Saturday, October 18, 2014

2-Wire and 4-Wire Transmitters and Control Loops

field transmitter
Digital Meters
Industrial process control transmitters commonly provide analog signals, such as 4-20 milliamps (mA), 1-5 volts DC, and 10-50 milliamps as outputs which can be scaled to the control range of the process variable they are sensing. Industrial transmitters are either “4-wire” or “2-wire” which defines how the transmitter gets its supply or “excitation” voltage.

4-wire transmitters have four wires exiting the device. Two wires are for the supply power of the device, typically 120 volts AC, 240 volts AC or and external 24 volts DC supply. The other two wires provide the analog output signal provided by the transmitter circuitry.

loop power
2-wire, loop power example
2-wire transmitters have only two wires exiting the device and rely upon the control “loop” for the excitation voltage - typically 24 volts DC which normally comes from the loop controller, PLC or DCS.

4-wire devices are also classified as “active” (supplying power) devices, while 2-wire devices are classified as “passive” (loop powered) devices.

For example, a digital meter (active) may provide loop power to a pressure transmitter. The pressure transmitter regulates the current on the loop to send the signal back to the digital meter, but since the transmitter does not provide power to the control loop, it is deemed passive. In another example, a passive (loop powered) digital meter and a passive pressure transmitter may be used in the same loop, but uses a 24 V battery as the active device to power the loop.