In industrial automation control applications, typical processes consist of the following components:

1. Field sensors (temperature, pressure, flow, etc)
2. Controllers (PLC’s, computers, programs, etc.)
3. Field control elements (control relays, actuators, valves, etc)

The most common control signal used by field sensors is the 4-20 mA current loop. For sensors that do not require large amounts to operate (temperature, pressure, level, electrochemical gas sensors, etc.) the most common type of sensor transmitter is the 2 wire transmitter.

Basic Operation of a 2 Wire Transmitter

A 2-wire transmitter is loop-powered, which means it is powered by a low voltage, low current, DC power supply, typically 24 VDC. A 4-20mA current is usually supplied by the control system and connected to the transmitter via 2 wires.

The base line 4 mA typically represents the 0% value of the variable being measured. The transmitter derives it’s operating power from the 4 mA (typically 3.5 mA to allow for calibration at the 0% level).

The three components that make up the measurement loop:

1. Field power supply (typically 24 VDC)
2. 2 Wire transmitter
3. Controller or receiving module input channel load resistor

Each controller or receiver module will have a load resistor (ranges between 100 Ohms to 250 Ohms). The typical wiring will be as follows (PI-SPI-8AI+ Module shown as an example)
  For our examples, we will use a Rosemount Model 2088 Pressure Sensor 2 Wire transmitter

 

PI-SPI-8AI+ Analog Input Module 2 Wire Transmitter Connection

PI-SPI-DIN-8AI Analog Input Module 2 Wire Transmitter Connection

 

 

In this example, the field 24 VDC power supply is wired to the PI-SPI-DIN-RTC-RS485 module which powers the Raspberry Pi and provides the field power and communications to the expansion modules via the 16 point ribbon cable. The 2088 + (RED) is wired to terminal block 11 (+V) and the – (BLACK or SIGNAL) in this example is wired to terminal block 3 (A1). Please note: any of the 8 mA inputs could be used, this module would support a qty of 8 transmitters.

PI-SPI-DIN-8AI 4-20mA Analog Input I/O Module

VP-EC-8AI Modbus RS485 Analog Input Module 2 Wire Transmitter Connection

 

 

In this example, the field pwoer supply (24 VDC) power up the VP-EC-8AI module. The 2088 + (RED) is wired to terminal block 1 (+V) and the – (BLACK or SIGNAL) in this example is wired to terminal block 2 (A1). Please note: any of the 8 mA inputs could be used, this module would support a qty of 8 transmitters.

VP-EC-8AI 4-20mA Analog Input I/O Module

WL-MIO VPE-6180 Analog Input Module 2 Wire Transmitter Connection