Why do analog devices prefer to use 4~20mA for signal transmission?

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4-20mA DC (1-5V DC) signal format is the analog signal transmission standard adopted by the International Electrotechnical Commission (IEC) for process control systems. Our country also adopts this international standard signal system; instrument transmission signals use 4-20mA.DC, while reception signals use 1-5V.DC, that is, it is a signal system that utilizes current for transmission and voltage for reception. The signal current for general instruments and meters is 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 across those wires, which in turn causes errors in the signal received at the other end. Therefore, current signals are generally used as the standard for signal transmission in transmitters. I. What is the 4–20mA.DC (1–5V.DC) signal standard? The 4–20 mA DC (1–5 V DC) signaling system is the International Electrotechnical Commission (IEC) standard for analog signals used 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. Principle of operation of the 4–20mA current loop: In industrial settings, using an instrumentation amplifier to condition the signal and enable long-distance transmission can lead to the following problems: First, since the transmitted signal is a voltage signal, the transmission line is susceptible to noise interference ； Second, the distributed resistance of the transmission line causes a voltage drop ； Third, how to provide the operating voltage for the instrumentation amplifier on-site is also a problem. 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 denotes the full scale of the signal; signals below 4mA but above 20mA are used to indicate various faults. II. What are the advantages of the 4~20mA.DC (1~5V.DC) signal system? Field instruments can operate on a two-wire system. In this system, the power supply and load are connected in series, sharing a common point. Thus, only two wires are required for both signal transmission between the field transmitter and the control room instruments and power supply. Since the current at the signal start point is 4 mA.DC, this provides a static operating current for the transmitter. Meanwhile, the electrical zero point of the instrument is also 4 mA.DC, 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 why a 4–20 mA DC signal is used for communication between field instruments and instruments in the control room is as follows: given the considerable distance between the field and the control room, the resistance of the connecting wires is relatively high. If voltage signals were used for remote transmission, they would be subject to voltage division caused by both the wire resistance and the input resistance of the receiving instrument, resulting in significant errors. In contrast, when using a constant-current source signal for remote transmission, provided there are no branches in the transmission circuit, the current flowing through the circuit remains unchanged regardless of the length of the wires. This ensures the accuracy of the transmitted signal. The reason for using 1–5V DC as the communication signal between instruments in the control room is to facilitate multiple instruments in receiving the same signal, as well as to simplify wiring and the creation of various complex control systems. If a current source is used as the communication signal, when multiple instruments receive the same signal, their input resistances must be connected in series. This causes the maximum load resistance to exceed the load capacity of the transmitting instrument. Additionally, the potentials at the negative terminals of the signals received by each instrument differ from one another, which introduces interference; furthermore, it becomes impossible to provide a single, centralized power supply. 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.DC. Typically, a distributor is used to carry out this task. III. Why do transmitters choose 4~20mA.DC as the transmission signal? 1. First and foremost, there are safety considerations related to on-site use. The focus on safety lies in the use of explosion-proof, spark-free instruments, and it is based on the principle of controlling the energy used by these instruments, thereby reducing both the static and dynamic power consumption required for their proper operation to the minimum level. Transmitters that output a standard 4–20 mA DC signal typically use a 24V DC power supply. The main reason for using DC voltage is that it eliminates the need for large-capacity capacitors and inductors; one only needs to consider the distributed capacitance and inductance of the wires connecting the transmitter to the instruments in the control room. For example, the distributed capacitance of a 2 mm² wire is approximately 0.05 μF/km ； For a single wire, the inductance is around 0.4 mH/km ； **This value is lower than the threshold for igniting hydrogen; obviously, this is very beneficial for explosion prevention. 2. Current sources are superior to voltage sources for transmitting signals. Since the distance between the field site and the control room is large and the resistance of the connecting wires is high, using a voltage source for signal transmission results in significant errors due to the voltage division caused by the wire resistance and the input resistance of the receiving instrument. If a current source is used for transmission, as long as there are no branches in the transmission circuit, the current in the circuit remains unchanged regardless of the length of the wires, thereby ensuring accuracy in signal transmission. 3. Reasons for choosing 20mA as the maximum signal current: The choice of a maximum current of 20mA is based on considerations of safety, practicality, power consumption, and cost. Safety spark-proof instruments must operate using low voltage and low current levels. A current range of 4–20 mA and 24V DC are also safe for use with flammable hydrogen. The ignition current for hydrogen at 24V DC is 200 mA, which is significantly higher than 20 mA. Additionally, factors such as the distance between instruments at the production site and the loads they drive must also be taken into consideration ； There are also issues related to power consumption and cost, the requirements for electronic components, and the required power supply capacity. 4. The reason for choosing 4 mA as the current at the signal start point is that transmitters with an output of 4–20 mA are mostly of the two-wire type. In this configuration, the power supply and the load are connected in series, sharing a common point; only two wires are used for transmitting signals and supplying power between the field transmitter and the instruments in the control room. Why isn’t the starting point signal 0mA? This is based on two points: first, the transmitter circuit cannot function without a static operating current; the signal start current is 4 mA.DC, 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 reason why the lower limit is not set at 0mA is to enable detection of open circuits: under normal operating conditions, it never drops below 4mA; when the transmission line becomes open 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 source to supply power to them. The most typical case is that a transmitter requires two power wires plus two current output wires, for a total of 4 wires, which 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 the circuit, the transmitter acts as a special load; what makes it special is that the current consumed by the transmitter varies between 4 and 20 mA, depending on the sensor output. The display meter simply needs to be wired into the circuit. This type of transmitter requires only 2 external wires, which is why it is called a two-wire transmitter. The lower limit for industrial current loop standards is 4 mA; therefore, as long as it is within the range, the transmitter receives 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 many other forms of output signals available: 3.33MV/V ； 2MV/V ； 0-5V; 0-10V, etc.