What is a two-wire transmitter?
When a fixed DC voltage is applied to a resistive load, a current is consumed from the voltage supply that is proportional to the size of the load. If the load is increased, less current is drawn from the voltage supply. If the load is decreased, more current is drawn from the voltage supply. In this way, the two-wire transmitter controls the current flow (4-20 mA) from the voltage supply. A minimum current flow of 4 mA is required to actually power the instrument. Therefore, a two-wire transmitter (or “loop-powered” transmitter) operates on only two electrical connecting wires. The transmitter sends out a 4-20 mA current signal proportional to the measured input through the same two wires that are used to provide power. Two-wire transmitters are ideal for remote locations where 115/230 VAC is unavailable, since only low-voltage, inexpensive, two-conductor cable needs to be run to the instrument.
A two-wire transmitter is a transmitter that needs only two wires to be run from the control system out to the transmitter. These two wires carry both the power, as well as the signal information. The output signals from two-wire transmitters can be analog, typically 4-20mA. They can also be digital such as Fieldbus, or a combination of analog and digital, such as HART. Generally two-wire transmitters run on lower power levels than three or four-wire transmitters. They have the added advantage of needing only two wires run the long distances out to the field, which results in lower installation costs.
