# What is the amount of switching and analog, tell you by example! Figure 1 shows a typical device that can output a switching signal. When the pressure is high, the two contacts C and B are closed, and the output pressure is high. When the pressure is low, the two contacts C and A are closed to close the output pressure low signal. With such a signal, the local pressure signal is transmitted to the remote electrical control cabinet to participate in the automatic remote control, wherein C and B are a switching quantity, and C and A are also a switching quantity. So a switch contact is a switch that is characterized by either turning it on or off at the same time. Turning on is 1 means there is a signal, and disconnecting is 0, which means there is no signal. This is the so-called switching signal.

Although the pressure gauge can transmit the pressure signal to a distant place, it transmits only the signal with or without pressure, and it is impossible to know what the real-time pressure value is. The device in Figure 2 is called a pressure transmitter. Inside the pressure transmitter is a circuit board with a pressure sensor F connected to it. Its working principle is that the pressure sensor F transmits the detected pressure to the circuit board C. After the detection signal enters the circuit board, the pressure signal is converted into a current signal by A and B through the conversion and calculation of the circuit board. Point output. The right side of the figure is a schematic diagram of the conversion process. It converts a 0-10 kPa pressure signal into a 4-20 mA current signal, which is output by two points, A and B. At this point we say that the two points A and B output is an analog signal. The characteristic of an analog signal is that its value is continuously variable over a range of values.

Let's take a look at how analog signals are transmitted over long distances. We install a pressure transmitter with a range of 0-10kpa on the pipeline. The positive pole of the power supply is connected to point B of the pressure transmitter. The negative pole is connected in series with a multimeter to point A of the pressure transmitter, and the multimeter is driven to the current level. When the pressure at point C of the pressure transmitter is 5 kPa, the current reading of the multimeter is 12 mA. It is exactly the middle value of the 4-20 mA current signal, and 5 kPa is also the middle value of the 0-10 kPa pressure value. When the pressure at point C of the pressure transmitter is 10 kPa, the current reading of the multimeter is exactly 20 mA. Thus, the 0-10kpa pressure value corresponds to the current signal value of 4-20mA. We only need to extract the current signal value of 4-20mA through a receiving device in the distance, and then through a certain calculation, we can know the local pressure. What is the value?

Why convert the pressure signal to a 4-20 mA current signal instead of a 0-20 mA current signal or a 0-10 volt voltage signal?

The 1.0-10V voltage signal is susceptible to external electromagnetic interference, especially when the cable length is long.

2. With a current signal of 0-20 mA, it is impossible to judge whether the fault is 0 mA caused by the cable break when the current signal is 0 mA, or the normal 0 mA of the output is 0 kpa.

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