Converting 4-20 mA Receiver Circuit Current to Voltage Using a Shunt Resistor

Key Takeaways

  • Most industrial electronic process instruments utilize the 4-20 mA current loop bus.

  • The receiver circuit is designed to measure or read the regulated current flowing in a 4-20 mA loop.

  • The most effective current to voltage conversion method is to insert a precision shunt resistor across the terminals of the 4-20 mA receiver circuit. 

Industrial instrumentation and process control

The analog 4-20 mA current loop is one of the most commonly used standards for communication buses

It is critical that industrial systems are able to exchange information about environmental variables, sensor outputs, and control signals. As one of the most commonly used standards for communication buses in industrial systems, the analog 4-20 mA current loop makes this possible.

The 4-20 mA current loop bus is implemented in electronic control systems for signal transmission and process sensing. As the name suggests, the minimum and maximum values of the current in the bus are 4 mA and 20 mA, respectively.  The biggest advantage of the 4-20 mA current loop is its non-degradation of current value over long distances. Because of the various characteristics of the 4-20 mA current loop, most sensors utilized in industries are designed for an output of 4-20 mA to indicate their measurements. Usually, a 4-20 mA receiver circuit is incorporated in industrial systems to receive the current output from sensors or field instruments. In this article, we will explore the 4-20 mA current loop and its receiver circuit. 

The Compatibility of Analog Signals for Electronic Process Instruments 

Different instruments, sensors, actuators, valves, and process control systems are employed to enable the safe, reliable, and uninterrupted operation of systems. Most of these electronic process instruments utilize the 4-20 mA current loop bus. The current range is maintained within 4-20 mA to meet the requirements specified by the International Society of Automation (ISA).

One of the most widely used standards from ISA is ANSI/ISA-50.00.01, referred to as “Compatibility of Analog Signals for Electronic Industrial Process Instruments.” According to this standard, 4-20 mA is specified as the signaling range for analog data transmission of instrumentation, control, and other industrial electronic device signals. Most process instrument manufacturers stick to the standard 4-20 mA range to maintain compatibility between field instruments, control systems, and sensor signals. 

Why Is the Signaling Range Set to 4-20 mA?

In industrial process control and instrumentation systems, accuracy is an important factor. If voltage signals are used instead of current signals, they become prone to losses. The voltage signals transmitted over long distances encounter voltage drops due to the resistance of the wire. The voltage drop in the wire is proportional to the length of the wire. As the wire length increases, the voltage dropped across the length increases and degrades the accuracy of the signals. 

Enhanced Accuracy

As current signals are not vulnerable to any degradation over long-distance transmission, they are chosen over voltage signals for signaling in industrial process measurements, controls, and instrumentation systems. The current flowing through all the components in the current loop bus remains the same, which helps enhance accuracy throughout the loop. The current value in the loop is limited to 4-20 mA as per the ISA standards. 

Enhanced Signal-to-Noise Ratio of the Current

The minimum current value is set to 4 mA instead of 0 mA to enhance the signal-to-noise ratio of the current at low levels. The value of 4 mA is considered to be a “live zero” and this helps in evident identification of the loop failure. Any malfunction of the current loop results in a current value other than the 4-20 mA range. For example, an open termination leads to zero current flow, which is external to the range specified by ISA standards, and this makes loop failure identification easier. 

Easier Troubleshooting

Troubleshooting is also easier with the use of current loops. With current loops, the value read at the 4-20 mA receiver circuit is the same as the value coming from the transmitter or sensor. In the case of voltage signals, the voltage measured at the receiver is different from that of the transmitter, and it simplifies the troubleshooting or debugging process. 

The 4-20 mA Receiver Circuit

We have demonstrated that there is a transmitter and receiver in the 4-20 mA current loop. Apart from these two, a loop power supply is also essential for completing the current loop. All three components are connected in series in the loop using wires. The power supply provides the DC power to the current loop. The transmitter regulates the current flow in the loop within 4-20 mA. Usually, a transducer forms the transmitter that converts the physical or electrical quantity into a 4-20 mA analog signal. The receiver circuit is designed to measure or read the regulated current flowing in the 4-20 mA loop. The 4-20 mA receiver circuit used in the current loop can be a recorder, meter, actuator, SCADA module, or data acquisition device. 

Current to Voltage Conversion 

In most receiver circuits, a current signal is converted into a voltage signal. The current to voltage conversion is performed on the 4-20 mA receiver side to easily relate the receiver output with the transmitter’s original input. The most effective current to voltage conversion method is to insert a precision shunt resistor across the terminals of the 4-20 mA receiver circuit. The resistor connected across the current loop wires forms the simplest 4-20 mA receiver circuit. The voltage drop across the resistor (measured using a voltmeter) gives the equivalent voltage signal corresponding to the current in the 4-20 mA loop.

When developing an industrial control system or process sensing circuit, it is necessary to design a 4-20 mA receiver circuit and a suitable shunt resistor connection to it. The right PCB design software can simplify current loop receiver circuit design. With the features from Cadence’s PCB design and analysis software, you can quickly finish the design of an efficient and reliable 4-20 mA receiver circuit.

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