Current transducer Categories

Two-wire, three-wire and four-wire, these terms do not only refer to the wiring method of the transducer, but also describe the differences in the principles and structures of the transmitter that outputs analog DC current signals. Next, we will explore their definitions and characteristics one by one.

Two-wire transducer 
This type of transmitter transmits power and signal simultaneously with two wires. Its characteristic is that the load output by the sensor is connected in series with the power supply. This means that the power supply needs to be introduced from the outside and connected in series with the load to drive it to work.

One-wire transducer

Different from the two-wire system, the positive terminal of the power supply of the three-wire sensor is separated from the positive terminal of the signal output, but they are connected to a COM terminal. This design improves the flexibility of the circuit to a certain extent.

Two-wire transducer

more complex wiring method is adopted , in which the power supply uses two wires and the signal uses another two wires. This design enables the power supply and signal to work independently, thus providing higher stability and flexibility.

The working principle of the two-wire system, three-wire system and four-wire system
After the electronic amplifier was widely used in instruments, the term of several-wire system appeared, which was mainly to distinguish transmitters with different wiring methods. Initially, the four-wire transmitter dominated, in which two wires were dedicated to power supply and the other two wires were responsible for transmitting converted and amplified signals such as voltage or current. However, today, the two-wire transmitter has become the mainstream.

Next, we will delve into the specific differences between different wire transducer.

3. Working principle of two-wire transducer

To construct a two-wire transmitter, certain conditions must be met. This transmitter only requires two wires, one for transmitting the converted and amplified signal, and the other for both power and signal. The key is that these two wires provide both power and signal, thus simplifying the wiring method.

1.V≤Emin-ImaxRLmaxThe

output voltage V of the transmitter, that is, the converted and amplified signal voltage, is affected by multiple factors. In practical applications, this voltage must meet certain conditions, that is, less than or equal to the specified minimum power supply voltage minus the voltage drop caused by the current on the load resistance and the transmission wire resistance. Such a design ensures the stability and reliability of the transmitter in the two-wire working mode.

 

2.I≤Imin
The output current I of the transmitter, that is, the current provided by the transmitter during actual operation, must be less than or equal to its maximum output current Imin. Such a limit ensures the stable operation of the transmitter and prevents possible overload conditions.

 

3.P＜Imin(Emin-IminRLmax)
The minimum power consumption P of the transmitter must be less than the value specified by the above formula, usually not more than 90mW.

1. The parameters involved in this formula include: Emin (minimum power supply voltage, the Emin value of most instruments is 95% of 24V, that is, 22.8V, taking into account the 5% negative change); Imax (maximum output current, usually 20mA); Imin (minimum output current, usually 4mA); and RLmax (maximum load resistance, usually 250Ω plus the resistance of the transmission wire).
2. When the transmitter meets the above three conditions, two-wire transmission can be achieved. This transmission method connects the power supply and the load in series and shares a common point. Only two wires are required for signal communication and power supply between the field transmitter and the control room instrument, and these two wires also play the role of power line and signal line.
3. There are many advantages to using a two-wire transmitter. First, its signal starting current is 4mA DC, which provides a stable static working current for the transmitter. In addition, the instrument’s electrical zero point is also set to 4mA DC, which does not coincide with the mechanical zero point. This design makes the “live zero point” technology easier to identify faults such as power failure and wire breakage. At the same time, the two-wire system also simplifies the use of safety barriers, thereby enhancing safety and explosion-proof performance.

4.Two-wire transducerwiring diagram

 

The two-wire transmitter is powered by 24V DC, its output signal range is 4-20mA DC, and it is equipped with a 250Ω load resistor. In the wiring diagram, the negative line potential of the 24V power supply is the lowest, so it is used as the signal common line. In addition, for smart transmitters, the FSK keying signal of the HART protocol can also be superimposed on this signal to achieve more advanced functions.

