Although the title says “basics”, here are some advanced circuits about the 4-20 mA current loop transmitters.
In the first example a USB connection is shown. Nowadays almost every transmitter incorporates a microcontroller. The transmitters can be programmed through their display, or through HART protocol, or via DIP switches.
Transmitters with HART communication are expensive, using displays also increases the cost, and using DIP switches provides a not so flexible way of configuration.
Some manufacturers use USB as a programming / configuration interface, but most of them are not loop powered, so they need a separate power supply, which increases the cabling costs.
The following solution shows a way of how to include the USB communication in the transmitter design, while the transmitter is powered from the loop.