How to Connect a 4–20 mA Sensor with a PLC: A Comprehensive Guide

Four to twenty milliamp (4–20 mA) sensors are widely used in industrial settings due to their reliability and the ease of transmitting data over long distances. This article will explore the process of connecting a 4–20 mA sensor to a Programmable Logic Controller (PLC), including the necessary hardware and steps involved. We will also discuss alternative setups if your system does not support 4–20 mA inputs.

Understanding the 4–20 mA Protocol

The 4–20 mA protocol is a standard for transmitting process variable data over a long-distance circuit. It is defined by the American Society of Mechanical Engineers (ASME) in ANSI/ASME P186.1 and IEC 61375. This protocol ensures consistent data transmission, requiring:

Twisted pair wire for optimal signal integrity Proper grounding and surge protection for long-distance installations The ability to work with a wide range of manufacturers

The Connection Process

Connecting a 4–20 mA sensor to a PLC involves a series of steps, including hardware selection and proper wiring. Here is a detailed guide:

Step 1: Hardware Requirements

Ensure you have the following components:

A 4–20 mA sensor A PLC or data logger that supports 4–20 mA inputs Twisted pair wire Terminal blocks or connectors (Optional) Surge protection

Step 2: Wiring the Sensor and PLC

1. If you have a PLC or data logger that supports 4–20 mA inputs:

Attach the sensor to the twisted pair wire, ensuring polarity is correctly maintained. Connect the wire to the appropriate terminal block or input terminal of the PLC or data logger. (Optional) Add surge protection to the sensor wires if installed outside a building.

2. If your PLC or data logger does not support 4–20 mA inputs, you can use a resistor:

Place a resistor in series with the sensor and a voltage source appropriate for the sensor. Calculate the current based on the voltage drop across the resistor. Use an Analog-to-Digital Converter (ADC) or similar device to read the voltage and compute the current.

Alternative Setup: Resistor-Based Method

If your system does not have built-in support for 4–20 mA inputs, you can use a resistor and voltage-based method:

Select a known resistor value that will allow you to read the current accurately. Connect the sensor in series with the resistor and a voltage source (e.g., a 0–5V ADC). Measure the voltage drop across the resistor and use the following formula to calculate the current:

Current (mA) (Voltage Drop (V) / Resistor Value (Ω)) * 1000

This method can be useful when upgrading old systems or if you need to retrofit existing sensors.

Conclusion

Connecting a 4–20 mA sensor to a PLC is essential for efficient data collection in industrial applications. By following the guidelines outlined in this article, you can ensure that your system is properly configured to transmit accurate data. Whether you use a PLC with native 4–20 mA support or a resistor-based method, the key is to maintain proper wiring, grounding, and protection to maximize the reliability and performance of your system.

Key Takeaways

4–20 mA sensors are used for reliable and long-distance data transmission. Proper wiring and surge protection are essential for outdoor installations. The resistor-based method can be used as an alternative for systems lacking 4–20 mA input support.