What is a Transmitter?
A transmitter is a device that converts a small signal from a sensor into a signal that represents the variable being measured.
What is 4 wire transmitters?
A 4-wire transmitter is powered by either a 110V or a 220V power supply. This enables the direct activation of relays, pumps, solenoids, and other devices. However, flammable or combustible vapors, fumes, or dust are present in a hazardous environment and may ignite under specific conditions.
A “4 wire” transmitter has two pairs: one pair used to transmit 4-20 ma signal and a separate pair to carry the power (typically either 24 VDC or 120 VAC).
How to wire 4 wire transmitters?
4-wire transmitters are referred to as self-powered instruments because it has their own internal power supply. 4-wire transmitters do not require connection to the DC supply. The receiving end is connected to the power supply of 120V AC. These are typically employed when an instrument is added to the DC supply load.
A 250-load resistor is applied in the image below. In most instrumentation systems, process controllers are not equipped to handle milliamp input signals directly, but rather voltage signals. As a result, a precision resistor is installed across the controllers’ input terminals to transform current signals from transmitters into conventional analog voltage signals that the controllers can understand.
1 to 5-volt voltage signals are considered as standard however in some controllers use various voltage ranges and it requires different precision resistor values. The precision resistor value must be 250 ohms if the voltage range is between 1-5 volts and 4-20 mA current range.
A voltage signal range of 1-5 V and a standard current signal of 4-20 mA are considered in the transmitter wire configurations detailed above.
Wiring Diagram of 4 wire transmitter
Difference between 3 wire and 4 wire transmitters
| 3 wire Transmitter | 4 wire Transmitter |
|---|---|
| 0 V DC lines for both receiver and transmitter | Uses separate 24 DC power supply |
| 4-20 mA is supplied to the PLC by transmitter | 4-20 flows through different cables |
| It doesn’t require additional cables for power supply | It requires additional cables for power supply |
| Less wiring requirement | More wiring requirement |
| It can’t supply power to LEDs and relays | It can supply power to LEDs and relays |
| Less power consumption | High power consumption |
| Supply isolated from output | Supply not isolated from output |
Advantages and Disadvantages of 4 wire transmitter
Advantages
- It can accommodate more power
- Easy to understand
- No voltage drops
- Can power easily by plugging to an electric socket or from a battery
Disadvantages
- Expensive
- Difficult to install
- Difficult to troubleshoot and maintenance
- In contrast to two-wire connections, four-wire connections necessitate a separate power supply for the device.
Advantages and Disadvantages of 3 wire transmitter
Advantages
- Less expensive
- Easy to install
Disadvantages
- Can’t be powered by AC (Alternating Current)
- Due to electrical interference, this arrangement may create erroneous warnings.