Understanding 2-Wire, 3-Wire, and 4-Wire 4-20mA LVDT or LVIT Wiring

Hello and thank you for choosing Harold G. Schaevitz Industries – The Sensor Connection. Today, we’ll explore the wiring configurations of 2-wire, 3-wire, and 4-wire 4-20mA LVDT or LVITs. Whether you're a technician or an instrument engineer, understanding these wiring methods will help you make informed decisions for your industrial automation applications.

What is a 4-20mA LVDTor LVIT?

An LVDTor LVIT (Linear Variable Differential Transformer) is a type of position sensor that converts linear displacement into an electrical signal.

The 4-20mA signal is the most widely used analog signal in industrial applications. At calibrated zero displacement, the LVDT outputs 4mA, and at calibrated full-scale displacement, it outputs 20mA. Now, let’s break down the different wiring configurations.

2-Wire 4-20mA LVDT

A 2-wire LVDT is powered by a low-voltage DC power supply, typically 24V DC. What makes this system unique is that it draws power from the 4-20mA loop itself while also transmitting the measurement signal.

How does it work?

• The positive terminal of the power supply connects to the positive terminal of the LVDT.
• The negative terminal of the LVDT connects to the positive terminal of the PLC analog input.
• The negative terminal of the PLC connects back to the negative terminal of the power supply, completing the loop.

3-Wire 4-20mA LVDT

A 3-wire LVDT has three dedicated connections:

1. A power wire (+V)
2. A signal wire (4-20mA output)
3. A ground wire (GND or -V)

How does it work?

• The power supply (+V) provides 24V DC to the LVDT.
• The LVDT outputs a 4-20mA signal to the PLC analog input.
• The ground wire is shared between the LVDT and PLC, forming the return path.

4-Wire 4-20mA LVDT

A 4-wire LVDT has two pairs of wires:

• One pair for the power supply
• One pair for the control signal

Unlike 2-wire systems, a 4-wire LVDT is externally powered and is isolated since there is no connection between power and ground.

How does it work?

• The power supply provides voltage to the LVDT.
• The LVDT generates a 4-20mA signal, which is sent to the PLC analog input.
• The signal wires form a parallel connection rather than a series circuit.

Conclusion

Now that you understand the differences between 2-wire, 3-wire, and 4-wire 4-20mA LVDT systems, you can select the best configuration for your application.