog current loop is to transmit the signal from an analog sensor over some distance in the form of current signal. Only two wires are required to send the analog signal and also supply power to the sensor. A loop supply voltage is used to power the remote sensor. The remote sensor regulates the loop current such that the loop current represents the value of the parameter being measured by the sensor. A series resistor RL at loop power supply converts this current to a voltage that can be used by the electronics to record or distribute the parameter being measured.
HART® 4 to 20 mA Current Loops
Figure 12 is another example of a type of 4 to 20 mA combined analog & digital current loop. This current loop uses HART® Communications protocol. The HART® (Highway Addressable Remote Transducer) protocol is used for SMART remote transducers that are compatible with 4 to 20 mA analog current loops but also have digital communications on the same two wires. This is accomplished by superimposing a two-tone Frequency Shift Keyed (FSK) digital current signal on the 4 to 20mA analog signal
Why Use a Current Loop?
The 4-20 mA current loop shown in the Figure is a common method of transmitting sensor information in many industrial process monitoring applications. A sensor is a device used to measure physical parameters such as temperature, pressure, speed, liquid flow rates, etc.
The transmission of sensor information through a current loop is particularly useful when the information must be sent to a remote location over long distances (1000 feet, or more). The operation of the loop is simple: the output voltage of the sensor is first converted to a proportional current, with 4 mA which normally represents the zero level output of the sensor and 20 mA representing the full scale output of the sensor. Then, a receiver at the remote end converts the 4-20 mA current back into a voltage which in turn can be processed by a computer or display module.
Advantages of 4 to 20mA
- It can be run over long distances with minimal signal losses compared to voltage type signals
- A varying current loop load impedance or supply voltage will not significantly affect the signal as long as it does not exceed recommended component limits
- Rugged signal with low electromagnetic susceptibility
- Saves on cable wire becasue it only needs 2 wires to function
- Live zero reading verifies sensor is electrically functional
Disadvantages of 4 to 20mA
- High power consumption compared to other analogue signal types
- Elevated output at zero reading
- Supply not isolated from output
- Increasing circuit load resistance, will reduce the supply voltage available to power the transmitter that is generating the 4-20mA signal.
The Future of 4-20mA
Looking ahead, the future continues to look bright for the 4-20mA signal transmission standard in industrial environments. Its lossless nature, lower-sensitivity to induced noise, its live-zero offset, fail-safe operation, and easy scalability contribute to its longevity. Plus its adaptability to many different wire conductors and connectors, and its relative immunity to poor quality connections, all contribute to its popularity. And because it is so widely supported by thousands of compatible devices, including wireless transducers, it would be difficult to unseat it as the leading analog transmission standard for industrial I/O. Likewise, modern variations of 4-20mA such as HART1 continue to drive support for the standard.