Minimalistic Current Loop Receiver Circuit Design

Key Takeaways

• The 4-20mA is the most robust analog signaling standard used in industries. 

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

• Typically, 250 Ω fixed-resistance is utilized for a current loop receiver circuit. 

Which came first, the chicken or the egg? This is a commonly stated paradox for situations where it is difficult to propose the dependency of two separate things on each other. This dilemma applies to sensors and current loops, which are inevitable in industrial process control systems. It is a baffling task to prove the existence of sensor circuits without current loops and vice versa.

To convey the sensor data acquired to the control system, industries rely on 4-20mA current loops. The 4-20mA is a universally accepted signaling range, and the 4-20mA current loop is a series connection of transmitter, receiver, DC power supply, and connecting wires. The current loop receiver circuit forms the final end of the loop and either displays the physical quantity measurements or performs the intended action on the signal acquired. We will discuss the current loop receiver circuit in this article.   

Current Loops in Industrial Process Controls 

It is often necessary to measure environmental variables and process variables to control a production or manufacturing system. For example, in the chemical industry, you need to measure the temperature, pressure, pH, flow, etc. There will be some intended action to perform when the temperature is above a reference value or when the pressure drops below a set value. It is not always practical to manually control the process by monitoring the physical variable fluctuations. Usually, PLC systems are utilized in industries for process control. 

PLC Systems

Most PLC systems receive inputs from the 4-20mA current loop. 4-20mA current loops can be hardwired between the sensor and process control system without a PLC. The current loop is the interface between the sensor and the display instrument or any embedded system on the control side. There is a minimalistic design approach available for sensor-current loop circuits. Let's understand the fundamentals of the current loop for designing a simple current loop circuit. 

4-20mA Current Loops

The 4-20mA is the most robust analog signaling standard used in industries. In the current loop, analog current signals carry the sensor data to the control panel, which can be a PLC, microcontroller, or an FPGA. The current signals are highly accurate and remain the same throughout the current loop from the field end to the control end.

In case voltage signals are used, the voltage drop caused by the internal resistance of the current loop components and wires alters the value of the physical quantity by the time it reaches the control end. The current signals are inherently immune to electrical disturbances or noises.

A major advantage of the 4-20mA current loop is live zero. The zero output from the sensor corresponds to 4mA and forms the live zero value. Assigning 4mA as live zero allows easy detection of the failures in current loops.

The simple design and ease in connection, configuration, and debugging are some of the highlights that lead to the extensive use of the 4-20mA current loop. Generally, the current loop consists of the following elements:

1. DC power supply - The power for the operation of the current loop is provided by a battery or regulated DC power supplies. 

2. Transmitter - The transmitter converts the sensor output into the current flow within 4-20 mA and translates the sensor’s zero-level output to 4 mA and full-scale output to 20 mA. 

3. Receiver - 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. 

4. Loop wiring - The connecting wires used for making the series connection of power supply, transmitter, and receiver are called loop wiring.

In the upcoming section, a briefing is given on the minimalistic current loop receiver circuit.  

Minimalistic Design of a 4-20mA Current Loop Receiver Circuit

Let's design a minimalistic temperature sensor system.

Sensor - The temperature sensor measures the temperature. It can be a thermocouple sensor and is connected to the current loop transmitter.

Current loop transmitter - The current loop transmitter measures the temperature sensor data and translates it into an analog current signal. The current loop transmitter can be programmed to regulate the current within the 4-20mA range. Assume that the change in current between 4-20 mA corresponds to 0-200℃.

Current loop receiver - You can utilize microcontrollers or precision IC-based current loop receivers to get the exact value of the temperature measured by the temperature sensor. However, the minimalistic current loop receiver circuit can be realized using a precision resistor. The resistor-based current loop receiver is minimal in terms of its complexity as well as cost. 

This cost-efficient current loop receiver circuit is based on Ohm’s law. Typically, 250 Ω fixed-resistance is utilized for the current loop receiver circuit. The voltage drop across the resistor can be converted into temperature readings. The proportionality can be obtained from the relationship 1V (4mA x 250 Ω ) corresponds to 0℃ and 5 V (20mA x 250 Ω ) to 200℃. You can use a voltmeter to measure the voltage drop and manually do the voltage-to-temperature conversion. If you don't want manual calculations, use an analog-to-digital converter to send the analog voltage values into the microcontroller. The calculation part is taken care of by the microcontroller and you can easily obtain temperature readings.

There is no sense in a sensor system without a current loop receiver circuit. Regardless of the current loop receiver circuit type, you can utilize Cadence’s PCB design and analysis software for current loop circuit design.

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