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.

Led Driver ICs may merit consideration, e.g. https://uk.rs-online.com/web/p/led-driver-ics/7377975 for 20 mA. However, they usu have a not insignificant voltage drop (a few Volts). I have used lots of 10 mA versions for sensing switch / relay closures, typically with an ACPL-217 as sense element. And, these days, the catalogs are replete with LED drivers. The classic TLR solution, e.g. 1.25 V drop with an LM317, is written up by Horrowitz and Hill as Current Sources - Three terminal regulator as current source [3rd Edn $9.3.14 A, pp620] and many data sheets / ANs. The LED drivers seem to work just as well, 10+ years ago the LM317 was my standard design. Martin -----Original Message----- From: Tony Duell via cctalk [mailto:cctalk@classiccmp.org] Sent: 02 May 2023 14:06 To: General Discussion: On-Topic and Off-Topic Posts < cctalk(a)classiccmp.org&gt; Cc: Tony Duell &lt;ard.p850ug1(a)gmail.com&gt; Subject: [cctalk] Re: Current Loop Schematics On Tue, May 2, 2023 at 1:34 PM Paul Koning via cctalk < cctalk(a)classiccmp.org&gt; wrote: https://forum.vcfed.org/index.php?threads/rs-232-to-current-loop-interface.… is close. 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. The common version of the Model 33 Teletype with a current loop interface [1] has a bit of electronics inside. A little driver PCB with a couple of transistors on it, a power transformer for it and so on. The result is that although the interface is 20mA or 60mA current loop you are not driving the receive magnet directly and there are no high voltage spikes at the interface terminals. [1] This is by far the most common version of the Model 33 in the UK. If you do have to drive the solenoid directly then (as you're in the UK) look out for the RSGB [2] Teleprinter Handbook. Although, not surprisingly, this is biased towards amateur radio RTTY operation, it has a lot of information on mechanical teleprinters, how to drive them (with circuit diagrams) and so on. [2] Radio Society of Great Britain. Basically our equivalent of the ARRL. -tony