Simple 4-20mA current Sense Readout

Elegant analog design often comes down to simplicity. This 4–20 mA current-sense circuit shows how a single JFET can do the job with minimal parts.

"Sometimes the simplest solution is the best. Using a single N channel JFET as a current sink, a 4-20mA instrument monitor loop can be read with a minimum of components. A 2N4392 diverts a current of 4mA until the load circuit voltage exceeds the Zener threshold where the remaining current up to 20 mA is diverted giving a linear indication from 4-20mA. Given that the two-wire polarity is known, 

the Bridge rectifier can be eliminated, further simplifying the circuit. The bridge simply makes it fool proof...." - Kirkwood Rough