Today’s electronic transmitters including pressure transmitters are connected in different wire configurations. These connection methods are of great concern to the instrument engineer/technician. The ‘two wire, three wire and four wire classifications are often used to describe the method of connection of electronic transmitters:
Two wire transmitters:
These are the simplest and most economical and should be used wherever load conditions will permit. They are often called loop powered instruments. In a two wire system, the only source of power to the transmitter is from the signal loop. The 4 mA zero-end current is sufficient to drive the internal circuitry of the transmitter and the current from 4 to 20 mA represents the range of the measured process variable. The power supply and the instruments are usually mounted in the control room. The schematic diagram below shows the wire transmitter configuration:
Three wire transmitters:
Some transmitters require more power than the signal loop ( 4-20 mA. etc) can supply their internal circuitry. A DC common wire is run from the instrument to the transmitter. This permits the transmitter to draw whatever power it needs from the power supply and produce the desired signal current at the transmitter output. The schematic diagram for a three wire transmitter is shown below:
Four wire transmitters:
Four wire transmitters have their own internal power supply hence they are often referred to as self-powered instruments. They require no connection to the DC power supply. A 120 Vac sources is connected only to the receiving instrument. These are often used where an instrument is added to the load of the DC supplies. The disadvantage is the need for AC power at the instrument site. Below is shown the wire configuration of a four wire transmitter:
Note that in all the transmitter wire configurations shown above, a load resistor of 250Ω is used. Usually process controllers used in instrumentation systems are not equipped to directly accept milliamp input signals, but rather voltage signals. For this reason a precision resistor is connected across the input terminals of the controllers to convert current signals from transmitters into standardized analog voltage signals that controllers can understand.
A voltage signal range of 1 to 5 volts is standard, although some models of controllers use different voltage ranges and therefore require different precision resistor values. If the voltage range is 1-5 volts and the current range is 4-20 mA, the precision resistor value must be 250 ohms.
In the transmitter wire configurations discussed above, it is assumed that a voltage signal range of 1-5V and a standard current signal of 4-20mA are used.