Control.com - Nerds in Control

Can anybody explain to me exactly how the 2 wire loop powered system works?

In this case I suppose the field transmitter which sends 4-20 mA will be acting as a resistance in the loop?


favouritin @ rediffmail. com

This is essentially correct. The transmitter acts a current regulating mechanism. It's "resistance" can be calculated by measuring the terminal voltage across the loop connections at the transmitter and the current in the loop using Ohm's law. (I=E/R)

It is important to note, however, that the transmitter is not "calculating" the required apparent resistance. It is regulating current, and in most modern applications it is some kind of configuration of an op-amp maintaining the inverted input voltage to match a signal from the sensing element. So if you drop the loop power supply from 24VDC to12VDC with a fixed loop current, then the apparent resistance changes to half of it's original value to maintain the current. In reality it isn't quite as simple as the resistance changing linearly with the loop PS voltage since you usually have some signal developing resistance at the input of the A/D converter (traditionally 250 Ohms, but other exist.)

Just draw yourself two loops with a 250 Ohm signal developing load at the analog input, and the transmitter as a variable resistor, and finish the loops with a 24VDC supply in one and a 12VDC supply in the other and use Kirchoff's to analyze the loops.

MB

Thank you so much for your reply.