[cctalk] Re: Current Loop Schematics

It seems surprisingly hard to find that, though this https://forum.vcfed.org/index.php?threads/rs-232-to-current-loop-interface.… is close. Conceptually it's really easy. The main tricky part, certainly if you're driving a mechanical terminal like a Teletype, is the current source. You need something that can drive 20 mA into an inductive load. The classic answer was to use a fairly high voltage with a big series resistor so the inductive impedance is much less than the resistance. A current regulator would be a modern replacement. Also, for the case of the inductive load, you'd need a shunt diode across the switch to absorb the voltage spike from the inductor when the current is switched off, otherwise the switching transistor will be quickly destroyed. paul