Feature: Sensors
It is very important that the loop load resistor power rating is sufficient to ensure that any heating caused by current flowing through it won’t change its value and thereby the voltage developed across it
elimination, or for operation in hazardous locations.
• Derivatives of four-wire transmitters such as loop isolators or current loop repeaters. In some cases these devices are incorporated into national-agency-approved safety barriers for intrinsically-safe (IS) systems that can be safely operated in specific hazardous environments.
Two-wire current loop powered transmitters Two-wire loop-powered transmitters are devices that can be connected in a current loop without a separate or independent power source, since they get their power from the loop current. Typical loop-powered devices include sensors, transducers, transmitters, repeaters, isolators, meters, recorders, indicators, data loggers, monitors and many types of field instruments. Loop-powered devices are important because for some systems
it is difficult to supply separate power to all the devices and instruments in the loop. Te device might be located in an enclosure where access is difficult, or in a hazardous location where power cannot be allowed or must be limited. Te two-wire device in Figure 1 is connected to a current loop,
and so is considered a current-sinking device. Te power to drive it is supplied entirely by the unused current below 4mA in the loop. Two-wire loop powered transmitters are popular, but usually more costly than three-wire.
Three-wire current transmitters Tree-wire transmitters are different from loop-powered transmitters because their loop current is developed from a DC power supply that makes more than just the loop current. Te entire transmitter operates from this supply and may consume much more current than typical two-wire loop-powered devices. However, a three-wire system is a sourcing element, so it supplies the current loop in additon to what it uses itself; see Figure 3. It is important to note that a three-wire transmitter should never
be connected to a two-wire loop powered system. Notice the high side of the power supply is not directly connected
to the loop, but that its return side is connected via a grounded point, so a three-wire transmitter requires careful consideration of
grounding issues to prevent potential ground loops. If an application using a three-wire transmitter requires isolation
in the loop, there are several paths to follow. One is to use a separate DC power supply for each three-wire loop output device, so there is no interaction with other current loops. Another way is to use a loop isolator module. These devices use various methods to achieve galvanic isolation, typically with transformers or optical couplers. They accept a 4-20mA signal, function as a repeater or re-transmitter, and deliver a reconstituted 4-20mA current loop signal that is fully isolated. A third way is to use a four-wire transmitter, which has isolation already built in.
Four-wire current transmitters Four-wire transmitters offer the current sourcing advantages of a three-wire device, but also provide galvanic isolation for the current loop output. Four-wire devices are substantially more expensive than
three-wire devices, so they are generally used where the isolation is needed, or they are part of a combination device with an approved safety barrier for current loop operation in a specific hazardous location. A four-wire transmitter block diagram is shown in Figure 4.
Note that the four-wire device itself uses a separate DC power supply for operation, just like a three-wire transmitter, and supplies loop current that way.
Review of 4-20mA data transmission It is useful to conclude with a summary of the features and benefits of the 4-20mA data transmission process, as well as its limitations. Advantages include: • The 4-20mA current loop is recognised as the simplest analogue data transmission method to connect and configure, and is the dominant standard in many industries.
• It uses less wiring and connections than other methods, greatly reducing startup and setup costs.
• It is superior over long distances, since current unlike voltage won’t diminish over long connections.
• It is relatively insensitive to most electrical noise and related electromagnetic interference.
• It allows both local and remote readout or monitoring devices to be inserted into the loop.
• It is easy to detect a fault in the system because 4mA is equal to 0% system output, so any loop current much lower than 4mA is a direct indicator of a loop fault.
Limitations include: • 4-20mA current loops can only transmit one specific sensor or process signal per loop.
• Multiple loops are required for applications with many sensor or process outputs. • A lot of field wiring will be needed, which can lead to serious issues with ground loops if the current loops are not properly isolated from each other.
www.electronicsworld.co.uk April 2021 33
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44
