hello zz : 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

You must study Ohms Law some more.  Voltage =  Current times Resistance.  So, if you knew the voltage across a known resistor you could compute the current (I = V/R).

Yes, you can measure current if you have an ammeter but you must break the circuit in order to put the ammeter in series with the current flow. And, in some cases the resistor is inaccessible so the voltage measurement allows computation of the current.

Make sure you fully understand Ohms Law.  It is powerful once you get the hang of it.

Let me know if you have further questions.  


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Electronics questions about AC, DC and digital theory.


Graduate electrical engineer with over 40 years in electronic design, manufacturing, project organization and patent review. Experience in fields of industrial and consumer electronics (audio, video, acoustics, etc.)

IEEE (Institute of Electrical and Electronics Engineers); Senior Life member AES (Audio Engineering Society), Fellow Life member

BSEE University of North Dakota

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