Instruments: [Weekly Question] Week 23 of 2011: What is the difference between 4–20 mmA current signals and 1–5 V voltage signals?

This post was last edited by denghl on 2012-12-21 at 21:13. The 4-20MA current signal is suitable for long-distance transmission; the 1-5V voltage signal is suitable for communication with electronic devices. The MHM-04B signal conditioner can be used to convert and relay signals.

This post was last edited by denghl on 2012-12-21 at 21:14. There is no voltage drop in the 4-20MA transmission, and multiple instruments can be connected in parallel using 1-5V in the control room

This post was last edited by denghl on 2012-12-21 at 21:15. The 4-20mA.DC(1-5V.DC) signal standard is an 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 transmit signals using 4-20mA.DC, while the communication signals are transmitted at 1-5V.DC – in other words, it is a signal system that uses current for transmission and voltage for reception. The advantages of this signaling system are as follows: field instruments can operate in a two-wire system, where the power supply and the load are connected in series with a common point; 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.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 facilitates the use of safety barriers, contributing to safety and explosion prevention. The instruments in the control room use a voltage parallel signaling system; the instruments within the same control system share common terminals, which facilitates their use in conjunction with sensing instruments, control instruments, computers, and alarm devices, as well as simplifies wiring. The reason why 4-20mA.DC is used for the communication signals between the 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-based signals are used for transmission, significant errors occur due to the voltage division caused by the wire resistance and the input resistance of the receiving instrument. In contrast, when a constant 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 the transmission. 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 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 instrument. 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 single 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 by connecting a 250-ohm standard resistor in series in the current transmission circuit, thereby converting 4-20 mA.DC into 1-5 V.DC. Typically, a distributor is used to carry out this task.

This post was last edited by denghl on 2012-12-21 at 21:16. The communication signal between the field instruments and those in the control room is 4-20mA DC. This is because the distance between the field area and the control room is large, resulting in high line resistance. If voltage signals were used for transmission, the presence of line resistance would cause significant voltage drops, leading to large errors. In contrast, with current signals, 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, thus ensuring accurate transmission. The use of 1-5V.DC signals between instruments in the control room is intended to enable multiple instruments to receive the same signal, thereby facilitating wiring and the creation of various complex control systems. If current is used as the communication signal, 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 load 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 a single centralized power supply.

Both are analog signals; 1---5V should be the required level signal

4--20mA is the most commonly used in industrial-level control, and it has relatively the strongest resistance to interference. Since it is a current output, a certain amount of power is required, and ordinary interference levels are very low, making it difficult to cause interference with such transmission. Voltage transmission is generally used in electronic products; its advantage is low power consumption, but of course its interference resistance is poor. The main characteristic of common-mode interference and differential-mode interference is that they propagate along the power lines or signal lines to various related devices; using low-pass filters is a common method for suppressing high-frequency interference

This post was last edited by denghl on 2012-12-21 at 21:18. The reason for using 4-20mA DC 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 connecting wires. If voltage-based signals are used for long-distance transmission, significant errors occur due to the voltage division caused by the wire resistance and the input resistance of the receiving instrument. In contrast, when a constant current source is used for transmission, as long as there are no branches in the circuit, the current remains unchanged regardless of the length of the wires, thereby ensuring accuracy in the transmission. 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 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 instrument. 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 single 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 by connecting a 250-ohm standard resistor in series in the current transmission circuit, thereby converting 4-20 mA.DC into 1-5 V.DC. Typically, a distributor is used to carry out this task.

This post was last edited by denghl on 2012-12-21 at 21:19. The 4-20MA current signal is suitable for long-distance transmission, while the 1-5V voltage signal is appropriate for communication with electronic devices. The MHM-04B signal conditioner can be used to convert and relay signals

This post was last edited by denghl on 2012-12-21 at 21:21. At greater distances outdoors, due to the long length of the wires and their high resistance, if the signal is transmitted as a voltage, there is significant voltage drop, which reduces accuracy. However, the current in the same circuit remains constant, allowing signals to be transmitted over greater distances using current compared to voltage signals.