[GreenKeys] An advanced approach to driving a 60mA Teletype from a USB port

[David Ross]

John -

  You mention:
> If you model the
> standard "120V supply through a 10K resistor" approach to driving a Model
> 15
  Common 60mA loops use a 120 VDC supply through a 2k Ohm dropping 
resistor.  1/5 the resistance provides 5x the di/dt of your 10k loop 
resistance.

Dave Ross    N7EPI




John Nagle wrote:
>     Some time ago, I did a design for a Model 15 Teletype
> driver that would be powered entirely from a USB port, eliminating
> the need for a 120-130V power supply. I never built this, because
> the parts required are surface mount only and I don't have the
> tools to solder surface mount.  But I thought I'd share the approach.
>
>     Here's the LTSPICE file:
>
> http://www.aetherltd.com/public/flashsupply6.asc
>
>     The free version of LTSPICE
> ("http://www.linear.com/designtools/software/ltspice.jsp") will read this,
> and you can run the simulation.
>
>      Graph the current through L3, the selector magnet.
> (Point the cursor at a device on the schematic while the
> simulation is running, and the icon turns to a clamp-around ammeter.
> Click and you get the current added to a graph.  Pointing to a wire
> gets you a voltage on the graph.) You'll see that it delivers about 60mA through
> the whole MARK period.  Look at the voltage at the top of L3, and
> you'll see that it goes up to 120V at the start of the MARK period,
> then drops to 2 to 3V, still delivering 60MA.
>
>      What's going on here is that the LT3484-1 IC, which is a photoflash
> controller form of a switching power supply, charges C2 up to 120V
> during SPACE periods.  L1/L2, which is a stock pulse transformer, is
> doing the step-up.  On a transition to MARK,
> S1 closes, dumping C2 into the selector magnet, providing the
> necessary 120V to overcome the 4H inductance of the magnet coil.
> LT3080 is just a linear regulator, providing 3.3V to sustain the
> selector magnet once the initial pulse has pulled it in and
> C2 has discharged.  You really need 120V only for the first 1ms
> or so of each MARK period.  Once the current has stabilized,
> the voltage needed is only 60mA * 55 ohms = 3.3V.
>
>     The design needs a little work; the inductive kickback from
> the selector magnet needs to be snubbed without stalling the
> switching power supply.  A full system would need a USB to
> serial converter chip, a keyboard input side, and motor control.
>
>     The design is a clear demonstration that you could run a Model 15 Teletype
> from power taken from a USB port. The circuit never draws more than
> 250MA at 4.75V, (the voltage you're guaranteed from a USB port) and that's only
> half the current a powered USB port can deliver.   If you model the
> standard "120V supply through a 10K resistor" approach to driving a Model
> 15, you get waveforms with current rise times no better than this provides.
> So this approach should work.
>
>     Note that this is designed for 45.45 baud.  At higher baud rates, there
> may not be enough SPACE time to recharge the capacitor.  Also note that
> this provides only enough energy for one 55 ohm 4 H selector magnet.  It
> won't power multiple machines on a loop.
>
>     This is how to approach the problem using modern technology.
> The resulting unit would be tiny.  None of the parts are expensive or difficult
> to obtain.  Power consumption is around 1 watt.
>
>                 				John Nagle
>
>
>
>
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