[GreenKeys] Model 28 Drive Voltage

[jhhaynes at earthlink.net]

The standard circuit for selector magnets is a 60ma loop, using 120 volts
and 2000 ohms in series with the magnet.  The reason for this is that the
magnet has to operate rapidly.  The time constant of an inductive circuit
is L/R, so you need to make R high to make the time constant small.
I'm not going to try to write out the expression for the current in 
plain ASCII.  The current starts out at zero and builds up to a final
value of V/R at a rate controlled by L/R.  If you try to operate at a
lower voltage and lower resistance the range will suffer.  Of course you
can tolerate that if you are driving the machine with perfect signals.  

You can do better than this with a pentode or a transistor.  These devices
have a constant-current property that can make the current build up 
somewhat faster.  But with a tube you still need about 120 volts because
tubes drop so much voltage.  There was an article in QST on a transistor
circuit, but I can't lay my hands on it right now.  I believe it used
about 60 volts.

Models 32, 33, and 35 use a low-voltage high-current selector magnet.
There seem to be a few Model 28 machines and even Model 15 machines
equipped with such magnets.  These use a transistor constant-current
circuit and a 20 volt 500ma. loop.  If you could find a set of Model 35
coils you could put them on a 28.



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jhhaynes at earthlink dot net