The conditions that the transmitter must meet at the same time to realize the two-wire system

What conditions must be met at the same time to realize a two-wire transmitter? To realize a two-wire transmitter, the following conditions must be met at the same time: 1. The output voltage V of the transmitter is equal to the specified minimum power supply voltage minus the current at the load resistance. and the voltage drop across the transmission wire resistance. That is: V≤Emin-Imax×RLmax2, the normal working current I of the transmitter must be less than or equal to the output current of the transmitter. That is: I≤Imin3, the small power consumption P of the transmitter cannot exceed the following formula, usually P<90mW. P<Imin(Emin-Imax×RLmax) where Emin is the lowest power supply voltage, for most instruments Emin=24×(1%-5%)=22.8V, 5% is the allowable negative change of 24V power supply; Imax= 20mA; Imin=4mA; RLmax=250Ω+ transmission wire resistance. If the transmitter is designed to meet the above three conditions, two-wire transmission can be realized. The so-called two-wire system means that the power supply and the load are connected in series, with limited common points, and the signal connection and power supply between the field transmitter and the control room instrument only use two wires, which are both power lines and signal lines. . The two-wire system also facilitates the use of safety barriers, which is conducive to safe explosion-proof. Two-wire transmitter wiring diagram The two-wire transmitter is shown in the figure, its power supply is 24VDC, the output signal is 4-20mA, the load resistance is 250Ω, the negative line potential of the 24V power supply is the lowest, it is the signal common line, For smart transmitters, the FSK keying signal of the HART protocol can also be loaded on the 4-20mA signal.