[GreenKeys] energizing relays and questions on voltage and current

[jhaynes@alumni.uark.edu]

First, congratulations on acquiring a rare and beautiful artifact.  Enjoy
the congratulations you get from here; if you get it running your 
household will tell you it belongs in the woodshed, if not in the garbage,
because of the noise it makes.

What you want to know is the current to operate those electromagnets, not
the voltage.  Of course if you can measure the resistance accurately you
can calculate one from the other.  But the point is they were operated in
series circuits with high voltage and high resistance, to get fast rise
times on the current.  If you use just enough voltage to operate things
the response will be too slow.  Figure the design operate current to be
about twice the minimum current to make an armature pull in.  It will
probably turn out to be in the 50-100 ma. range.

Now it has been over fifty years since I saw one of those things in 
operation, and I was pretty young at the time, but here's how I remember 
it working.  You'll notice that one electromagnet drives the print hammer 
that pushes the tape into contact with the print wheel.  The other 
probably has a permanent magnet to make it polarity sensitive and
drives a lever that causes the typewheel shaft to rotate as the lever 
moves from side to side.

In the idle condition they transmit polarity reversals at a rate of
about 20 Hz.  The print magnet is too massive to respond to these
reversals, but the stepping mechanism causes the typewheel to rotate.
A thing that looks like a worm gear on the shaft moves an arm to the
side until a pin on the shaft fits into a notch on the arm, stopping
rotation.  At this point all the typewheels in all the printers in the
system are synchronized.

To print, they first interrupt the reversals so that the print magnet
operates.  This moves the stop arm out of the way of the pin.  Then
they resume reversals until the desired character is over the print
hammer.  Another delay in the reversals allows the print hammer to
operate, printing the character, and then the reversals resume until
the next character to be printed is over the hammer, and again delaying
the reversals prints that character.  And so on.  All the typewheels in 
the system should remain synchronized while printing goes on.  But if
one of them should get out of sync for any reason it will get back in
when there is a pause in the printing that lasts long enough for the
stop arm to be engaged again.  Since the print magnet is not polarized
it can operate on current in either direction, provided the current
continues long enough in the same direction.

I guess the trick now is to program a microcontroller to generate the
current reversals and pauses.