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

[info at drukknop.nl]

Well done, and in Kicad, my design tool of choice.

On 29-08-17 06:18, John Nagle wrote:
>     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
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-- 
Met vriendelijke Groet,

Simon Claessen
drukknop.nl