theory of 4 to 20ma

we all know that field transmitter minimum voltage require to operate is 12vdc

and the maximum loop resistance design is for 600ohms

thus voltage drop for the maximum resistance is 12vdc =600x20ma hence this is the maximum limit that the transmitter will operate can't go further design limits.

secondly when current signal meets the receiver it is converted to voltage signal.Also all transistors and diode works on the voltage between base emitter and collector which operates between 0.3vdc to 0.6vdc so inorder to have all the transistors and diode in conducting phase 1.0 vdc minimum is preferred ie;4ma signal for minimum so that the reading of the transmitter remain stable no drift for long period of time ,

communicate with 2 wire system for smart transmitter HART, is used depend on the protocol of MARK and SPACE ; mark as 0 =2200hz and space 1=1200hz ,this works over 4-20 ma signal to communicate with SMART transmitter in a digital format.