Question

15. On the diagram below, show The connections for a 4-20mA transmission signal to allow a loop powered transmitter to give a controller a measurement signal a. b. Where you would connect a meter to measure the signal produced by the transmitter Xmtr 24Vdc Controll

Answer 1

a. 4-20mA is robust and most popular sensor signalling s normally a 2 wire current loop and in order to operate it needs to consume some of the current , and this is provided by the minimum current of 4mA.

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

The main advantages of the 4–20mA current control loop are the accuracy of the signal is not affected by the voltage drop in the loop,

To help us to understand the operation of a two-wire current loop, let’s consider each element of the current loop separately-

We have 2 wire,3 wire and 4 wire connections.I have taken the above connection for 2 wire connection only.

There are four basic elements -

1.Power supply

2.The 2 wire transmitter(4-10mA)

3.The receiver (Of Resistance R)

4.Loop Wires

1.Power Supply-

The current loop uses DC power because the magnitude of the current represents the signal level that is being transmitted. For 4-20mA current loops with 2-wire transmitters, the most convinient loop power supply voltage is 24V DC.But one thing keep in mind, the power supply must be set to a level that is greater than the sum of the minimum voltage required to operate the Transmitter, plus the IR drop in the Receiver.

2.Transmitter-

This device transmits data from a sensor over the two-wire current loop. It acts like a variable resistor with respect to its input signal and is the key to the 4-20mA signal transmission system. It is the transmitter that converts the real world signal, such as flow, speed, position, level, temperature, humidity, pressure, etc., into the control signal necessary to regulate the flow of current in the current loop.

3.Receiver-

This is the device at the other end of the transmission line that receives the transmitted signal. We often use a resistor to represent the Receiver in a 2-wire current loop since its easy to measure voltage than current.Most of the Receiver devices, loop current is commonly passed through a resistor to produce a voltage for measurement by the device. hence a simplest receiver can be represented by a load resistance connected in parallel with a voltmeter.   

Depending on the source of current for the loop, receiver devices may be classified as active (supplying power), or passive (relying on loop power).

Generally  The Receiver integrates different devices, such as: a panel meter, actuator valve, motor speed control, a PLC (Programmable Logic Controller), or other DCS (Digital Control System) etc.

4.The loop wires-

Connections between the elements, or the wire used to wire the elements together.

Make sure the loop supply voltage is always large enough to drive the minimum transmitter voltage Vt, plus the IR voltage drop in the wire, plus the IR voltage drop in the receiver, and at maximum loop current as shown in below fig.

b.The 4 mA to 20 mA current loop is a common method of transmitting sensor information in many industrial process monitoring applications especially in systems monitoring pressure, temperature, pH, flow, or other physical factors.

These systems employ a two-wire, 4 mA to 20 mA current loop.

To measure a 4-20 mA loop signal with multimeter - locate the mA signal wire, disconnect one lead of the signal wire and put the meter in series with the mA signal wire and view the mA measurement generally done for calliberation.

Current loops are ideal for industrial environments where long cable lengths are required or large amounts of electromagnetic interference exist between the transducer and data acquisition device.