Why Use a Current Loop? Many process sensors are voltage output devices.
From basic electronics, we know that attempting to transmit a voltage over long distances has numerous drawbacks - losses due to wiring and interconnect resistance will produce lower voltages at the receiving end.
Using devices with high impedance inputs to reduce loading is not practical because of the dramatic increase in noise sensitivity.
Shielded wires could be used to minimize noise pickup, but this gets costly when long distances are involved.
Industrial process monitoring applications traditionally solved this problem using current loops to send sensor data.
Cumulative loop drops will not reduce the accuracy of a current signal until the transmitter has reached its maximum output voltage.
Loop operation is straightforward.
A sensors output voltage is converted to a 0 to 20 mA (milliamps) proportional current.
Four milliamps represents the sensors zero-level output, and twenty milliamps represents the sensors full-scale output.
A receiver at the remote end converts the current back into a voltage that is available for further processing.
The live-zero represented by four milliamps allows the receiving instrument to detect wiring failures and permits transmitters to be loop powered (a.
k.
a.
two-wire transmitters) Typical Current Loop Components A typical 4-20mA current-loop circuit combines a sensor/transducer, a transmitter, a loop power supply and a receiver/monitor.
In loop powered applications, all four elements are connected in a single series circuit.
Sensors provide an output voltage representing the measured parameter.
The transmitter amplifies and conditions the sensors output then converts this voltage to a proportional 4-20mA dc current.
The receiver/monitor converts the current signal back into a voltage that can be further processed and/or displayed.
In loop-powered applications, the power supply's negative furnishes a path for closing the series loop.
24V is still the most widely used power supply voltage in process monitoring applications.
Lower supply voltages, such as 12V, are popular in specialized applications ( fire, safety, security and computer-based DAQ systems).
Burden or Loop Drop A process monitor presents a load or loop burden to a transmitter's output driver.
Most data sheets will specify a maximum loop resistance the transmitter can drive while providing a full-scale 20mA output.
Given that the voltage drop developed across a current carrying resistor can be found by multiplying the resistor's value by the current passing through it and that the sum of the voltage drops around a series loop has to be equal to the supply voltage.
To calculate the loop drop: Vd (voltage drop) = I (current through the resistor in amperes) x R (value in Ohms).
Every component through which the current passes will develop a maximum voltage drop equal to that component's resistance multiplied by the maximum current or 0.
020 Amperes (20mA).
For example, a loop powered monitor with a 249 ohm resistance yields a maximum voltage drop: Vd = 0.
020A x 249 ohms = 4.
98V Determining Transmitter Power Dissipation Transmitter specifications usually provide both minimum and maximum operating voltages.
The minimum voltage is that required to ensure proper transmitter operation (typically 8V), while the maximum voltage is determined by its maximum rated power dissipation.
A transmitter's power dissipation can be determined by multiplying its maximum over range loop drop by the largest expected output current.
For example, if a transmitter drops 24V at an over range output of 30mA, its power dissipation is: 24V x 0.
030A = 0.
72 watts Losses Due to Wiring Resistance Copper wires exhibit a dc resistance proportional to their length and diameter.
Applications in which two or more devices are connected over long wiring distances (1000 - 2000 feet) normally use 24V supplies.
The voltage drop developed along a given length of wire is found by multiplying the wire's total resistance by the current passing through it.
A wire's resistance can be found by looking up its resistance (ohms per 1000 feet) in a wire spec table.
For example, assume a transmitter is connected to a remote process monitor using 1000 feet (330 meters) of 26 AWG solid copper wire (40.
8 ohms per 1000 feet).
Since the current must travel 1000 feet to the process monitor and another 1000 feet back to the transmitter the total loop resistance (R) is equal to: 2000 feet x (40.
8 ohms /1000 feet) = 81.
6 ohms.
The total voltage dropped over the 2000 feet of wiring is therefore, Vd = 0.
020A x 81.
6 ohms.
Therefore, the wire drop = 1.
63V.
Total Loop Power Supply Requirement Since the process monitor has a loop drop of 249 Ohms x 20mA = 4.
98V, the total loop drop seen by the transmitter will be the sum of the 1.
63V wire drop and the 4.
98V process monitor drop, for a total of 6.
61V.
If the transmitter requires a minimum of 8V for normal operation, the system will require at least 6.
61V + 8V = 14.
61V.
A standard 24V loop supply would be more than adequate.