[GreenKeys] Current loops

[John Nagle]

> From: "Steve Garrison" <steve.n4tty at gmail.com>
> 
> I haven?t had any issues with John Nagle?s device as far as RFI or
> anything else.  Although it will only handle one device not a several
> machines as my REC 45. Steve G./N4TTY

Right. It works by charging up 2uf to 120VDC and dumping that into the
selector magnet, then following up with a 15V sustain voltage. That's
enough to maintain 60mA through the selector magnet.

The 120V is only needed for about the first 2ms, to push through the
5H inductance of the selector magnet. To drive two Teletypes, you'd
need to double the power, and that would draw more 5V input power
than a USB port can supply.

Schematic:
https://raw.githubusercontent.com/John-Nagle/ttyloopdriver/master/board/images/schematic.png

> To: Mattis Lind <mattislind at gmail.com> Cc: greenkeys at mailman.qth.net;
> Jim Haynes <jhhaynes at earthlink.net> 120 volts at 60 mA is only 7.2
> Watts. I agree a switching regulator is more efficient but one has to
> watch for RFI.

Yes, you do. Board layout matters, and there are ferrite beads at
various places. It took a fair amount of SPICE analysis to get the
spikes down. Not just voltage spikes. It's important to avoid current
spikes on the source side, to avoid tripping USB port protection
circuits.

Mattis:
> I agree with this. You need the high voltage so that the electro
> magnet in the teletype react as fast as possible. But after this all
> the line voltage will be dissipated in the resistor. Using a linear
> current source will not make any difference. What I did was to make a
> switch mode current source. It will require considerably less power.
> There are some losses of course due to the switching process.
>  
> Some info is here, work in progress though:

> https://github.com/MattisLind/teleprinterHat

Nice. Classic boost topology with current feedback. This
is simpler than what I did. I wanted full power supply isolation
between  the input and output sides, which meant a switcher with
a transformer.  This was probably unnecessary.

Where does the "J2 current sense" input connect?

I see you have a 1K load on the keyboard side. That's something some
Model 15 machines need, if they have the RF suppression kit. That
has a capacitor across the keyboard, and you have to draw more
current on the keyboard side to avoid pulses getting stretched.
I didn't have that in V3.3 of my board, and one Model 15 needs it.

It looks like you can get all the parts for yours in through-hole.
That simplifies life for hobbyists. Makes sense.

			John Nagle