[GreenKeys] Current limiting

[Claude Frantz]

Chris Elmquist wrote:

> I'm a little green on current loop fundamentals-- having grown up in
> the RS232 age...  but is the high voltage really neccessary if you are
> not running a long distance with the loop?
> 
> In other words, could a modern constant current source using say, LM317
> be built that provides a nice stable 20 or 60mA but with only 12, 24 or
> 48v DC source?
> 
> Is there something in the older TTYs that requires a higher voltage on
> the loop?

I was using a constant current source in a TU I have built. Excellent 
results ! It can be switched from 40 to 60 mA for european an american 
TTY machines.

Best 73 de Claude