4-20mA transmitter - basic

There are several devices used in the industry, which convert mechanical quantities into electrical quantities. The mechanical magnitudes can be converted into the most varied forms, or electrical magnitudes, which we can mention:

  • Voltage Signals
  • Current signs
  • Power Signals
  • Resistivity Measures
  • Others

In a closed loop control system, the input of this corresponds to the value of the variable to be controlled, be it temperature, level, speed. The output will be the actuator that affects the state of the variable being controlling, and as feedback it will be common to find sensors, which take the current state of the variable to be controlled in the system. The analog signals from these sensors are susceptible to noise, so they can not transmit their signal at remote distances to some controller, PLC, this is solved by placing current loop transmitters 4-20 mA.

The standard 4-20mA current loop has its origin in -The American national Standard Institute (ANSI) and The Instrumentations Systems (ISA)

Why 4-20 ma signal is used in Industrial instrumentation?

The 4-20mA transmitter:

4-20 mA Current loop components:

  • Sensor / transducer: The sensors provide a voltage output whose value represents the variable physical that is measured example a thermocouple is a sensor type that provides a level very low voltage to its outputs, which can be proportional to the temperature at the one that is submitted.

  • Transmitter: The transmitter amplifies and conditions the sensor output, then converts the voltage to a direct current level in a range of 4-20 mA that circulates in series through a closed loop. The output on the transmitter is current and is proportional to the physical variable.4 mA will be one end of the physical variable and another 20 mA the other opposite end.

  • The receiver/monitor: that converts the current between 4-20mA back to voltage, which may be used in some subsequent process.

  • The loop power supply: usually provides any energy requirement to both the transmitter and the receiver. A source of 24 Volt is widely used in 4-20mA monitoring applications. As well There are 12 volt power supply for computers

The 4-20mA transmitter is a current loop which simplified with an ideal Norton current source composed of Signal, and R signal models the 4-20 mA transmitter.

The line resistance is shown as Rline, and Vnoise represents randomly induced loop noise. In this example, a 500-ohm controller and a 250-ohm digital display are connected in series with the signal current. The circuit is powered by a DC supply of 24 volts.

Signal voltage at any load is (I signal * R load), which is independent of supply voltage variations and line resistance (Rline).

Types of transmitters:

Type 2:

They are transmitters energized by the current loop, where the voltage of Power is included in the receiver. The transmitter is floating and the earth is found in the receiver.

Type 3:

They correspond to transmitters of 3 cables fed by a source of voltage in the transmitter. For this case, the transmitter is the source for the current loop. The common transmitter is connected to the common receiver.

Type 4:

They correspond to 4-wire transmitters, energized by a voltage source in the transmitter. The transmitter feeds the current loop and the receiver behaves like a floating load.

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