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Knowledge extension:

Why do flow switches use 4-20mA current to transmit analysis signals?

The reason why flow switches use current signals is that they are not easily disturbed, because the example of noise voltage in industrial sites may reach several volts, but the power of noise is very weak, so the noise current is manually lower than the nA level, resulting in very small errors in 4-20mA transmission; The internal resistance of the current source tens to infinity, and the series connection of the wire resistance in the circuit does not affect the accuracy, so it can transmit hung of meters on ordered twisted pairs; 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 retain 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

The upper limit of 20mA is due to exploration 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 break: 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 break Current type transmitters convert physical quantities into 4-20mA current outputs, and external power sources are necessary to supply them A typical transmitter requires two power lines, plus two current output lines, to connect a total of four lines, which is called a four wire transmitter Of course, the current output can be shared with the power supply on the same wire (VCC or GND), which can save one wire Theremore, four wire transmitters are now fundamentally 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 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 standard lower limit of industrial current loop is 4mA, so within the range, the transmitter is manually powered only by 24V and 4mA (there are, 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 transformer under light load conditions This makes the design of two wire sensors possible and challenging

Generally, a VI converter needs to be designed, with an input of 0-3.3v and an output of 4mA-20mA It can use an operational amplifier LM358 and supply+12v

Let's take a systematic look at why flow switch analyzer devices preference to transmit signals with 4-20mA~

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

The signal current of general instruments and meters is 4-20mA, which refers to a small current of 4mA and a 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 Theoretical, current signals are generally used as the standard transmission for transmitters

Why did the transmitter choose 4-20mA. DC as the transmission signal?

1. First, considering the safety of on site applications

The safety focus is to consider exploration proof safety spark type instruments, and to reduce the static and dynamic power consumption that maintains normal operation of the instruments to a minimum, with the promise of controlling instrument energy Transmitters that output 4-20mA. DC standard signals have a power supply voltage of 24V. DC. The main reason for using DC voltage is that large capacity capacity capacity and indicators are not needed, and only the distributed capacity and indicators of the connecting wires between the transmitter and control room instruments need to be considered For example, a 2mm2 wire has a distributed capacity of 0.05 μ/ About km; For a single line, the index is about 0.4mH/km; The value is significantly lower than that of hydrogen, which is objectively very beneficial for exploration proof

2. A current source is superior to a voltage source for transmitting signals

Due to the distance between the site and the control room, when the resistance of the connecting wires is large, if a voltage source signal is used for remote transmission, there will be a significant error due to the voltage difference between the wire resistance and the input resistance of the receiving instrument If a current source signal is used for remote transmission, as long as there are no branches in the transmission circuit, the current in the circuit will not change with the length of the wires, so ensuring the accuracy of transmission

3. Reason for selecting 20mA signal current

The selection of a current of 20mA is based on considerations of safety, practicality, power consumption, and cost Safety spark instruments can only be used with low voltage and low current 4-20mA current and 24V DC are also safe for flammable hydrogen gas For 24V DC hydrogen gas, the current is 200mA, far above 20mA In addition, factors such as the connection distance between production site instruments and the load carried should be comprehensively considered; There are also factors such as power consumption and cost issues, requirements for electronic components, and requirements for power supply

4. Reason for selecting 4mA as the starting current of the signal

Transmitters with an output of 4-20mA are mostly in a two wire system, where the power and load are connected in series and have a common point How, the signal connection and power supply between the on site transmitter and the control room instruments only use two wires Why is the starting signal not 0mA? This is based on two points: first, the transmitter circuit cannot operate without static working current, and the signal starting current is 4mA DC, which does not involve with the mechanical zero point This ^ ^ active zero point ^ ^ is conductive to identifying faults such as power output and wire break