AN42: 0-20mA or 4-20mA current loop

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When using LAN and GSM sensors by HW group, it is important to understand the difference between a 0..20mA current loop and a 4..20mA current loop. This Application Note explains the difference.

4-20mA / 0-20mA analog current loop is the output interface of many industrial sensors (water level measurement, current transformers, gas concentration sensors, and so on). The output current is proportional to the measured value.


Connecting a sensor with a 4-20 mA output

  • The analog sensor typically has no other power connection (it is powered from the minimum 4mA current).
  • HW group converter provides power for the sensor (4mA / max 36V).

Is it possible to use “Sensor 0-20mA 1W-UNI”?

Yes, the HW group “0-20mA” converter can also be used, as long as an external current source is added:

Connecting a sensor with a 0-20 mA output

  • Analog sensor (a current probe in this case) does not require external power.
  • The HW group “Sensor 0-20mA 1W-UNI” converter does not provide power for the sensor.

How to connect multiple analog outputs

  • Converters can be daisy-chained on the 1-Wire UNI bus.
  • The number of converters is limited by the maximum number of sensors supported by the particular Poseidon2 or Ares unit.

Technical notes

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  • The output unit format is configurable in HW group converters. The subsequent Ares or Poseidon2 unit can then, for example, display the water level as “38 cm” instead of the “17 mA” raw value.
  • The input to output value conversion using a curve with up to 8 points can be stored directly into the “Sensor 4-20mA 1W-UNI” or “Sensor 0-20mA 1W-UNI” converter.
  • To monitor sensors over GSM, the Poseidon2 unit can be replaced by an Ares unit.
  • Caution: This application note only concerns the analog value. It does not apply to digital communication over the current loop.