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4 to 20 mA Current Loop Output Signal

4-20mA Current loop output is a type of electrical signal that is used in a series circuit to provide a robust measurement signal.

The sensor or instrument generating the signal acts like a current source providing a constant current output signal for a given measurement independent of supply voltage or circuit impedance.

Unlike voltage outputs, the current output is the same at any particular point in the circuit and is a relatively high power analogue output which can be used in measurement circuits over long distances.

A typical current output is 4 to 20 mA in a 2 wire configuration, but there are 3 wire configurations which can provide 0 to 20mA or 4 to 20mA current loop output signal with a separate positive supply and output connection.

4 to 20 mA current loop signal measurement products
  • 4-20mA Output Hydrostatic Level Transmitters & Sensors
  • SIAi-8 8 Channel Analogue Signal Input to RS-485 Modbus RTU Interface Converter
  • Universal Strain Gauge Bridge Input Transmitter – STRAIN
  • DMK 331 Ceramic Diaphragm Industrial Pressure Transmitter

Request info on 4 to 20 ma current loop signal measurement products for your application.

The current can be converted to voltage output for measurement purposes at any point in the circuit by adding a load resistor in series. The voltage drop across the resistor will then vary proportionally with pressure and current. For example a 250 ohm resistor will produce a 1 to 5 volt output signal with 4-20mA current loop signal.

When connecting up a 4-20mA transmitter to a circuit with a measurement load resistor it is important to ensure that there is sufficient supply voltage available across the transmitter positive and negative supply contacts particularly at full scale output.

4-20 milliamps is used extensively throughout the process control industry and is also being increasingly used for manufacturing, aerospace and automotive applications because of the convenience of 2 wire connections, robustness to RFI, and similar size to other types of output devices .

4 to 20 mA current loop signal measurement products
  • DMK 331 Ceramic Diaphragm Industrial Pressure Transmitter
  • Barometric Pressure Transmitters
  • 18.605 G Low Cost Submersible Diesel Fuel and Water Tank Level Sensor
  • ATEX Approved Pressure Transmitters

Request info on 4 to 20 ma current loop signal measurement products for your application.

Contents

  • 4-20mA Help
    • 2 wire 4-20ma pressure transmitter wiring configuration
    • Series or parallel 250Ω resistor to create 1-5Vdc from 4-20mA
    • 2 & 4 wire comparison
    • Convert 4-20mA to 1-5 volt output
    • Why start from 4mA and not lower
    • 4mA vs 0mA

4-20mA Help

2 wire 4-20ma pressure transmitter wiring configuration

The diagram below below shows a simple wiring configuration for current loop pressure transmitter.  It is assumed that the measurement device includes a sufficient load resistance for measuring a current loop. If the measurement device is a multi-meter it is unlikely to include sufficient load resistance, so in this case you will need to include a separate load resistor, e.g. 250Ω added in series to the current loop.

Series or parallel 250Ω resistor to create 1-5Vdc from 4-20mA

If a 4-20mA signal is connected to a DCS circuit that accepts a 1-5 Vdc input, which of these statements are correct?

  1. Connect 250 Ohms resistor in parallel
  2. Connect 250 Ohms resistor in series

Either statements can be true because it depends on which part of the circuit you are referring to.

Statement 1 would be true if you were relating the connection of the resistor to the 1-5Vdc signal input – Connect 250 Ohms resistor in parallel with the 1-5Vdc input wires.

Statement 2 would be true if you were relating the connection of the resistor to the current loop circuit – Connect 250 Ohms resistor in series with the 4-20mA current loop circuit.

If you were to ask what is the most usual way of describing how a 4-20mA current loop signal is converted to a 1-5Vdc output signal, then statement 2 would be the one that most people use.

2 & 4 wire comparison

What is difference between a two wire and a four wire 4 to 20 milliamp transmitter?

Two wire 4-20mA transmitter will have a single loop circuit with the transmitter [+/-], power supply [+/-] and signal conditioning unit (SCU) [+/-] all connected together in series.

