Process Transmitters Voltage and Power Requirements in a Two-Wire 4 – 20mA Current Loop ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

Process Transmitters Voltage and Power Requirements in a Two-Wire 4 – 20mA Current Loop

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In two-wire 4-20mA control loops which are the most popular these days, the 2-wire transmitters convert various process signals representing flow, level, temperature, pressure, etc., to 4-20mA DC current for the purpose of transmitting the signal over some distance with little or no loss of signal.

Relationship between the Components in the 4 – 20mA Control Loop
There are three key components in the 4 – 20mA loop as shown below:

They are:

(1) The power supply
(2) The Transmitter
(3) The Receiver or load
                             Read  basics of the 4 – 20mA Loop
Power Supply
The 4 – 20mA current loop is supplied power by a DC power supply. 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, and for long transmission distances, the IR drop in the wire. Calculating this drop must consider the maximum level of current that can flow in the 4-20mA current loop and not 20mA, but the over-scale or alarm limit of the transmitter

 A transmitter is the device used to transmit data from a sensor over the two-wire current loop. There can be only one Transmitter output in any current loop. The transmitter converts the real world signal, such as flow, level, temperature, pressure, etc., into the control signal necessary to regulate the flow of current in the current loop. The level of loop current is adjusted by the transmitter to be proportional to the actual sensor input signal.

This is the device that receives the transmitted signal. In a 4-20mA process loop, the Receiver could be located thousands of feet from the transmitter. Because it is much easier to measure voltage than current, we often use a resistor to represent the Receiver in a 2-wire current loop. This resistor can be a physical resistor, or simply the input impedance of the receiver channel. The receiver could be any number of different devices, such as: a panel meter, actuator valve, motor speed control, a PLC (Programmable Logic Controller), or other DCS (Digital Control System)

Transmitter Voltage Requirements
The transmitter typically uses 4mA output to represent the calibrated zero input or 0%, and 20mA output to represent a calibrated full-scale input signal or 100%. Because the lowest level of the transmission signal is 4mA or less, the transmitter must be able to operate from less than 4mA. Consequently a two-wire transmitter will have minimum and maximum output voltage ratings that must be observed. The lower voltage is the minimum voltage required to operate the transmitter, and the maximum transmitter voltage is the highest voltage it can withstand.

4 – 20mA Current Loop Supply Voltage Limitations
For the 4 – 20mA loop to operate correctly without problems, the loop supply voltage should be large enough to drive the minimum transmitter voltage Vtmin, plus the IRw voltage drop in the wire, plus the IRcv voltage drop in the receiver, and at maximum loop current.

In the loop diagram below, the simplified current loop has been modified to account for loop wire resistance by depicting this resistance as a single series-connected resistor Rw in the loop. The length of the cable, which is usually shielded twisted pair in many installations, must account for the resistance of
both wires combined, or the resistance of the “round-trip” length of the two-wire loop:

In the above circuit,
Vs = Vt + Vw + VR                              

    = Vt + IRw + IRcv

For the current loop to operate correctly without roll off effect on the receiver ,

Vs > Vtmin + Vw + VR

Where Vtmin is the minimum voltage required to operate the transmitter.

Operating Region of a Transmitter
The maximum loop resistance is determined by the voltage level of the external supply.  Every transmitter has an operating region which makes effective and efficient operation of the transmitter possible. In practice, the loop resistance or number of connected loads in the current loop should be limited if the transmitter must operate with the correct voltage for a given power supply voltage. The operating voltage and load range of a typical transmitter is given in the chart below:
As can be seen above:
Vmin = Minimum operating Voltage of Transmitter
Vs   = External Supply Voltage to Transmitter Loop
Vmax = Maximum operating Voltage of Transmitter
Rmax = Maximum loop resistance
Rmin = Minimum loop resistance
RL   = Loop resistance at External Supply Voltage, Vs

For a standard 4 – 20mA Rosemount transmitter, with a minimum and maximum operating voltage of 10.5V and 55V at no load, the maximum loop resistance is given by:
                  Maximum Loop Resistance = 43.5 (Power Supply Voltage – 10.5)