How to Connect a 4-20mA Current Loop Pressure Transmitter

All devices connected to make up a 4 to 20 milliamp current loop are connected in series.

The devices that are typically connected together include:

1. Pressure Transmitter
2. Input Device to accept the signal from the Pressure Transmitter such as a Programmable Logic Controller (PLC), Controller, Panel Meter, Analog to Digital Converter (ADC) or Signal Conditioning Unit (SCU)
3. Sensor Power Supply to power the Pressure Transmitter such as a Programmable Power Supply unit, DC Power Supply Unit (PSU), AC/DC Voltage Regulated Transformer or a DC Battery

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A) If the 4-20mA Input Device includes an Internal Sensor Power Supply and has 2 input connections:

1. Connect Positive (+) Input connection to Positive (+) Pressure Transmitter connection
2. Connect Negative (-) Input connection to Negative (-) Pressure Transmitter connection

B) If the 4-20mA Input Device requires an External Sensor Power Supply and has 2 input connections:

1. Connect Positive (+) External Sensor Power Supply connection to Positive (+) Pressure Transmitter connection
2. Connect Negative (-) Pressure Transmitter connection to Positive (+) Input connection
3. Connect Negative (-) Input connection to Negative (-) External Sensor Power Supply connection

C) If the 4-20mA Input Device includes an Internal Sensor Power Supply and has 2 input connections plus 2 supply connections:

1. Connect Positive (+) Internal Sensor Power Supply connection to Positive (+) Pressure Transmitter connection
2. Connect Negative (-) Pressure Transmitter connection to Positive (+) Input connection
3. Connect Negative (-) Input connection to Negative (-) Internal Sensor Power Supply connection

Some Input Devices bridge the Negative (-) Input connection to the Negative (-) Internal Sensor Power Supply via an internal / external jumper or link.

If the input device only includes a Voltage input, it can be easily adapted to measure a 4-20mA signal by adding a 250 Ohm load resistor across the Positive (+) and Negative (-) Input connections.  This will generate a voltage drop of between 1 and 5 Volts which is directly proportional to the 4-20mA signal.

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