[GreenKeys] LTSpice

[John Nagle]

> Date: Fri, 19 Jul 2013 02:19:12 -0500
> From: Jeffrey D Angus <jdangus at att.net>
> To: greenkeys at mailman.qth.net
> Subject: [GreenKeys] LTSpice
> 
> If you've got a flair for design, I highly recommend this program.
> <http://www.linear.com/designtools/software/>
> 
> I was playing with it tonight optimizing a snubber to go across
> the loop keying.
> 
> looks like 0.22 uF and 22 ohms in series will keep things civilized
> with adding too much distortion.

   That's good to know.  I've been using 1uf and 100 ohms.  Same
R/C ratio.  Note that you need at least 500V ceramics for that
snubber; there's a big voltage spike when the selector magnets
dump.

   I once designed a 60mA Teletype loop supply in LTSpice, intended
to be run off a USB port.  I never built it, though; the parts are all
very small surface mount and I don't have a full surface mount
soldering station.  The basic idea is to use a camera photoflash
switching power supply to charge up a cap during SPACE.  At the
SPACE/MARK transition, the cap is dumped into the selector magnet.
After a few milliseconds, the sustain supply takes over to maintain
the current.  You really need 120VDC only for the first few
milliseconds after turn-on to maintain 60mA.  The high voltage
is needed only to overcome the inductance of the selector magnet.
Model 33 Teletypes use a pulse system something like that, but
they charge up the cap from a line voltage.  That's what that
PC board with the big power transistor in a 33 does.

    This approach eliminates the usual loss of 95% of the energy
in the ballast resistor, and could potentially be run from 5V 250mA
taken from a USB port.

				John Nagle