You may be very familiar with RS232, RS485, CAN and other commonly used buses in the industry. They are all ways of transmitting digital signals. So, what method do we use to transmit analog signals? Industry generally needs to measure various types of non-electrical physical quantities, such as temperature, pressure, speed, angle, etc., which need to be converted into analog electrical signals to be transmitted to a control room or display device a few hundred meters away. The most widely used in the industry is to use 4 ~ 20mA current to transmit analog quantity.
The reason for using the current signal is that it is not easy to be disturbed, because the amplitude of the noise voltage in the industrial site may reach several V, but the power of the noise is very weak, so the noise current is usually less than nA level, so the error brought to 4-20mA transmission is very Small; the internal resistance of the current source tends to infinity, and the series resistance of the wire does not affect the accuracy in the loop, so it can transmit hundreds of meters on the ordinary twisted pair; due to the large internal resistance of the current source and the constant current output, we only need at the receiving end Place a 250 ohm resistor to ground to get a voltage of 0-5V. The benefit of a low input impedance receiver is that the input current noise of the nA level only produces very weak voltage noise.
The upper limit is 20mA because of the explosion-proof requirements: the spark energy caused by the 20mA current on and off is not enough to ignite the gas. The reason why the lower limit is not 0mA is to be able to detect the disconnection: during normal operation, it will not be lower than 4mA. When the transmission line is disconnected due to a fault, the loop current drops to 0. 2mA is usually taken as the disconnection alarm value. The current-type transmitter converts the physical quantity into 4 ~ 20mA current output, which must be powered by an external power supply. The most typical is that the transmitter requires two power lines, plus two current output lines, a total of 4 wires, called a four-wire transmitter. Of course, the current output can be shared with a power line (common VCC or GND), which can save a line, so now basically four-wire transmitters are called three-wire transmitters. 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. This transmitter only needs to connect 2 wires, so it is called two wires. Transmitter. The standard lower limit of the industrial current loop is 4mA, so within the range, the transmitter is usually only 24V, 4mA power supply (thus, high efficiency DC / Dc Power Supply (TPS54331, TPS54160) under light load conditions, low power consumption sensors and Signal chain 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.
Generally need to design a VI converter, input 0-3.3v, output 4mA-20mA, can use op amp LM358, power supply + 12v.
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