4-20mA Sensor Interface Guide

Introduction

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4-20 mA sensors:

• 4-20 mA is an electrical standard for connection sensors to a data acquisition device.
• 2 wires are used, one wire to power the sensor and the other wire is a"ground".
• The sensor sends data by changing the amount of current it consumes
• This is not very efficient, but it is much more immune to noise, and can be used for long distances between sensor an DAQ.
• 4-20 mA sensors often run off high voltages, often 12-24v but check sensor data sheet to be sure
• 4-20 mA sensors are typically expensive and industrial. There are often 0-5v versions available.
• Why to use 4-20mA sensors?
  • You have one already
  • You want to do a really long run of wire
  • Environment has lots of EMI
  • Sensor is only available in 4-20 mA format

Advantages:

• If wire is broken, no current is consumed (sensor returns 0 A), normal operation is minimum 4mA, therefore failure is obvious.

• Q: the Phidget 4-20 mA adapter can only supply 15V? What if my sensor needs 24V?
  • A: Use 24v external power supply, hook + side to sensor and - side to ground on 1018. The 15V on the 1132 is left unused.

• Remember: on very long wire runs, you may lose too much voltage on the wire. The minimum varies by sensor; check the datasheet.

to measure the effective voltage, use a multimeter across the two wires close to the sensor, when the sensor is returning a large current (as close to 20mA as possible)

• Twisted pair wire is best for long-wire applications