[GreenKeys] HRS312 TTY Interface

[Jeffrey D Angus]

John Nagle wrote:
> I've been chatting with some people on sci.electronics.design about
> alternative drive circuits.  I'm trying to eliminate the need for
> a 120VDC supply.  It only takes 3.3v to maintain 60mA through 55 ohms;
> the 120VDC supply is only needed for the first 2ms or so of each
> bit time to overcome the 4H inductance of the coil.  A design
> that only requires a +5 supply with a boost supply to make the
> small amount of 120VDC required would be a neat solution to the
> problem.  The idea is to charge up a capacitor during the 95% of
> the time that kick isn't required, then discharge it during turn-on.
> I've been playing with SPICE models of this.
A similar and unrelated technique was mentioned back in the early '50s
for driving model railroad switch machines. (And which I've found very
useful for dealing with solenoid access door locks etc.)

Although it didn't use different supply voltages. It used a series 
resistor to
limit the holding current of the coil, and a shunt capacitor (large 
value) to
supply the initial current surge to actuate it.

*grins* I knew sooner or later I'd be able to sneak something in about
model railroads.

Jeff-1.0
wa6fwi