Explain the working principle and structural difference of the two-wire system, three-wire system and four-wire system

Today's Xiaobian and the two-wire system, three-wire system, and four-wire system are all kinds of transmitters that output analog DC current signals. The working principle and structure are different, not just the transmitter. Wiring form.

First, let’s take a look at their definitions.

Two-wire system: Two wires and transmission power supply transmit signals, that is, the load and power supply of the sensor output are connected in series. The power supply is externally introduced and connected in series with the load to drive the load.

Three-wire system: The three-wire sensor is the positive terminal of the power supply and the positive terminal of the signal output, but they share a COM terminal.

Four-wire system: two wires for power and two wires for signals. The power and signal work separately.

The name of the two-wire system was only available after the birth of the two-wire transmitter. This is the result of the widespread use of electronic amplifiers in meters. The essence of amplification is an energy conversion process, which is inseparable from the power supply. Therefore, the first to appear is a four-wire transmitter; that is, two wires are responsible for the supply of power, and the other two wires are responsible for outputting the signals that are converted and amplified (such as voltage, current, etc.). But at present, many transmitters use two-wire system. Below, let's take a look at the differences between different line transmitters.

Differences in different line transmitters

One, two-wire system

To implement a two-wire transmitter, the following conditions must be met:

1. V≤Emin-ImaxRLmax

The output voltage V of the transmitter is equal to the specified minimum supply voltage minus the voltage drop across the load resistance and transmission line resistance. 2. The normal operating current I of the I ≤ Imin transmitter must be less than or equal to the output current of the transmitter. 3. The minimum power consumption P of the P<Imin(Emin-IminRLmax) transmitter cannot exceed the above formula, usually <90mW. Where: Emin = minimum supply voltage, for most meters Emin = 24 (1-5%) = 22.8V, 5% is the negative change allowed by 24V power supply; Imax = 20mA; Imin = 4mA; RLmax = 250Ω +Transmission wire resistance. If the transmitter is designed to meet the three conditions above, a two-wire transmission can be achieved. The so-called two-wire system, that is, the power supply and the load are connected in series, has a common point, and the signal communication and power supply between the field transmitter and the control room meter uses only two wires, which are both a power line and a signal line. The two-wire transmitter provides a static working current for the transmitter due to the signal starting current of 4 mA DC. At the same time, the instrument's electrical zero is 4 mA DC, which does not coincide with the mechanical zero. This “live zero” is useful for identifying power outages and Faults such as disconnection. Moreover, the two-wire system also facilitates the use of safety barriers, which is conducive to safety and explosion protection.

Figure 1 Wiring diagram of two-wire transmitter

The two-wire transmitter is shown in Figure 1. The power supply is 24V DC, the output signal is 4-20mA DC, the load resistance is 250Ω, and the negative line potential of the 24V power supply is the lowest. It is the signal common line for smart transmitters. The HART protocol FSK keying signal can also be loaded on the 4-20 mA DC signal.

Second, three-wire system

In order to reduce the size and weight of the transmitter, improve the anti-jamming performance, and reduce the wiring, some instrument manufacturers change the power supply of the transmitter from 220V AC to low-voltage DC power supply, such as the power supply from the 24V DC power box. Since the low-voltage power supply creates conditions for the negative line sharing, there is a three-wire transmitter product.

Figure 2 Schematic diagram of three-wire transmitter wiring

The three-wire transmitter is shown in Figure 2. The so-called three-wire system uses one wire for the positive terminal and one wire for the positive terminal of the signal. The negative terminal and the negative terminal share a single wire. The power supply is mostly 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 have mA and mV signals, but the load resistance or input resistance, because The output circuit is different in form and the values are different.

Third, four-wire system

Due to the popularity and application of the 4-20 mA DC (1-5 V DC) signal system, in order to facilitate the connection in the control system application, the signal system is required to be unified. For this reason, some instruments that are not combined with electric units are required, such as online analysis and machinery. The quantity, power and other instruments can be used as the 4-20mA DC signal system. However, due to the complicated conversion circuit and high power consumption, it is difficult to fully meet the three conditions of the two-wire transmitter design. For the wire system, only the external power supply can be used to make a 4-wire transmitter with a 4-20 mA DC output.

Figure 3 Wiring diagram of four-wire transmitter

The four-wire transmitter is shown in Figure 3. The power supply is mostly 220V AC, and the power supply is 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 have mA and mV signals, but the load resistance or input resistance is different depending on the output circuit form. It is different.

In the above three figures, the input receiver is a current signal. If the resistor RL is connected in parallel, the voltage signal is received. As can be seen from the above description, due to the different working principles and structures of various transmitters, different products appear, which determines the two-wire, three-wire, and four-wire wiring forms of the transmitter. For the user, the selection should be based on the actual situation of the unit, such as the unified signal system, explosion-proof requirements, receiving equipment requirements, investment and other issues to consider the choice. It should be pointed out that the 4-20mA DC signal output by the three-wire and four-wire transmitters is different from the two-wire system because of its output circuit principle and structure. Therefore, can the output negative terminal and 24V be used in the application? The negative line of the power supply is connected? Can you share the land? This is to be noted, if necessary, isolation measures can be taken, such as the use of distributors, safety barriers, etc., in order to share power with other instruments, and to avoid the occurrence of additional interference.

Two-wire system and four-wire system

If you want to change the four-wire transmitter with 0-10mA DC to two-wire system, the first problem encountered is that its initial current is zero. When the current is zero, the transmitter's electronic amplifier It is impossible to establish a working point, so it will be difficult to work properly. If DC power supply is used and the original constant current characteristic of the instrument is ensured, when the load resistance of the transmitter is 0-1.5KΩ, the feedback dynamic coil resistance in series with it is about 2KΩ, and when the output is 10mA, the voltage drop of these two parts It will be greater than 24V, that is to say, it will be powered by 24V DC. When the load is 0-1.5KΩ, it is impossible to ensure constant current characteristics, so it cannot be transmitted by two-wire system. In the 1970s, there was a meter factory that changed the 4-wire transmitter of 0-10mA DC to a two-wire transmitter. The specific method is to improve the original transmitter circuit and increase the power supply voltage. Up to 48 VDC, but the initial current of the transmitter is still not zero. For this reason, a negative current is used to cancel the current output of 4 mA on the load resistor. However, such products have not been promoted and applied. If you want to change the two-wire system to the four-wire system, there is no need at all. Again, this is a technical retrogression.