Physics/need of resister
hello james: i do not get this concept of need a resister to measure voltage in a loop. I am studying instrumentation and in a 4 to 20 milliamp loop they have 250 omh resister in the controller to measure the signal it changes it to voltage. Why is this necessary , could not just measure the amperage in the signal?? Thanks
The problem may be that the current in the loop is very high! If so the power dissipated could be very problematic. If there was no resistance the current would be only limited by the capability of the current source. In addition, large current flows reduce the terminal voltage of the source due to the source's internal resistance. By putting a significant load in the measurement circuit you limit the current output of the source without generating a significant internal voltage drop in the power supply.