[GreenKeys] New 60ma current loop driver board design

Thu Oct 27 21:57:27 EDT 2016

"This has roughly the same functionality as my previous board,
but does not require an external 120VDC power supply.  It
generates the selector magnet power from the USB port power."

This comment threw me for a moment, until I read on, and came across this
caveat:

"  It works in LTspice, but may take some tuning
in the real world.  More later."

So you're counting on the limited duty cycle to bridge the gap between the
120 @ 60mA (7.5W) and the USB power of 5V @ 250mA (1.25W)? That seems like
a trick, but I can see how it might work - but the loop length will have to
be fairly short, eh?

Is there any reason you're not using the full 2.0 spec, for 500mA? I'd
think that would really help to close the power gap.

On Thu, Oct 27, 2016 at 7:31 PM, Ed Sharpe Archivist for SMECC via
GreenKeys <greenkeys at mailman.qth.net> wrote:

> Absolutely great and amazing John !
> did  not  know about  option to get   more current out of the usb that
> way!
> Keep up the great  work!
>
> Ed#
>
> In a message dated 10/27/2016 8:34:24 A.M. US Mountain Standard Time,
> nagle at animats.com writes:
>
>     I'm working on a new circuit board design to drive
> 60mA current loop Model 14 and 15 machines.  Here's
> what it looks like:
>
> https://github.com/John-Nagle/ttyloopdriver/blob/master/
> board/ttydriver01.jpg
>
> The schematic:
>
> http://www.aetherltd.com/public/misc/loopdriverv1.0.png
>
> This has roughly the same functionality as my previous board,
> but does not require an external 120VDC power supply.  It
> generates the selector magnet power from the USB port power.
>
> No difficult surface mount soldering is required.  It's
> all through-hole parts, except for one surface-mount
> transformer, and that has big pins.
>
> I'm having some boards fabbed now as a test.  Waiting for
> delivery from Shentzen.
>
> Here's full documentation for the project:
>
>     https://github.com/John-Nagle/ttyloopdriver
>
> If you have KiCAD (which is free) installed, you
> can look at the detailed design, bill of materials,
> etc.  BOM cost is about $20 - $25.  From that info,
> anyone can build this. But hold off until I have
> one working; some components may change.
>
> How it works:
>
>     U3 is a CP2102 USB to serial converter. This
> is reprogrammed to talk 45 baud, and to ask for
> 250mA of power from the USB port.  Once the USB
> port has come up and the other end has agreed to
> supply 250mA, ENABLE goes high and U2 turns on power.
>
>      U5 is a 555 timer, set up to generate 100KHz
> at 25% duty cycle. This drives the boost converter
> composed of Q1, T1, D2, and 1uF cap C2, which
> is pumped up to 120VDC during SPACE periods.
> Zener D4 limits the voltage to 120V.
>
>      On a SPACE to MARK transition, the opto-
> isolator U4 turns on, and C2 is dumped
> into the selector magnet output, providing the
> energy needed to get 60mA through the magnet
> and pull it in fast. The boost converter continues
> to run, but under load, can only produce about 3-4V
> continuous at 60mA into a 55 ohm load.  That
> provides the sustain current to hold in the selector
> magnet for the remainder of the MARK period.
>
>     On MARK to SPACE, power to the selector magnet
> is turned off by U4, and the capacitor starts to charge
> up for the next cycle. This takes about 18-20ms, just
> in time for the next MARK period in the shortest case.
>
>     It works in LTspice, but may take some tuning
> in the real world.  More later.
>
>                 John Nagle
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