[GreenKeys] Current loops

[Harold Hallikainen]

>
> 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.
>

I don't know how it can be done with a low voltage. E=L*di/dt. If you need
a certain current slew rate (di/dt), and you have a certain L, you need a
certain E (voltage). In our loop, that full voltage is across the magnet
when the keyboard closes, then tapers off the voltage determined by the
resistance (I*R). The full voltage is needed only for a short time, but it
is still needed.

Back when I built my neighborhood Teletype loop in high school, I found I
could get 60 mA with a low voltage supply (I think around 12V), but the
text was garbage. Going up to higher voltage with series resistance fixed
it.

That's what I think, though I could be wrong!

Harold



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