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Physics/need of resister

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Question
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

Answer
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.

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James J. Kovalcin

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I am teaching or have taught AP physics B and C [calculus based mechanics & electricity and magnetism] as well as Lab Physics for college bound students. I have a BS in Physics from the University of Pittsburgh and a Master of Arts in Teaching from same. I have been teaching physics for 34 years. I am constantly updating my skills and have a particular interest in modern physics topics.

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