4-20 mA Transmitter Wiring
4-20 mA Transmitter Wiring Types: 2-Wire, 3-Wire, 4-Wire
In this article, we’re going to take a close look at the 2-wire, 3-Wire and 4-wire transmitter and discuss where they are used and why?
We’re also going explore transmitter wiring configurations and how transmitters are connected to a PLC.
What is a Transmitter?
A transmitter is a device that sense signals or data in and translates them into a form that can be transmitted, usually over a distance.
Transmitters are available with a wide variety of signal outputs. The 4-20mA analogue signal is so far the most commonly used in industrial applications. Several physical 4-20mA wiring options exist.Industrial transmitters are available for monitoring many parameters these including pressure, temperature and flow etc. Gas detectors / transmitters offer 4-20mA outputs, where 4 mA equates to a zero reading and 20 mA equates a full scale reading of the calibrated range.This signal is sent to a remotely located control panel. The control panel uses this signal and activates executive actions via relay contacts, e.g. audible and visual alarms or initiate some trips or even plant shut down procedures.Transmitters come in all different shapes and sizes and connect with several types of sensors.The transmitter output signal representing the variable being measured can be voltage or current.
Transmitter analog output loops
A transmitter analog output loop contains the transmitter, power supply, and the receiving device which could be a PLC or DCS.
Just like any other instrument, transmitters need a power supply to operate. But, is it a 2-wire 3-wire and 4-wire transmitter? 4-20 mA Transmitter Wiring
Defining 2, 3, and 4 Wire Connections
All devices in a 4-20 mA current loop need to be supplied power from somewhere in order to function. Two-wire devices receive their power from the process signal loop itself. The power for the loop usually comes from the transmitter power supply or some other kind of external power supply, and all of the power for the system travels through the wires that also carry the signal. Since this setup only requires two wires, loop-powered instruments are also referred to as two-wire devices.
- Simple and easy display for 4-20 mA transmitter
- Low cost solution for display
- Easy Agency approvals(safety purpose)
- Local power not required
- Limited output options
- Does not support relays
- Does not support LED displays
The simplest form of 4-20 mA measurement loop is one where the transmitter has two terminals for the 4-20 mA signal wires to connect, and two more terminals where a power source connects. These transmitters are called “4-wire” or self-powered. The current signal from the transmitter connects to the process variable input terminals of the PLC to complete the loop.
The power supply can be AC or DC depending upon the model.
Advantages of 4-wire transmitter
- More capabilities than 2 wire (relays, LEDs, serial communications)
- Easier to understand the wiring
- No need to worry about voltage drop
- Excellent isolation (power from input/outputs)
Disadvantage of 4 wire transmitter
- Requires a separate local power supply
- Generally more expensive
- More wiring requirements
- Limited hazardous area options
A three-wire connection is essentially the same as a four-wire connection except that the isolation just discussed is not present; a three-wire device does not float in comparison to the current loop. In a three-wire connection, the process signal return from the device and the common of the power supply are a shared connection.
- Lower cost than 4 wire
- Easier to wire (fewer connections)
- No isolation, very susceptible to ground loops
- May be confusing to wire.