A transmitter is an electronic device that would convert the process variables such as frequency, pressure, temperature, and flow to a 4-20mA signal. So this device would convert the sensor input signals to a control signal which is capable to regulate the current flow. The 2,3, and 4 wire transmitters play a major role in the 4-20mA signal transmission system. The output of the transmitter would be current. In the transmitter, there won’t be any degradation of the signal output with distance. So the transmitters are capable to do long-distance signal transmission. Mostly the transmitter current loop won’t be affected by the noise. The transmitter would convert the value of the measured variable to a standard signal and this can be easily sent to other devices, a transmitter can be described as a combination of the transducer, amplifier, and signal conditioning.
What are the major functions of a transmitter?
In an industrial process, the sensor would transmit a physical signal like pressure or temperature and this signal will be converted to an electrical signal. The electrical signal will be transmitted over a long distance without any signal loss to the controller. So a transmitter does the transduction, amplification, and also signal conditioning. A transmitter would sense the process variable with the help of a sensor and the output value of the transmitter would vary as a predetermined function of the process variable.
Why analog signals are used in transmitters and what is an analog signal in instrumentation?
An Analog signal is an electrical signal and its current or voltage magnitude corresponds to sensors inputs such as temperature, flow, etc. The 4-20mA current signal is the most common way of transmitting the analog signal. So in this transmission, the 4mA would represent the lowest measured value and the 20mA represents the highest measured value.
Click here to know why the 4-20mA signal is used for analog transmission
What are the types of transmitters used in process instrumentation?
Two types of transmitters are used in process instrumentation
Pneumatic and electronic transmitters
Electronic transmitters are classified into analog or digital transmitters
Why pneumatic transmitters were replaced by electronic transmitters in process instrumentation?
| SL NO | PNEUMATIC TRANSMITTERS | ELECTRONIC TRANSMITTERS |
| 1 | A lot of moving parts | Less number of moving parts |
| 2 | Slow response | Quick response |
| 3 | Pressure signals are used for transmission | Electrical signals are used for transmission |
| 4 | Not suitable for long-distance transmission | Suitable for long-distance transmission |
| 5 | There will be mechanical vibrations and noise | No mechanical vibrations and noise |
| 6 | Low accuracy | Good accuracy |
| 7 | Frequent maintenance would be needed | Low maintenance |
| 8 | Non-linear and non-repeatable | Good linearity and repeatability |
| 9 | It does have external leakage and hysteresis | Doesn’t have any external leakage or hysteresis |
What are 2-wire, 3-wire, and 4-wire transmitters?
Transmitter configuration
Two-wire transmitter
The 2 – wire transmitter would receive the power and transmit data signal through the same pair of cables. These two wires are used to power the transmitter and also to transmit the output signal.
Two-wire transmission is only possible in the current transmitters. These transmitters are energized by loop current, these transmitters depend upon the loop current from the receiving device. This transmitter would convert the process signals to 4-20 mA DC so that this signal can be transmitted without any loss. The loop current would energize the two-wire transmitter and there won’t be any external power source required for this process. So the two-wire transmitter is dependent on the loop power.
Advantages of two-wire transmitters
- The two-wire system only needs half the wire runs for their operation
- It doesn’t need a local power source
Disadvantages of two-wire transmitters
- Low impedance capability is the major limitation of the two-wire transmitter
- The type and number of devices which can be driven by the system and also the distance is limited
Three-wire transmitter
The 3 -wire transmitter would transmit the data signal and the power with respect to the common ground. The three-wire transmitters are energized by the supply voltage in the transmitter and the transmitter would source the loop current. The receiver common is connected to the transmitter common. In this transmitter, the current loop can also be operated on a measuring instrument that has high input impedance. The three-wire arrangement is not widely used but it would deliver more power to the module electronics.
Advantages of three-wire transmitter
- Higher accuracy than the two-wire transmitter
- 24v DC power supply; Effective for different power problems
- It can use higher Ohmic loads
Four-wire transmitter
In the case of a 4- wire transmitter, two wires are used for the data signal and two wires for the power supply. This is the simplest form of the 4-20mA measurement loop in which the transmitter has two terminals for the 4-20mA signal wires to connect and it also has two more terminals where the power source will be connected. In this arrangement the current signal from the transmitter would connect the process variable input terminals of the controller to complete the loop. 4 wire transmitters are powered by the external power supply at the transmitters.
Advantages of 4 wire transmitter
- These transmitters have higher signal side impedance and due to this it has better loading and signal-transmission distance characteristics
Disadvantages of 4 wire transmitters
- More wires are used in this
Comparison between 2-wire, 3-wire, and 4-wire transmitters
| SL NO | 2- Wire transmitters | 3 – wire transmitters | 4 – Wire transmitters |
| 1 | Energized by the loop current | Energized by the supply voltage in the transmitter | Powered by external power source |
| 2 | Less number of wire runs are needed when compared to the other two types | Less number of wire runs are used than 4 wire type but it has more number of wire runs than the two-wire type | More wires are used when compared to the other two types |
| 3 | Not suitable for long-distance transmission | Transmission distance is better than 2 -wire type | Suitable for long-distance |
| 4 | Low accuracy when compared to the other two types | Higher accuracy than the 2 -wire transmitter | Good accuracy than the other two transmitters |
| 5 | 2 wires are used for both the signal and the power supply | 3 wires are used commonly for the signal and the power supply | 2 wires are used for the data signal and the other two wires are used for the power supply |
| 6 | Signal and power supply are isolated | Signal and power supply are not isolated | Signal and power supply are isolated |
What are the advantages of using current as an electric signal for transmission?
- We can connect many loads at a different location in series, to a specified limit on the total resistance while the transmitter behaves as a current source
- The resistance and length of the transmission circuit won’t affect the sensitivity of the signal
- Electrical signals would travel a greater distance with a low time lag
- Multiple input signals can be handled
- These signals will be compatible with the digital computer
