Introduction

This guide explains the 4-20 mA Current Loop Sensor Board features and functions. This product was designed for Waspmote v12 and continues with no changes for Waspmote v15. There are no great variations in this library for our new product line Waspmote v15, released on October 2016.

Anyway, if you are using previous versions of our products, please use the corresponding guides, available on our LINK PENDIENTE Development website.

You can get more information about the generation change on the document LINK PENDIENTE "New generation of Libelium product lines".

The standard

The 4-20 mA current loop is one of the most robust sensor signaling standard. Current loops are ideal for data transmission because of their inherent insensitivity to electrical noise. In a 4-20 mA current loop, all the signaling current flows through all components; the same current flows even if the wire terminations are less than perfect. All the components in the loop drop voltage due to the signaling current flowing through them. The signaling current is not affected by these voltage drops as long as the power supply voltage is greater than the sum of the voltage drops around the loop at the maximum signaling current of 20 mA.

Transmitting sensor information via a current loop is particularly useful when the information has to be sent to a remote location over long distances (500 meters, or more). The loop's operation is straightforward: a sensor's output voltage is first converted to a proportional current, with 4 mA normally representing the sensor's zero-level output, and 20 mA representing the sensor's full scale output. Then, a receiver at the remote end converts the 4-20 mA current back into a voltage which in turn can be further processed by a computer or display module.

This list includes some of the most common uses of the standard:

  • Sensors and instruments

  • Remote transducers

  • Monitoring processes

  • Data transmission in industrial ambients

Figure : 4-20 mA standard typical connection diagram

Power Supply

The loop power-supply generally provides all operating power to the transmitter and receiver, and any other loop components that require a well-regulated DC voltage. In loop-powered applications, the power supply's internal elements also furnish a path for closing the series loop. +24 V is still the most widely used power supply voltage in 4-20 mA process monitoring applications. This is due to the fact that +24 V is also used to power many other instruments and electromechanical components commonly found in industrial environments. Lower supply voltages, such as +12 V, are also popular since they are used in computer based systems.

Transmitters categories

Depending on the source of current for the loop, devices may be classified as active (supplying power) or passive (relying on loop power).

Type 2 loop current

Type 2 transmitters are energized by the current loop, where the supply voltage is included in the receptor. The transmitter is floating and the ground is in the receptor.

Figure : 4-20 mA type 2 connection

Type 3 loop current

Type 3 transmitters have 3 wires powered by the source voltage in them. In this case the transmitter is the power source for the current loop. The transmitter common is connected to the common of the receptor.

Figure : 4-20 mA type 3 connection

Type 4 loop current

Type 4 transmitters have 4 wires powered by the source voltage in them. The transmitter powers the current loop and the receptor acts a floating load.

Figure : 4-20 mA type 4 connection