[GreenKeys] Arduino Code/Speed Converter

[ad7i]

I have found (sample of 1) that the M15 is much more tolerant of low
voltage, low resistance, drive than is the M28.  Bill Henry of HAL told me
years ago that the inductance of the M28 magnets is much larger than the
15, with the 28 being 2 Henrys.  My measurements for my one M28 show the
magnet inductance at 1.7 Henry, which is still a big number.

A model 28 selector magnet will pull in with as little as 6 volts.  The
reason that classic loop supplies are ~150V 2500 ohms is that the time
constant of an R-L circuit is in proportion to L/R.  If L gets bigger the
time constant gets longer.  To shorten the time constant, to make the
magnet action fast and "snappy", then R must get larger.  As R gets larger
then the driving voltage needs to get larger to maintain the same loop
current.  To get the time constant of the selector magnet to be under 1 mS,
the circuit resistance needs to be at least about 1.5K ohms.

For comparison purposes, when using the classic 170 VDC 2800 ohm loop
circuit, the selector magnet current takes about 600uS to go from zero to
30 mA (final value is of course is 60 mA).  Also when tested in a static
situation (sample of one, model 28) the pull in current for the selector
magnet is about 24 mA and the release current is about 12 mA.  I also found
that my M28 ran well with only a 30 mA loop (but that 30mA had to come from
a high voltage high resistance loop -- 120V 4000 ohms in this case), but I
didn't try adjusting the range control with the 30mA loop to see if I had
much latitude at that 30mA loop current.

In my 36 volt electronic magnet driver the system has three modes, (mode 0)
off, (mode 1) 36 volts with zero ohm loop and then (mode 2) 36 volts with
1200 ohm loop.  During mode 1 the resistance isn't really zero, it's the
sum of the magnet resistance (66 ohms), a 100 mA fuse (5 ohms) and a 10 ohm
current sense resistor (the MOSFET switch is less than one ohm when ON so I
ignore that) for a total of 81 ohms.  When the serial input to the magnet
driver goes from SPACE to MARK the magnet driver enters mode 1.  When in
mode 1 the magnet current has the classic exponential decay toward it's
final current value (36V/81ohms or 444mA), but from 0 mA to 60 mA we are so
low on the exponential curve that the curve looks like a straight-line
sawtooth rising current, rising at a slope of 15mA per miillisecond.  As
the loop current rises so does the voltage across the 10 ohm current sensor
resistor.  When voltage on that sense resistor reaches 600 mV we then have
60mA through the sense resistor (600mV/10 ohms equals 60 mA), and thus also
60 mA through the selector magnets) the device switches to mode 2 and the
magnet current then decreases at an exponential decay rate to it's final
value of 30 mA (36V/1200ohms) and stays at 30 mA until the serial input
goes SPACING.  When the input goes from MARK to SPACE a timer extends mode
2 from zero to 5 extra milliseconds (controlled by a pot) and then enters
mode 0 which causes the magnet driver to disengage and the magnet current
goes to zero at a rate determined by the snubber components (usually about
1 mS).  The purpose of the timer at the end of Mode 2 is to compensate for
the time it took at the start of Mode 1 to get the magnet current from 0 to
45 mA (about 3 mS).  I could probably replace that timer pot with a fixed
resistor that provided a fixed 3 mS delay and call it close enough.

This setup seems to work fine with my model 28 at 100 WPM as well as my
model 15 at 60 WPM.  The useful adjustment range of the "range" control on
the M28 is about the same with the 36 V electronic magnet driver as it is
with a 120V  2000 ohm loop.

Paul, ad7i





On Fri, Jan 24, 2020 at 9:52 PM Jim Haynes <jhhaynes at earthlink.net> wrote:

> On Fri, 24 Jan 2020, Gil Smith wrote:
>
> > 36 volts is probably a good choice also.  The purists may say it needs
> to be
> > over 100V to burn oil off contacts and such, but if a lower voltage loop
> is
> > working for you go for it.
> >
> A reference on this topic is Western Union Technical Review 15:4, October
> 1961, p. 149.
>
> Also Western Union Technical Review 5:1, January 1951, p. 32.
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