[GreenKeys] Just about all you need to know about Selector Magnet Drivers

[WA5CAB at cs.com]

Note that the screed says it will work with M15 holding magnet selectors.  
The instructions say (on the 8th page of the scan) not to use it with 
pulling magnet selectors.  I didn't see an explanation of why not.

In a message dated 09/30/2012 13:59:08 PM Central Daylight Time, 
nagle at animats.com writes: 
> >From: Don Robert House <k9tty at dls.net>
> >Date: Fri, Sep 28, 2012 at 3:54 PM
> >Subject: Just about all you need to know about Selector Magnet Drivers*
> >
> >Attached please find Teletype Corporation 50042S
> >This covers all information for the 177010 Selector Magnet Driver
> >used with the Models 15, 19, 20, and 28 machines.
> 
> >http://www.navy-radio.com/manuals/tty/sel_mag-15-19-20-28.pdf
> 
>     That's interesting.  It's useful to have an actual Teletype
> Corporation spec with waveforms for driving a 15 selector magnet.
> In the past, I've looked at designing more efficient drive
> circuits that wouldn't require such a big power supply and
> ballast resistor.  You really only need 120VDC for the first
> 1 or 2 MS of a space to mark transition, to overcome the
> inductance of the coils.  After that, the sustain voltage is
> very low.  (0.60A * 220 ohms = 13.2V).  The circuit shown
> does that.  They apply the full supply voltage through only
> a 27 ohm resistor (R11) and Q5, so for a brief period the
> selector magnet is almost directly connected to the power
> supply.  This would burn out the selector magnet if that
> state continued.  But once the selector magnet's inductance
> has been overcome and the voltage at the selector magnet
> has neared the 120 volt rail, Q5 cuts out and Q4 turns on.
> Then there's a 700 ohm (R10) resistor in series with the 120V
> supply, limiting the current to 60mA.  I would have expected
> R10 to be 2K, so I'm missing something.
> 
>     It's common today to drive relays and solenoids with circuits
> that supply much more voltage during turn-on, but this is the first
> time I've seen it for older Teletypes.
> 
>     They do maintain 60mA through the entire MARK period,
> so they're being conservative about holding current.  But they
> accept a slower rise time than I'd thought was necessary.
> 
>     It's interesting that the circuit has an active load dump
> (CR9 and Q2) to dump the energy in the coil on a MARK to SPACE
> transition.  (This is the effect which causes arcing on keyboard
> contacts in local loops.)  I'd been trying to design that entirely
> with passive components, but their approach is better.
> 
>     Much of the complexity of that circuit comes from the limitations
> of early transistors, which didn't handle either high voltages or
> high currents well.  It's a very '60s circuit.  Everything is somewhere
> in the middle between the power rail and ground, like tube circuits.
> The selector magnet connections have active components on both sides.
> Today, we tend to have active components connected to either
> GND or Vcc and outputs relative to ground.
> 
>                 John Nagle
> 

Robert & Susan Downs - Houston
wa5cab dot com (Web Store)
MVPA 9480
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