为什么说 4～20mA 信号更合适应用在工业现场

　　工业上普遍需要测量各类非电物理量，例如温度、压力、速度、角度等，这些都需要转换成模拟量电信号才能传输到几百米外的控制室或显示设备上。工业上最广泛采用的是用4~20mA电流来传输模拟量。

In industry, it is generally necessary to measure various non electrical physical quantities, such as temperature, pressure, speed, angle, etc., which need to be converted into analog electrical signals to be transmitted to control rooms or display devices hundreds of meters away. The most widely used method in industry is to transmit analog signals using 4-20mA current.

①

      采用电流信号的原因是不容易受干扰，因为工业现场的噪声电压的幅度可能达到数V，但是噪声的功率很弱，所以噪声电流通常小于nA级别，因此给4-20mA传输带来的误差非常小;电流源内阻趋于无穷大，导线电阻串联在回路中不影响精度，因此在普通双绞线上可以传输数百米;

The reason for using current signals is that they are not easily disturbed, as the amplitude of noise voltage in industrial sites may reach several volts, but the power of the noise is very weak, so the noise current is usually lower than the nA level, resulting in very small errors in 4-20mA transmission; The internal resistance of the current source tends to infinity, and the series connection of the wire resistance in the circuit does not affect the accuracy, so it can transmit hundreds of meters on ordinary twisted pairs;

　　由于电流源的大内阻和恒流输出，在接收端我们只需放置一个250欧姆到地的电阻就可以获得0-5V的电压，低输入阻抗的接收器的好处是nA级的输入电流噪声只产生非常微弱的电压噪声。
Due to the large internal resistance and constant current output of the current source, we only need to place a 250 ohm resistor to ground at the receiving end to obtain a voltage of 0-5V. The advantage of a low input impedance receiver is that the nA level input current noise only generates very weak voltage noise.

②

　　上限取20mA是因为防爆的要求：20mA的电流通断引起的火花能量不足以引燃瓦斯。下限没有取0mA的原因是为了能检测断线：正常工作时不会低于4mA，当传输线因故障断路，环路电流降为0。常取2mA作为断线报警值。电流型变送器将物理量转换成4~20mA电流输出，必然要有外电源为其供电。最典型的是变送器需要两根电源线，加上两根电流输出线，总共要接4根线，称之为四线制变送器。

The upper limit of 20mA is due to explosion-proof requirements: the spark energy caused by the current of 20mA is not enough to ignite gas. The reason why the lower limit is not set to 0mA is to detect wire breakage: during normal operation, it will not be lower than 4mA. When the transmission line breaks due to a fault, the loop current drops to 0. 2mA is often taken as the alarm value for wire breakage. Current type transmitters convert physical quantities into 4-20mA current outputs, and external power sources are necessary to supply them. The most typical is that the transmitter requires two power lines, plus two current output lines, to connect a total of four lines, which is called a four wire transmitter.

　　当然，电流输出可以与电源公用一根线(公用VCC或者GND)，可节省一根线，所以现在基本上将四线制变送器称之为三线制变送器。其实大家可能注意到， 4-20mA电流本身就可以为变送器供电，变送器在电路中相当于一个特殊的负载，这种变送器只需外接2根线，因而被称为两线制变送器。工业电流环标准下限为4mA，因此在量程范围内，变送器通常只有24V，4mA供电(因此，在轻负载条件下高效率的DC/DC电源(TPS54331,TPS54160)，低功耗的传感器和信号链产品、以及低功耗的处理器(如MSP430)对于两线制的4-20mA收发非常重要)。这使得两线制传感器的设计成为可能而又富有挑战。

Of course, the current output can be shared with the power supply on the same wire (VCC or GND), which can save one wire. Therefore, four wire transmitters are now basically referred to as three wire transmitters. In fact, you may notice that 4-20mA current itself can power the transmitter, and the transmitter is equivalent to a special load in the circuit. This type of transmitter only needs to be connected to 2 external wires, so it is called a two wire transmitter. The lower limit of the industrial Current loop standard is 4mA, so within the range, the transmitter usually only has 24V, 4mA power supply (therefore, under light load conditions, high-efficiency DC/DC power supply (TPS54331, TPS54160), low-power sensor and signal link products, and low-power processors (such as MSP430) are very important for two-wire 4-20mA transceiver). This makes the design of two wire sensors possible and challenging.

③

　　我们系统地来看看模拟量设备为什么都偏爱用4~20mA传输信号？

　　4-20mA. DC(1-5V.DC)信号制是国际电工委员会( IEC )过程控制系统采用的模拟信号传输标准。中国也采用这一国际标准信号制，仪表传输信号采用4-20mA.DC，接收信号采用1-5V.DC，即采用电流传输、电压接收的信号系统。

4-20mA DC (1-5V. DC) signal system is the analog signal transmission standard adopted by the International Electrotechnical Commission (IEC) process control system. China also adopts this international standard signal system, with instrument transmission signals using 4-20mA. DC and reception signals using 1-5V. DC, that is, a signal system that uses current transmission and voltage reception.

　　一般仪器仪表的信号电流都为4-20mA，指最小电流为4mA,最大电流为20mA 。传输信号时候，因为导线上也有电阻，如果用电压传输则会在导线内产生一定的压降，那接收端的信号就会产生一定的误差了，所以一般使用电流信号作为变送器的标准传输。

The signal current of general instruments and meters is 4-20mA, indicating a minimum current of 4mA and a maximum current of 20mA. When transmitting signals, because there is also resistance on the wire, if voltage is used for transmission, a certain voltage drop will be generated inside the wire, which will cause a certain error in the signal at the receiving end. Therefore, current signals are generally used as the standard transmission for transmitters.