2 to 4 wire 4-20ma comparison

Four wire 4-20mA transmitter will have a separate output and supply circuit. The transmitter output +/- will be connected to the SCU +/-. The transmitter supply +/- will be connected to the power supply +/-.

Convert 4-20mA to 1-5 volt output

I have a pressure transmitter with an output of 4-20mA / 2 wire. How could I change this current output to a voltage output?

Add a load resistor in series with the current loop and then measure the voltage drop across the load resistor.

For example a 250 ohm resistor will convert a 4-20mA signal to 1 to 5 Vdc.
change 4-20ma to 1-5vdc circuit
Also note that you will need to allow for the required additional power supply of 5 volts  over the sensor’s minimum supply voltage of 9 Vdc, to allow for the extra load resistance.

Why start from 4mA and not lower

Why do current loop transmitters provide a signal which starts from 4ma and not below this?

4-20mA is normally a 2 wire current loop and in order for the device to operate it needs to consume some of the current to function, and this is provided by the minimum current of 4mA.

Also it is useful to know whether the current output device is functioning, unlike a 0-10Vdc signal for example, a zero mA signal would suggest there is a failure in the current loop circuit at some point.

4mA vs 0mA

Why do current loop transmitters provide a signal which starts from 4ma and not 0mA?

The main reason is that 4-20mA is normally a 2 wire current loop and in order for the device to operate it needs to consume some of the current to function, and this is provided by the minimum current of 4mA.

4 to 20 mA current loop signal measurement products
  • DPT200 High One Side Overload Differential Pressure Sensor
  • STT-26 Submersible Temperature Probe and Transmitter
  • HART® Pressure Transmitters
  • DMK457 Marine Approved Pressure Transmitter

Request info on 4 to 20 ma current loop signal measurement products for your application.

Measurement Signal Related Terms

More pages related to measurement signal technical terms.

  • 2 Wire
  • 3 Wire
  • 4 Wire
  • Amplified Voltage Output
  • BFSG – Bonded Foil Strain Gauge
  • Deadband
  • Digital Interface
  • FSO – Full Scale Output
  • HART®
  • High Frequency Response
  • Intrinsic Safety
  • Measurement Signal
  • Millivolt Output Pressure Transducers
  • mV/V – Millivolts per Volt Output Signal
  • NC – Normally Closed
  • NO – Normally Open
  • Piezoresistive Strain Gauges
  • Pressure Transmitters
  • Rangeable
  • Ratiometric
  • Span
  • Span Offset
  • Span Sensitivity
  • Square Root Extraction
  • Threshold
  • Totalizer
  • Transducer
  • Transmitter
  • TSL – Terminal Straight Line
  • TSS – Thermal Span or Sensitivity Shift
  • Turndown Ratio
  • USB
  • Vented Cable
  • Voltage Output Pressure Transducers
  • Wheatstone Bridge Strain Gauge
  • Zero Offset
  • Zero Tare

Primary Sidebar

Product Selection Guides

  • Pressure Transmitters
  • Pressure Transducers
  • Liquid Level Sensors
  • Digital Pressure Gauges
  • Data Loggers

Application Selection Guides

  • Vacuum
  • Submersible
  • Low Pressure Range
  • Leak Testing
  • High Frequency Response
  • Dual Temperature and Pressure
  • Barometric Pressure

Help Guides

  • Reduce calibration costs of analogue pressure gauges
  • Measuring the difference in air pressure between rooms
  • How to get a 10 volt signal from a 4-20mA output pressure sensor
  • How to Connect a 4-20mA Current Loop Pressure Transmitter
  • Determining the hydrostatic pressure range for a tank level sensor
  • Choosing PVC, PUR, FEP and TPE cable types

Online Measurement Tools

  • Temperature Transmitter 4-20mA Current Output Calculator
  • Pressure Transmitter 4-20mA Current Output Calculator
  • Pressure Transducer 1-5V Voltage Output Calculator
  • Pressure Converter
  • Measurement Reading to 4-20mA Signal Converter
  • Liquid Depth/Level to Hydrostatic Pressure Calculator
  • Input to Output Measurement Reading Converter
  • Force & Area to Pressure Calculator
  • Density to Specific Gravity Calculator

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