The 4-20 mA Current Loop Explained: How a DC Current Transformer Helps

A simple 4-20 mA current loop consists of four parts: a DC power supply, a two-wire transmitter, a receiver resistor, and RWire. Unlike alternating current (AC), which changes all the time, direct current (DC) is consistent. Unless the loop uses a three-wire DC current transformer, the system may not recognize changes in current flow while in AC power.

The three-wire variant can hold its own in a voltage drop. The amount of voltage lost is relative to the length of the wire; the longer the wire, the more voltage lost. According to the National Electric Code, voltage drops must be no more than 3 percent of circuit voltage from the breaker box. Otherwise, operators risk pushing an appliance to its breaking point due to lack of current.
http://blog.aimdynamics.com/the-4-20-ma-current-loop-explained-how-a-dc-current-transformer-helps/

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