Resistor-Based 4-20 mA Receiver Design

Key Takeaways

• The 4-20 mA current loop is a robust signaling standard used for transmitting industrial sensor signals. 

• Many current loops use a simple receiver circuit consisting of a resistor connected in series with a transmitter and power supply.

• The selection of the receiver resistor is based on the maximum voltage that the receiver can deal with.

Many current loops use a simple receiver circuit consisting of a resistor connected in series with a transmitter and power supply

Most industries utilizing electronic devices and sensors for process control employ a 4-20 mA current loop. This current loop consists of a DC power supply, transmitter, and receiver connected in series using wires. In the current loop, the current within the range of 4-20 mA flows from the transmitter to the receiver, and the 4-20 mA receiver circuit measures and interprets the received current signal.

In most cases, 4-20 mA receiver design focuses on the conversion of the received current signal into equivalent voltage. Ohm’s law governs the 4-20 mA receiver design. Usually, precision shunt resistors are connected across the receiver terminals for current to voltage conversion. In this article, we will discuss 4-20 mA current loops and receiver designs. 

4-20 mA Current Loops in Industrial Control Systems

More than voltage, current loops are preferred for data or signal transmission by industrial sensor systems due to their inherent immunity to electrical disturbances and noise. 4-20 mA current loop signaling protocol is best suited for hostile environments. The current loop communication protocol is highly accurate, as the current remains the same throughout the loop, irrespective of the length. The requirement of only two wires makes the current loop even more popular. Both the power and signal are sent over the same pair of wires. 

Let's look at the arrangement and operation of a 4-20 mA current loop.

4-20 mA Current Loop Arrangement and Operation

A 4-20 mA current loop is a robust signaling standard used for transmitting industrial sensor signals. A sensor output signal is given to the 4-20 mA current loop, which constitutes a transmitter, a power supply, and a receiver. The sensor is interfaced with the transmitter present in the current loop to measure the process variable. The electrical output of the sensor is given to the transmitter that receives the power for operation from the DC power supply in the loop. The transmitter converts the sensor measurement into a current signal between 4-20 mA. The transmitter translates the sensor’s zero-level output to 4 mA and full-scale output to 20 mA.

The transmitter, receiver, and power supply are connected in series using wires in the current loop. Mostly, the 4-20 mA current loop employs a two-wire system where a single twisted pair cable is utilized for supplying the power to the transmitter and signal to the receiver. The receiver interprets the transmitter output. The purpose of the receiver is to either display the information received or to perform an action on the signal.

Let’s examine the receiver and its design in the upcoming section.  

4-20 mA Receiver Design

As we discussed, the function of the 4-20 mA receiver circuit is to measure the transmitter output current and interpret its value. When it comes to measurements in the receiver end of the 4-20 mA current loop, voltage measurements are easier compared to current measurements. Therefore, the receiver circuits convert the current signal to voltage for easy measurement. Many current loops use a simple receiver circuit consisting of a resistor connected in series with the transmitter and power supply. The voltage drops across the receiver resistor given by the product of current, and the resistance value (Ohm’s law) can be easily measured by the controllers.

Let's use an example of a simple receiver circuit design formed by a resistor:

The selection of the receiver resistor is based on the maximum voltage that the receiver can deal with. The maximum voltage corresponds to the full-scale sensor output or 20 mA current in the loop. Once the maximum voltage is known, the receiver resistor can be obtained.

Typically, 250 Ω resistors are used in the 4-20 mA current loop for current-to-voltage conversion.

The voltage drop across the receiver resistor is maintained such that it is less than the power supply voltage, and the power supply voltage is selected to be greater than the total sum of the voltage drops in the 4-20 mA current loop. There is an offset voltage associated with the receiver resistor voltage drop due to the consideration of 4 mA as live zero in the 4-20 mA current loop.

The offset voltage is critical in the design of the receiver circuit. When there is no sensor input, the voltage output at the receiver end should be equal to 0 V. However, it is not the case, as 4 mA is made as live zero. To remove the offset voltage corresponding to live zero or 4 mA current, an op-amp circuit is incorporated into the receiver resistor circuit. The op-amp circuit subtracts the received voltage signal (voltage across the receiver resistor) from the reference voltage. The reference voltage is the voltage corresponding to the 4 mA current signal.

4-20 mA receiver design is available as printed circuit boards or integrated circuits. Regardless of the design type, the receiver circuit should offer high reliability and high conversion accuracy at low design and manufacturing costs. Utilizing Cadence’s PCB design software can simplify current loop receiver circuit design.

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