[GreenKeys] low Voltage Loop Supply

[ad7i]

I haven't done a circuit simulation, but I have built and tested the
circuit and made measurements with both digital and analog scopes.  With
respect to circuit simulation, MicroCap is now free (
http://www.spectrum-soft.com/download/download.shtm ) so anyone so inclined
can do simulations on all types of circuits.  There are good tutorials on
YouTube that describe how to use Microcap.

Below is a PDF of the schematic for my non-MCU version of an electronic
selector magnet driver.  Basically, when the TTY line goes from spacing to
mark, both Q6 and Q4 become active (ON).  At this point the effect of Q4 is
insignificant and Q6 is doing the work.  At this point the loop resistance
consists of the ON resistance of Q6, the fuse resistance, the resistance of
the selector magnets and the current sense resistor (R1, 10 ohms) - all of
that totals about 80 ohms.  The voltage source is 36 to 48 VDC.  Because
the current that flows through the magnets also flows through R1, the
instantaneous voltage across R1 is directly proportional to the
instantaneous current through the magnets and thus is directly proportional
to the instantaneous magnetic flux produced by the selector magnets.  This
circuit, like all R-L circuits, will have an exponential current rise, and
the final (asymptotic) current of this circuit will be about 350 mA.   But
the circuit will never allow the selector magnet current to get anywhere
near 350 mA.  Instead, U2A is set to trip when 600 mV is across R1, which
is also when 60 mA of current is flowing through R1 (and thus 60mA through
the magnets).  This means that this circuit is operating on just the very
first part of the exponential curve, and that part of the exponential curve
is essentially a straight line.  So when Q6 goes active, the current
through the magnets rises in a straight-line linear way at a rate of about
17 mA per millisecond.  At about 3.5 ms after the TTY line went from space
to mark the instantaneous current through the selector magnets is about 60
mA and the voltage across R1 is about 600 mV.  When U2A "sees" 600 mV
across R1, it will clear the latch formed by U1C and U1D and will shut off
Q6, but Q4 remains active.  The magnets are still at 60mA but now the loop
circuit consists of a 12V DC driving voltage and a loop resistance of about
400 ohms (330 from R8 plus 66 from the magnets) so the magnet current falls
down to 30 mA and falls with a classic exponential curve because in this
case the final (asymptotic) current of 30 mA is reached.  Note that once
the magnet was fully pulled in with 60 mA of magnet current, only about 20
mA is required to hold it active, and thus the 30 mA is more than adequate
to hold the magnet.

When the TTY line goes from Mark to Space, Q4 remains active for a few more
milliseconds, as determined by U2B, C2 and the 100K trimmer, and when the
timeout has expired (~2.5ms), Q4 is cut off, and the magnet current goes
(almost instantaneously) to zero.  Normally, the trimmer is adjusted to
extend the mark time about 2.5 ms, to compensate for the time on the front
end when the magnet current was rising over a time period of about 3.5 ms.
I'm assuming (guessing) that the magnet is effectively pulling in at around
the 40 mA point (but it could be less).  Note that when Q4 is cut off, and
the magnet current is interrupted, a big voltage spike will be produced
across the magnet coil by the collapsing magnetic field.  The purpose of
the 1nF across the magnets is to limit the instantaneous voltage spike so
as not to produce (significant) clicking RFI, but the 1nF is not enough.
It just stops the step function, but the voltage across the magnets would
still go to a very large value (hundreds of volts) that could damage Q4
and/or Q6.  The purpose of the series combination of 470 ohms and 100nF
across the selector magnets is to limit the voltage excursion to something
reasonable, like 150 volts.  Because the R and C along with the L of the
selector magnets forms an R-L-C circuit, that circuit does "ring" when
current through the magnets is interrupted, but because of the series R the
oscillation dies out in about one cycle (3 mS or so if I remember right).
Note that these snubber values are right out of the ST-6000 schematic.

I built and tested this non-MCU version of the schematic on perfboard.  But
I now have PC boards back from the board shop for a version using a
microcontroller and that allows me to remove a lot of the discrete circuit
components.  The MCU version is not tested yet (I've only tested the step
up voltage converter) but I hope to get back to that project in a few weeks.

Paul - ad7i





On Thu, Mar 24, 2022 at 6:33 PM Jim Haynes <jhhaynes at earthlink.net> wrote:

> I've seen some published circuits for a selector magnet driver that
> takes advantage of the constant-current characteristics of a transistor
> to make the loop current rise faster than it would with a simple
> resistor.  What I've been wishing for is someone who knows how to
> run one of the circuit simulation programs to see just how fast it
> is, compared to the resistor.  Seems like it was said to work
> satisfactorily with a 45 volt supply.  See for example "The Modern
> Teleprinter Local Loop" by Frank Merritt, QST January 1972 page 40
> and feedback in QST March 1972 p. 57.
>
> Also I was told by Walt Zenner, the late V.P. for R&D at Teletype, that
> the original purpose of the holding magnet selector (he inventd it)
> was to allow TWX switchboards to operate with only 48V on the cord
> circuits.  There was concern about switchboard operators accidentally
> touching the bare plug tips (which is hard to avoid in a manual
> switchboard) and being exposed to 120V which could be deadly.  So if
> there is an experimenter among us perhaps you can take rangefinder
> readings on both types of selectors when operating at reduced loop
> voltages.  Of course in Walt's day they didn't have transistors, so
> couldn't use the constant-current properties.
>
>         ---
>
>         "Ya can argue all ya wanna, but it's dif'rent than it was."
>         "No it ain't! No it ain't!  But ya gotta know the territory."
>                 Meredith Willson, The Music Man
>
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