Understand the wiring methods and applications of various current transformers and other products

 

The concept of “wire system” comes from the number of wires used for instrument power supply and standard signal transmission. With the popularization of electronic amplifiers in instruments, the essence of amplification – energy conversion process, has prompted the need for power supply. Therefore, the first transmitters were four-wire systems, two of which were dedicated to power supply, and the other two were responsible for outputting converted and amplified signals, such as voltage, current, etc.

 

Instruments powered by 220VAC usually adopt a four-wire design. The characteristic of a four-wire instrument is that its power supply circuit and signal circuit are independent of each other and separated by an isolation circuit. This design is mainly used to transmit 4-20mA standard signals to ensure signal isolation and is usually not grounded. The power supply can be selected as 24VDC or 220VAC.

 

When the two-wire power supply is not enough to meet the needs of the instrument, a three-wire system is used. The three-wire instrument contains a power line (usually 24VDC+), a signal line, and a common line for voltage reference and current loop. Instruments that do not use a standard 4-20mA signal are not called two-wire.

 

In practical applications, the four-wire power supply method usually uses 220VAC, but some also use 24VDC. The four-wire transmitter is a typical representative of the four-wire instrument. Most of them are powered by 220VAC, the output signal is 4-20mA.DC, and they are equipped with corresponding load resistance. In addition, there are different output options such as 0-10mA.DC and mA and mV signals, but the specific value of the load resistance or input resistance will vary depending on the output circuit form.

 

The three-wire power supply method usually uses 24VDC. The design feature of the three-wire transmitter is that the positive end of the power supply uses 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 mostly powered by 24VDC and outputs a 4-20mA.DC signal. In terms of load resistance, for the 4-20mA.DC output, its value is usually 250Ω; for the 0-10mA.DC output, the load resistance range is 0-1.5KΩ. In addition, some three-wire transmitters also provide mA and mV signal options, but the specific load resistance or input resistance value will vary depending on the form of the output circuit. The two-wire power supply method also uses 24VDC. The design feature of the two-wire transmitter is that only two wires are used for power supply and signal output. Among them, the negative line of the 24V power supply is used as the signal common line with the lowest potential. In addition, the smart transmitter can also superimpose the FSK keying signal of the HART protocol on the 4-20mA.DC signal to achieve more advanced functions.

Due to the differences in the working principle and structure of the transmitter, a variety of products have been produced, which in turn have formed different wiring methods such as two-wire, three-wire and four-wire. When choosing, it is usually determined based on power requirements and accuracy requirements. When the power requirement exceeds 10W and the accuracy requirement is high, the four-wire instrument becomes the first choice.

The history of four-wire instruments can be traced back to the birth of the DDZ-II electric unit combination instrument, which was powered by 220V.AC, and the subsequent 0~10mA.DC four-wire transmitter has been widely used. Although the complexity of the conversion circuit and power consumption still exist, the four-wire instrument is still given priority in many occasions. For example, Coriolis mass flowmeters and electromagnetic flowmeters require vibration power consumption and excitation power consumption, and their power exceeds 10W, so they still use a four-wire design.

 

While meeting the power supply requirements of the instrument, in order to ensure safety, three-wire instruments are usually selected, and the power supply of the transmitter is adjusted from 220V.AC to low-voltage DC power supply accordingly, for example, through a 24V.DC power supply box. In this way, the power supply requirements of the instrument are met, and the safety of use is ensured, and the three-wire transmitter product is born.

 

For occasions with low power and economical, two-wire is usually selected. This instrument uses a 4-20mA.DC signal, which enables the field instrument to achieve two-wire power supply. At present, the application of two-wire transmitters in China has been quite extensive.

 

In summary, when selecting, users need to comprehensively consider the actual situation of their units, including signal system uniformity, explosion-proof requirements, receiving equipment characteristics, investment budget and other factors. It is worth noting that although three-wire and four-wire transmitters also output 4-20mA.DC signals, their output circuit principles and structures are different from those of two-wire transmitters. Therefore, in practical applications, special attention should be paid to whether the negative output terminal can be connected to the negative line of the 24V power supply and whether they can be used together. To ensure stability and safety, isolation measures should be taken when necessary, such as using power distributors or safety barriers to achieve common power and ground with other instruments, and effectively avoid the influence of additional interference.

 

 

--- END ---