Difference between revisions of "4-20mA Sensor Interface Primer"

From Phidgets Support
Jump to: navigation, search
(Created page with "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 po...")
 
 
(39 intermediate revisions by 6 users not shown)
Line 1: Line 1:
Things this page should cover:
+
<metadesc>Many industrial sensors use a 4-20mA output which allows the use of long wires while ignoring EMI or voltage dropoff. Learn more at Phidgets.com.</metadesc>
 +
[[Category:Primer]]
 +
__TOC__
  
'''4-20 mA sensors:'''
+
==Introduction==
* 4-20 mA is an electrical standard for connection sensors to a data acquisition device.
+
[[Image:DAQ1400_0.jpg|400px|right|link=]]
* 2 wires are used, one wire to power the sensor and the other wire is a"ground".
+
Normally, 4-20mA sensors are intended for industrial use, and are difficult to use with a computer. However, with a {{CT|420Adapter|4-20mA adapter Phidget}}, interfacing with these sensors is extremely simple.
* The sensor sends data by changing the amount of current it consumes
+
<br clear=all>
* This is not very efficient, but it is much more immune to noise, and can be used for long distances between sensor an DAQ.
+
==How it works==
* 4-20 mA sensors often run off high voltages, often 12-24v but check sensor data sheet to be sure
+
===Overview===
* 4-20 mA sensors are typically expensive and industrial. There are often 0-5v versions available.
+
[[Image:420Interface.png|500px|link=|thumb|Traditional setup for reading a 4-20mA sensor.]]
* Why to use 4-20mA sensors?
+
4-20mA is an electrical standard for connecting sensors to a ''data acquisition device'' (DAQ). A 4-20mA sensor has two wires. One wire is used to power the sensor, and the other is a ground. The sensor sends data to the DAQ in the form of the amount of current it consumes. For example, a 4-20mA velocity sensor might consume 4mA when it is sensing zero movement, and would output 20mA when the sensor is moving at the maximum velocity it can sense. 4-20mA sensors are usually expensive and intended for industrial purposes. There are often less expensive 0-5V analog versions available.  
** 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:'''
+
===Advantages of a 4-20mA Sensor===
* If wire is broken, no current is consumed (sensor returns 0 A), normal operation is minimum 4mA, therefore failure is obvious.
+
*You can use longer wires with a 4-20mA sensor, since the data values are sent in the form of current rather than voltage levels that drop off with distance. However, you should ensure that the minimum voltage required to power the sensor is present at the end of the wire. To check, use a multimeter to measure the voltage across the two wires connecting to the sensor, when the sensor is measuring it's maximum value (and therefore drawing close to 20mA of current). If the voltage is higher than the minimum required voltage of the sensor (found in the sensor's data sheet), then it should be fine. Please note that twisted pair wire is the best for these long wire applications.
 
+
*4-20mA sensors are less susceptible to electromagnetic interference (EMI). This is because the data is sent in the form of current, which is less susceptible to EMI than voltage.
* Q: the Phidget 4-20 mA adapter can only supply 15V? What if my sensor needs 24V?
+
*It is easier to tell when your system has failed with a 4-20mA sensor. When a 4-20mA sensor senses its minimum value, it consumes 4mA of current. If it ever consumes zero current, it means there is a broken wire or some other failure in the system. When a 0-5V sensor fails in this way, the data line would read zero volts, which could mean it's just sensing zero.  
** A: Use 24v external power supply, hook + side to sensor and - side to ground on 1018. The 15V on the 1132 is left unused.
+
===Disadvantages of a 4-20mA Sensor===
 +
* Usually more expensive than 0-5V sensors.
 +
* 4-20mA sensors are not very efficient, because they only need 4mA to function, and any additional current draw exists solely to communicate the value it is sensing
 +
===When to use a 4-20mA Sensor===
 +
* If you already own a 4-20mA sensor and don't want to buy an analog version.
 +
* You need to use a very long wire between the sensor and the power source.
 +
* The environment has a lot of electromagnetic interference.
 +
* The sensor is only available in the 4-20mA format.

Latest revision as of 15:52, 7 June 2018

Introduction

DAQ1400 0.jpg

Normally, 4-20mA sensors are intended for industrial use, and are difficult to use with a computer. However, with a 4-20mA adapter Phidget, interfacing with these sensors is extremely simple.

How it works

Overview

Traditional setup for reading a 4-20mA sensor.

4-20mA is an electrical standard for connecting sensors to a data acquisition device (DAQ). A 4-20mA sensor has two wires. One wire is used to power the sensor, and the other is a ground. The sensor sends data to the DAQ in the form of the amount of current it consumes. For example, a 4-20mA velocity sensor might consume 4mA when it is sensing zero movement, and would output 20mA when the sensor is moving at the maximum velocity it can sense. 4-20mA sensors are usually expensive and intended for industrial purposes. There are often less expensive 0-5V analog versions available.

Advantages of a 4-20mA Sensor

  • You can use longer wires with a 4-20mA sensor, since the data values are sent in the form of current rather than voltage levels that drop off with distance. However, you should ensure that the minimum voltage required to power the sensor is present at the end of the wire. To check, use a multimeter to measure the voltage across the two wires connecting to the sensor, when the sensor is measuring it's maximum value (and therefore drawing close to 20mA of current). If the voltage is higher than the minimum required voltage of the sensor (found in the sensor's data sheet), then it should be fine. Please note that twisted pair wire is the best for these long wire applications.
  • 4-20mA sensors are less susceptible to electromagnetic interference (EMI). This is because the data is sent in the form of current, which is less susceptible to EMI than voltage.
  • It is easier to tell when your system has failed with a 4-20mA sensor. When a 4-20mA sensor senses its minimum value, it consumes 4mA of current. If it ever consumes zero current, it means there is a broken wire or some other failure in the system. When a 0-5V sensor fails in this way, the data line would read zero volts, which could mean it's just sensing zero.

Disadvantages of a 4-20mA Sensor

  • Usually more expensive than 0-5V sensors.
  • 4-20mA sensors are not very efficient, because they only need 4mA to function, and any additional current draw exists solely to communicate the value it is sensing

When to use a 4-20mA Sensor

  • If you already own a 4-20mA sensor and don't want to buy an analog version.
  • You need to use a very long wire between the sensor and the power source.
  • The environment has a lot of electromagnetic interference.
  • The sensor is only available in the 4-20mA format.