How To Bring 4-20mA Loop Powered Sensors to the IoT World

4-20mA Sensors and IoT

4-20mA is one of the oldest and most used wired communication protocols out there. These types of sensors are easy to use, easy to set up, reliable, and noise immune. Almost every manufacturing facility will have numerous sensors that output a 4-20mA signal. Generally, these sensors connect to a PLC and from there users have to jump through a few hoops in order to take it to the cloud. There are two ways users can push data to the cloud:

Install new sensors which can connect to the cloud directly
Retrofit old sensors with IoT technology

Types Of 4-20mA Sensors

There are two types of 4-20mA signal output devices:

4-20mA Current Loop Type ( Active )
4-20mA Current Output ( Passive)

In this article we will be covering the first type, 4-20mA Current Loop ( Active )

4-20mA Current Loop Type ( Active )

4-20mA Current Loop devices are the oldest type of 4-20mA devices. In this type of device, there are two wires. These two wires carry voltage as well as signal. These kinds of devices are also known as 2-wire devices.

These types of devices do not need any external power supply. The NCD transmitter supplies 16V/20mA power.

Product Link: Industrial IoT Long Range Wireless 4-20mA Current Loop Receiver

Hardware Setup

One of the main reasons behind the popularity of 4-20mA devices is that the hardware setup is extremely simple. Sensors will have two wires. One will be +ve and another one will be Ground.

Power Switch – The switch position should be towards the enclosure wall if it’s powered by internal batteries and it should be away from the enclosure wall if its powered by an external power supply

Radio Module – The antenna cable should be plugged into the radio module. Make sure your modem/gateway radio module looks exactly like the device radio module( same frequency).

Antenna — Antenna should be installed all the time. It should be pointing towards the sky. If possible mount the device away from walls.

M12 connector — This connector contains signal and power wires

“R” Button — Reset Button. When this button is pressed and released devices will send a welcome message followed by a data message

‘C‘ Button — It is a configuration Button.

How to put into Configuration Mode

Press and release R button
Immediately hold “C” button
Keep holding the “C” Button for 6-7 sec

Hardware M12 Cable Connection

White — Power -ve
Brown — Power +ve
Blue — 4-20mA +Ve
Black — 4-20mA -Ve
Gray — DNC

M12 Connector Connection

VIN-
VIN+
4-20mA+
4-20mA-
DNC

There are a large number of sensor types that come with 4-20mA output. For example:

Pressure sensor
Temperature sensor
Flow sensor
Tank level sensor
pH Sensor
Vibration sensor
Limit Sensors
Light Sensor

All of these sensors can be hooked directly to the ncd.io 4-20mA Current Loop Transmitter Data Acquisition Device.

How it Works

The ncd.io 4-20mA Current Loop transmitter comes with a high-resolution 16bit ADC and onboard 16V power supply which powers the loop device.

This device is designed to work in low power mode. Here is how it works in the background

After a fixed interval the on-Board MCU will wakeup
It will turn on the Loop Supply
It will wait for sensor data
Turn off the Loop power to save power
Turn on the radio module
Send data over wireless
Turn off the radio module
Go back to sleep
Repeat 1-8

The sensor will then send the following data in the Wireless Packet:

Devices MAC address
Device node id
Device Type
Firmware version
Battery Level
Transmission Counter
Raw ADC value
4-20mA Value

Once the hardware is connected, turn on the device and it will send a welcome message followed by a Data message. As an example, let’s say a pressure sensor is connected to the 4-20mA loop transmitter. This pressure sensor can measure pressure between 0-100psi. When the pressure is 0psi the sensor will output 4mA and when the pressure is 100psi it will output 20mA.