[GreenKeys] Current loops

[Jim Haynes]

Now we all know the canonical circuit is 120V power supply in series with
2K ohms to produce a 60 ma loop current with acceptable rise time.

I've seen circuits published using the constant-current properties of a
transistor to reduce the rise time and allow a lower voltage power supply.
What I haven't seen is any simulation or measurement of these circuits,
and I'm not young and smart enough to be up on things like SPICE.  Wish
someone who knows how would do this and show us how well it works.

 	---

 	"Ya can argue all ya wanna, but it's dif'rent than it was."
 	"No it ain't! No it ain't!  But ya gotta know the territory."
 		Meredith Willson, The Music Man