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The 4-20mA(1-5V) signal standard is an analog signal transmission standard adopted by the International Electrotechnical Commission (IEC) for process control systems. Our country also adopts this international standard signal system, with instrument transmission signals using 4-20mA and reception signals using 1-5V; in other words, it is a signal system that employs current for transmission and voltage for reception. The signal current for general instruments is typically 4-20mA, meaning the minimum current is 4mA and the maximum current is 20mA. When transmitting signals, since there is also resistance in the wires, using voltage for transmission results in a certain voltage drop within the wires, which causes errors in the signal received at the receiving end. Therefore, current signals are generally used as the standard for transmitting signals from transmitters. I. What is the 4-20mA (1-5V) signal standard? The 4-20mA(15V) signal standard is established by the International Electrotechnical Commission (IEC): it is a standard for analog signals in process control systems. Our country began to adopt this international standard signal system with the DDZ-Ⅲ type electric instruments; the instruments use 4–20mA.DC for signal transmission, while 1–5V.DC is used for communication signals, that is, a signal system that employs current for transmission and voltage for reception. The working principle of the 4-20mA current loop: In industrial settings, using an instrument amplifier to process the signal and enable long-distance transmission leads to several problems. First, since the signal transmitted is a voltage signal, the transmission lines are subject to noise interference ； Second, the distributed resistance of the transmission line causes a voltage drop ； Third, how to supply the operating voltage for the instrumentation amplifier at the site is also an issue. To address the aforementioned issues and avoid the impact of related noise, we use current to transmit signals, as current is not sensitive to noise. In a 4-20mA current loop, 4mA represents a zero signal, while 20mA indicates the full scale of the signal; signals below 4mA but above 20mA are used to trigger alarms for various faults. II. What are the advantages of the 4-20mA(1-5V) signal system? Field instruments can operate in a two-wire system; in this system, the power supply and the load are connected in series with a common point, and the signal communication as well as power supply between the field transmitter and the instruments in the control room are accomplished using only two wires. Since the current at the signal start point is 4 mA, this provides a static operating current for the transmitter. Meanwhile, the electrical zero point of the instrument is also 4 mA, and it does not coincide with the mechanical zero point; this \"dynamic zero point\" facilitates the detection of faults such as power loss and wire breaks. Moreover, the two-wire system also facilitates the use of safety barriers, contributing to safety and explosion prevention. The instruments in the control room use voltage parallel signal transmission; the instruments belonging to the same control system share a common terminal, which facilitates the use of measuring instruments, control instruments, computers, and alarm devices, as well as simplifies wiring. The reason for using 4-20mA as the communication signal between field instruments and those in the control room is that the distance between the field area and the control room is large, resulting in high resistance in the connection wires. If voltage signals are used for transmission, the voltage division caused by the wire resistance and the input resistance of the receiving instrument leads to significant errors. In contrast, using a constant current source for transmission ensures that the current in the circuit remains unchanged regardless of the length of the wires, as long as there are no branches in the circuit, thereby maintaining the accuracy of the transmission. The reason for using 1-5V for the communication signals between instruments in the control room is to enable multiple instruments to receive the same signal easily, as well as to facilitate wiring and the creation of various complex control systems. If a current source is used as the signaling source, when multiple instruments receive the same signal, their input resistances must be connected in series; this can cause the maximum load resistance to exceed the loading capacity of the transducer. Moreover, the potential at the negative terminal of the signal for each receiving instrument varies, which introduces interference, and it is not possible to provide power in a centralized manner. Voltage source signaling is used for communication; the current signals used to communicate with field instruments must be converted into voltage signals. The simplest way to do this is to connect a 250Ω standard resistor in series in the current transmission circuit, thereby converting 4-20mA.DC into 1-5V. Typically, a distributor is used to carry out this task. III. Why is 4-20mA chosen as the transmission signal for transmitters? 1. First and foremost, from the perspective of safety in field applications, the focus is on using explosion-proof, spark-free instruments. Based on the principle of controlling the energy of these instruments, efforts are made to minimize both the static and dynamic power consumption required for their proper operation. Transmitters that output 4-20mA standard signals typically use a supply voltage of 24VDC. The reason for using direct current is that it eliminates the need for large-capacity capacitors and inductors; instead, only the distributed capacitance and inductance of the wires connecting the transmitter to the instruments in the control room need to be taken into account. For example, a 2mm2 wire has a distributed capacitance of around 0.05μ/F/km ； For a single track, the inductance is around 0.4 mH/km ； **It is below the value at which hydrogen ignites, which is clearly very advantageous for explosion prevention. 