[GreenKeys] Current loops

[Peter Gottlieb]

I did this some time ago using the free LTSpice. Given the inductance of the selector magnet there is only so low you can go, a constant current source can’t get around that. I don’t remember the exact results but I ran into not knowing how much degradation in rise time mattered. 

I ended up using a high voltage (120) supply with a constant current keyer in loop. This is a constant current transistor circuit but keyable, so in idle it caused a constant current in the loop but I could use it to insert data into the loop from other devices. I used a high voltage darlington, avalanche rated, with a big Zener as protection, IIRC something like 300 volts. You need to let the spike get pretty high to allow for rapid and nearly complete unimpeded collapse of the selector magnet field. 


Peter

> On Jun 24, 2019, at 11:47 PM, Jim Haynes <jhhaynes at earthlink.net> wrote:
> 
> 
> 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."
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