 

Four, three-wire system

In order to further reduce the size and weight of the transmitter, while enhancing its anti-interference ability and simplifying the wiring method, some instrument manufacturers choose to convert the transmitter power supply from 220V AC to low-voltage DC power supply. For example, they may get power from a 24V DC power box. This low-voltage power supply method makes it possible to share the negative line, thus giving birth to three-wire transmitter products.

Schematic diagram of three-wire transducer wiring

The three-wire transmitter, as shown in the figure, is characterized by the fact that the positive end of the power supply only needs one wire, the positive end of the signal output also uses one wire, and the negative end of the power supply and the negative end of the signal share one wire. This type of transmitter is usually powered by 24V DC, its output signal is 4-20mA DC, and is equipped with a load resistor of 250Ω or 0-1.5KΩ. In addition, some transmitters also provide mA and mV signals, but the specific value of the load resistor or input resistor will vary depending on the output circuit.

 

Five, four-wire system

In control system applications, in order to achieve unified signal connection, it is usually required that instruments with non-electric unit combinations, such as online analyzers, mechanical meters, and power meters, can use 4-20mA DC signal system for output. However, since the conversion circuits of these instruments are often more complex and have higher power consumption, it is difficult to fully meet the design standards of two-wire transmitters. Therefore, in order to meet the output requirements of 4-20mA DC signal system and avoid the limitations of two-wire system, we use an external power supply to design a four-wire transducer.

 

Schematic diagram of four-wire transducer wiring

The power supply of the four-wire transducer is usually 220V AC, and some are powered by 24V DC. The output signal can be selected as 4-20mA DC, in which case the load resistance should be 250Ω; or 0-10mA DC, corresponding to a load resistance of 0-1.5KΩ. In addition, some transmitters also provide mA and mV signals, but the specific value of the load resistance or input resistance will vary depending on the output circuit.

 

In the above three wiring methods, the current signal is input to the receiving instrument. If the resistor RL is connected in parallel, the received signal will be converted into a voltage signal. It can be seen that due to the differences in the working principles and structures of various transmitters, different products are produced, which in turn determines the two-wire, three-wire and four-wire wiring methods of the transducer.

 

For users, when selecting a model, they need to comprehensively consider the actual situation of the unit, such as the uniformity of the signal system, explosion-proof requirements, the capacity of the receiving equipment and investment. It is worth noting that the 4-20mA DC signal output by the three-wire and four-wire transmitters is different from the two-wire system in terms of output circuit principle and structure. Therefore, when using it, it is necessary to pay attention to whether the negative end of the output can be connected to the negative line of the 24V power supply, whether it needs to be grounded, and how to avoid additional interference. If necessary, isolation measures can be taken, such as using distributors, safety barriers and other equipment to ensure common power, common ground and avoidance of interference with other instruments.

 

In addition, if a four-wire transducer with a transmission signal of 0-10mA DC is to be changed to a two-wire transmitter, there will be a challenge: since the transmitter’s starting current is zero, and the electronic amplifier cannot establish an operating point when the current is zero, it is difficult to work properly. If a DC power supply is used and the constant current characteristic of the instrument is maintained, when the transmitter load resistance is 0-1.5KΩ, the feedback moving coil resistance in series with it is about 2KΩ. When the output is 10mA, the voltage drop between these two parts will exceed 24V, which means that it is impossible to maintain the constant current characteristic under the conditions of 24V DC power supply and 0-1.5KΩ load, so two-wire transmission cannot be achieved.

In the 1990s, some instrument manufacturers did try to transform a 0-10mA DC four-wire transmitter into a two-wire transmitter. They solved the problem of the transmitter’s starting current not being zero by improving the original transmitter circuit and increasing the power supply voltage to 48VDC. To do this, they introduced a negative current to offset the 4mA starting output current on the load resistor. However, such products have not been widely promoted and applied.

 

On the other hand, trying to change a two-wire transmitter back to a four-wire system is not only unnecessary, but is actually a step backwards from a technological perspective.

 

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