4-20mA Sensor Interface Primer

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Revision as of 15:10, 21 December 2011 by Mparadis (Talk | contribs) (Introduction)

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Introduction

A 4-20mA sensor interface is a small device that adapts a 2-wire 4-20mA sensor's output to an analog output.

Normally, 4-20mA sensors are intended for industrial use and are difficult to use with a computer. However, with a 4-20mA sensor interface and a Phidgets Interface Kit (or any device with an Analog Input), these sensors can be used as easily as an ordinary analog sensor.

How it works

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Basic Use

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Things this page should cover:

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