As we know, RS232, RS485 and CAN BUS are the commonly used carriers for digital signal transmission and conditioning. While how about the analog signal data transmission mode? In industrial site, we have to measure various type of non-electric quantity, like temperature, pressure, speed, angel, etc. In order to transmit it to controlling and display devices in controlling room remotely, all of these signals must be converted into analog electric signal firstly. The most commonly used transmission carrier is 4-20mA current signal.
Current signal has strong anti-interference capacity. In industrial site, amplitude of noise voltage is up to several volts, but the power consumption of noise is very weak, the noise current is less than nA class usually, so it causes very little error in 4-20ma data transmission process. And the current internal resistance tends to infinite, lead resistance in series connection in loop has no influence on accuracy, so the current can be transmitted to several hundred meters through common twisted pair. Current source has large internal resistance and constant current output, in receiving terminal, only add a 250Ohm resistance to GND, user can obtain 0-5V voltage signal easily. The benefits of low input impedance receiver are that it only generates very weak voltage noise in nA class current noise input.
The upper limit is set to 20mA to meet the anti-explosive requirements, because the flame energy of 20mA current is not big enough to ignite gas. The lower limit is not 0mA, because it needs to detect break line. In normal operation status, the current is not less than 4mA, thus when the transmission line is breakdown, the loop current descend to 0mA. Usually use 2mA as break line alarm value.
Current Transmitter can convert physical quantity into 4-20ma current output, but it requires external power supply to drive it. The typical transmitter needs two power supply wires and two current output wires, total 4 wires, so it is called 4-wire transmitter. While the current output terminals and power supply terminals can share the same common wire VCC or GND, so the 4-wire transmitter also called 3-wire transmitter. In application circuits, transmitter equals to a type of special load and can be powered by 4-20ma current itself, so this type of transmitter only needs 2 wires, it is called 2-wire transmitter. In the industrial standard, lower limit of current loop is 4mA, so within its measuring range, the power supply of transmitter is 24V, 4mA. All in all, in light load conditions, high efficiency dc to dc power module, low power consumption sensor, signal channel module, and low consumption MCU are very important factors which affect the performance of 2-wire 4-20mA transmission and receiving, and the design of 2-wire sensor becomes challenging and difficult.
Signal calibration kits for 4-20mA current signal:
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Pocket Size Precision 4-20mA 0-5V 0-10V 24V Signal Generator Simulator