4-20mA Sensor Interface Primer
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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How to buy
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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
- 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