[GreenKeys] Current limiting

[Joe Stevens]

The voltage makes it easier for contacts, such as in a keyboard, to 
work in the presence of oil and dirt.  For instance, it was very 
popular to use 5 volts on a model 33 keyboard when hooking it up to a 
computer way back three decades or so.  They were very unreliable at 
that low voltage but worked flawlessly at 130 volts.  Telephone 
relays work best with 48 volts on their contacts, so that sounds like 
a good voltage for a TTY too.

I don't know if anybody already mentioned this but on the selector 
magnet: it must be able to release quickly.  This means the reverse 
EMF, or voltage spike, upon release must not be hampered.  It must be 
allowed to spike, perhaps to 600 volts or more, for a quick 
release.  If you want a slow releasing relay, just short the 
coil.  This high-voltage spike is what a keying transistor has to 
survive.  Any TV horizontal output transistor or IC should be able to 
handle it.

You can demonstrate this high-voltage spike by putting your fingers 
across the interrupter contacts on a buzzer powered by a single D 
cell.  It will shock you noticeably with only a 1.5 volt supply.

      ...j0e  WL7AML

>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?