The sensor selects the origin of the two-wire 4-20mA output Â
Normally we choose 4-20mA output is selected in many cases, when the sensor, especially in the choice of load sensor, a temperature sensor when the sensors are generally 4-20mA output. As for why the industry uses 4-20mA output instead of other numerical outputs? I believe many people don't know why.
The reason for using the current signal is that it is not susceptible to interference. And the internal resistance of the current source is infinite, the wire resistance is connected in series in the loop without affecting the accuracy, and can be transmitted hundreds of meters on the ordinary twisted pair. The upper limit is 20 mA because of the requirements of explosion protection: The spark energy caused by the current interruption of 20 mA is not enough to ignite the gas. The reason why the lower limit is not taken 0mA is to detect the disconnection: it will not be lower than 4mA during normal operation, and the loop current will drop to 0 when the transmission line is disconnected due to a fault . Usually take 2mA as the disconnection alarm value. Â
The current-type transmitter converts the physical quantity into a 4~20mA current output, which must be supplied by an external power supply. The most typical is that the transmitter needs two power lines, plus two current output lines, a total of four lines, called a four-wire transmitter. Of course, the current output can be shared with the power supply by a common line VCC or GND , which saves one line and is called a three-wire transmitter. In fact, you may notice that the 4-20mA current itself can power the transmitter. The transmitter is equivalent to a special load in the circuit. The special point is that the current consumption of the transmitter varies between 4 and 20 mA depending on the sensor output. The display meter only needs to be stringed in the circuit. This transmitter requires only two external wires and is called a two-wire transmitter. The industrial current loop standard lower limit is 4 mA , so the transmitter has at least 4 mA supply as long as it is within the range . This makes the design of the two-wire sensor possible. In industrial applications, the measuring points are generally on site, while the display or control devices are typically located in the control room or control cabinet. The distance between the two may be tens to hundreds of meters. Calculated by the distance of one hundred meters, the elimination of two wires means that the cost is reduced by nearly one hundred yuan! Therefore, two-wire sensors are definitely the first choice in applications. Â
Therefore, the sensor is generally 4-20 mA signal output when selected , so that the signal is not easily interfered and safe and reliable. The two-wire 4-20 mA output can save the sensor cost. These reasons make the sensor commonly used in the industry. Wire 4-20mA output. Of course, many sensors now have other styles of output such as 0-5V , RS485/RS232, etc., all of which are designed to handle sensor signals very well.
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