[GreenKeys] 60ma current loop driver board powered from USB port

[John Nagle]

    As many of you know, I've been working on building a 60ma current 
loop driver board that's powered entirely from a USB port.  I had
a working one a few months ago, but it was a bit underpowered and would
not drive some machines.

    Now I have a new design.  This one has plenty of power.  I've tried
it on a Model 15 and two different Model 14 machines, one wired for a 55 
ohm selector and one wired for a 220 ohm selector. They
all type without error.

    Here are all the design files:

      https://github.com/John-Nagle/ttyloopdriver

This isn't the final design; I'm going to do another round because
the USB overcurrent protection isn't quite right yet, and occasionally
trips due to noise spikes.  I'll fix that in the next board.
The current board will run with a jumper soldered in place to
bypass that.

    This board needs no external current loop supply.  It generates
60mA at 120V to drive the selector magnet while only drawing 300
mA from the USB port.   This  is possible because you only need
a high voltage for the first 2-3ms of each bit time; after that,
12V is enough. The board charges up 2uf to 120 V (it was 1uf in
the previous board, and that wasn't quite enough), and at the
beginning of a MARK period, dumps that into the selector magnet.
Then a sustain supply takes over to keep the selector magnet pulled
in for the remainder of the cycle.

    This runs cool. It's an efficient switching power supply, and
it's not throwing power away.  A laptop running on battery
power can drive a Teletype with this.

    The board is 75mm x 120mm, including two 1/4" phone jacks and
a USB connector.  This fits a standard aluminium slide-in case.

    Anyone is free to make this.  The design files are public.
I'll make some blank boards available, but you have to be able to
solder 0.5mm spacing surface mount to use them.  BOM cost for this
is about $50.  All parts are from DigiKey except for a transformer
from Coilcraft.


				John Nagle