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Instrument knowledge|How to wire the two-wire system and four-wire system of the instrument?

DATE: 2021-04-20

'Wire system' refers to the number of wires used for power supply and standard signal transmission of the instrument. The name of the several-wire system only came into being after the birth of the two-wire transmitter. This is the result of the wide application of electronic amplifiers in meters. The essence of amplification is an energy conversion process, which is inseparable from power supply. Therefore, the first to appear is the four-wire transmitter, that is, two wires are responsible for the power supply, and the other two wires are responsible for outputting the converted and amplified signals (such as voltage, current, etc.).

Four wire

A four-wire signal meter refers to a 4-20mA standard signal that is independent of the power supply circuit and the signal circuit, and is separated by an isolation circuit. So the signal must be isolated and usually not grounded. The power supply can be 24VDC or 220VAC.

The four-wire transmitter is shown in the figure below. Most of its power supply is 220V.AC, and some are powered by 24V.DC. The output signal has 4-20mA.DC, the load resistance is 250Ω, or 0-10mA.DC, the load resistance is 0-1.5KΩ; some also have mA and mV signals, but the load resistance or input resistance is different due to the output circuit form The values are different.

Three wire

When the power provided by the two-wire system is not enough to power the instrument, the three-wire system is adopted and the signal is a 4-20mA standard signal.

The three wires of the three-wire system are: one is the power wire (usually 24VDC+), one is the signal wire, and the other is the common wire between the power and the signal, which is used for the voltage reference point and the current loop.

The three-wire transmitter is shown in the figure below. The so-called three-wire system means that the positive terminal of the power supply uses one wire, the positive terminal of the signal output uses one wire, and the negative terminal of the power supply and the negative signal terminal share the same wire. Most of its power supply is 24V.DC, the output signal is 4-20mA.DC, the load resistance is 250Ω or 0-10mA.DC, the load resistance is 0-1.5KΩ; some also have mA and mV signals, but the load resistance or input Resistance, the value is different due to different output circuit forms.

Two wire

The two-wire system transmits 4-20mA standard signals while supplying power by two wires. Signals other than this standard are not called a two-wire system. The minimum power supply capacity of the two-wire system is: 4mA×24V-receiving instrument power consumption-line loss.

The two-wire transmitter is shown in the figure below. Its power supply is 24V.DC, the output signal is 4-20mA.DC, the load resistance is 250Ω, and the negative wire potential of the 24V power supply is the lowest. It is the signal common line. For smart transmitters The FSK keying signal of HART protocol can also be loaded on the 4-20mA.DC signal.

How to choose different wire systems

Due to the different working principles and structures of various transmitters, different products have appeared, which determines the two-wire, three-wire, and four-wire wiring form of the transmitter.

1. When the power is greater than 10W and the accuracy is high, a four-wire instrument is generally used.

The application of four-wire meters began with the emergence of DDZ-II electric unit combination meters. The four-wire transmitter with a power supply of 220V.AC and subsequent output signals of 0-10mA.DC has been widely used. Due to the complexity of the conversion circuit and the high power consumption, many meters still prefer the four-wire system. The most common ones are Coriolis mass flowmeters and electromagnetic flowmeters. They are still four-wire systems, because mass flowmeters require vibration power. The electromagnetic flowmeter needs excitation power consumption, and the power of the two meters is above 10W, so a four-wire system is required.

2. Reduce unsafe voltage supply, generally choose three-wire instrument.

For the application of the three-wire system, in order to meet the design of the power supply required by the instrument, the power supply of the transmitter is changed from 220V.AC to low-voltage DC power supply, and the power supply is taken from the 24V.DC power box to reduce unsafe voltage power supply. In this way, there is a three-wire transmitter product.

3. The power is small and economical, and the two-wire system is generally used.

The application of two-wire system instrument, adopts 4-20mA.DC signal, the field instrument can realize the two-wire system, now the domestic two-wire system transmitter products are widely used.

In summary, for users, the selection should be based on the actual situation of the unit, such as the unification of the signal system, explosion-proof requirements, requirements for receiving equipment, investment and other issues to comprehensively consider the selection.

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Instrument knowledge|How to wire the two-wire system and four-wire system of the instrument?

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