2. Current sources are superior to voltage sources for transmitting signals, as the distance between the field site and the control room is large and the resistance of the connecting wires is high. When using voltage source signals for long-distance transmission, voltage division caused by the wire resistance and the input resistance of the receiving instrument leads to significant errors. In contrast, when current source signals are used for long-distance transmission, as long as there are no branches in the transmission circuit, the current in the circuit does not change depending on the length of the wires, thereby ensuring accuracy in signal transmission. 3. The reason for choosing a maximum signal current of 20mA is that this value is determined based on considerations of safety, practicality, power consumption, and cost. Safety spark instruments must use low voltage and low current; a current range of 4-20mA along with 24VDC is safe even for flammable hydrogen. The ignition current for hydrogen at 24VDC is 200mA, which is well above 20mA. In addition, factors such as the connection distance between instruments in the production area and the load they carry must also be taken into consideration ； There are also issues related to power consumption and cost, the requirements for electronic components, and the requirements regarding power supply capacity. 4. The reason for selecting 4 mA as the current at the signal start point is that transmitters with a 4-20 mA output are mostly of the two-wire type. In this configuration, the power supply and the load are connected in series, sharing a common point, and the signal communication as well as power supply between the field transmitter and the instruments in the control room are accomplished using only two wires. Why isn’t the start signal 0mA? This is based on two points: first, the transmitter circuit cannot function without a static operating current; the current at the signal start point is 4 mA, which does not coincide with the mechanical zero point. This \"dynamic zero point\" facilitates the detection of faults such as power loss and wire breaks. IV. Where did the 4-20mA sensor come from? The reason for using current signals is that they are less susceptible to interference, and the internal resistance of a current source is infinite; thus, the resistance of the wires in the circuit does not affect accuracy, allowing transmission over hundreds of meters using ordinary twisted pairs. The reason for using current signals is that they are less susceptible to interference. While the amplitude of noise voltages in industrial environments can reach several volts, the power of such noise is low; as a result, the noise current is usually below the nA level. Hence, the error introduced by 4–20mA transmission is very small ； The internal resistance of the current source tends to infinity, and the resistance of the wires in series within the circuit does not affect accuracy; therefore, it is possible to transmit over hundreds of meters using ordinary twisted pairs ； Due to the high internal resistance and constant current output of the current source, we only need to place a 250-ohm resistor from the receiver to ground in order to obtain a voltage of 0–5V. The advantage of a receiver with low input impedance is that input current noise on the order of nA results in only very slight voltage noise. The upper limit of 20mA is set due to explosion-proof requirements: the spark energy generated by the on/off of a 20mA current is not sufficient to ignite gas. The lower limit is not set to 0 mA in order to enable detection of a broken connection: under normal operation, the value will be above 4 mA, but when the transmission line breaks due to a fault, the loop current drops to 0. 2mA is commonly used as the wire-break alarm value. Current-type transmitters convert physical quantities into a 4–20mA current output, and therefore require an external power supply to operate. The most typical case is that a transmitter requires two power wires and two current output wires, for a total of 4 wires, which is what is known as a four-wire transmitter. Of course, the current output can share the same VCC or GND wire as the power supply, which saves one wire; such transmitters are known as three-wire transmitters. In fact, as everyone may have noticed, the 4-20mA current can itself power the transmitter. In a circuit, the transmitter functions as a special type of load; what makes it special is that its current consumption ranges from 4 to 20 mA and varies depending on the sensor’s output. To display the meter, it only needs to be connected in series in the circuit. This type of transmitter requires only 2 external wires, which is why it is called a two-wire transmitter. The minimum value for the industrial current loop standard is 4 mA; therefore, as long as it is within the range, the transmitter will receive at least 4 mA of power supply. Therefore, 4-20mA signal output is generally not prone to interference and is safe and reliable; as a result, two-wire 4-20mA power output signals are widely used in industry. But in order to handle sensor signals more effectively, there are currently other forms of output signals available: 3.33 MV/V ； 2MV/V ； 0-5V; 0-10V, etc. Source: Changhui Instruments http://yunrun.com.cn/