5V Powered 4-20mA Current Loop Generator
Voltage step-up also employed in figure below, but for a different purpose. The common requirements are transmission of industry standard 4-20mA current loop signals to valves and other actuators.
Current transmitters is required by resistive line losses and actuator impedance to be able to force a compliance voltage of at least 20V. 5V powered systems usually cannot meet current loop transmitter requirements because of this, but the following figure shows a way to do this. To generate the compliance voltage necessary for loop current requirements, this 5V powered circuit utilizes a servo controlled DC-DC converter. It will drive 4-20mA into loads as high as 2200Ω (44V compliance) and is inherently short circuit protected. Easy interface to digital systems is permitted because it has a digital inputs. [Circuit’s schematic diagram source: Linear Technology Application Note]