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:
- Pressure Transmitter
- 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)
- 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
DMK 331 Ceramic Diaphragm Industrial Pressure Transmitter
DMP343 Low Range Pneumatic Pressure Sensor
DMP331 Precision Pressure Transmitter
DMK351 Liquid and Gas Resistant Intrinsically Safe Low Range Gauge Pressure Sensor
A) If the 4-20mA Input Device includes an Internal Sensor Power Supply and has 2 input connections:
- Connect Positive (+) Input connection to Positive (+) Pressure Transmitter connection
- 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:
- Connect Positive (+) External Sensor Power Supply connection to Positive (+) Pressure Transmitter connection
- Connect Negative (-) Pressure Transmitter connection to Positive (+) Input connection
- 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:
- Connect Positive (+) Internal Sensor Power Supply connection to Positive (+) Pressure Transmitter connection
- Connect Negative (-) Pressure Transmitter connection to Positive (+) Input connection
- 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.
DPS300 User Switchable Pressure Range, Volts or Current Output Low DP Sensor
PrimAtü 10 Low Cost Low Range Differential Air Pressure Transducer
LMK 487 Submersible Ballast Tank and Draught Level Pressure Transmitter
Differential Pressure Transmitters