[GreenKeys] New 60ma current loop driver board design

[COURYHOUSE at aol.com]

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.jp
g

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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