[GreenKeys] Teletype DRPE tuned reed mechanism

[Jim Haynes]

Yeah, I don't know if there ever was a CRPE - which would have been
pronounced "crappy" so maybe that's why there was not one.

The idea of the DRPE goes back to 1959 or so.  You have a steel reed
that is is held up by an electromagnet.  Cut off the current and the
reed tip swings down and pushes the punch pin through the tape.  Then
the reed tip swings back up near the electromagnet and you turn the
current on and grab it.

One problem is that a lot of energy is dissipated in the process, so
you need a pretty strong current to pull the reed back up.  Also when
you turn the power off the reeds fall to the mid position and put the
punch pins into the paper.  So it takes extra power when you first turn
it on to get the reeds up against the magnet poles.  A third problem that
was not discovered until fairly late is that eddy currents heat the
reed close to red hot.  So they had to mill out slots and put in ferrite
slugs to keep the eddy currents down.  And of course there is the
problem of the inductance of the electromagnets that slows current 
buildup, the same problem we have with selector magnets in printers
that requires the use of high voltage and high resistance to get
the rise time down.  And there is the problem of the thing being
awfully noisy with the reeds beating against the pole pieces.  Like
a one-cylinder gasoline engine (per reed) with no exhaust muffler.

The original demonstrator simply used high voltage and high resistance
and used an awful lot of power, but it worked.  The first production
used a circuit that stored energy in inductors - patent 3,191,101
by Al Reszka.  A later driver by Gene Sullivan, apparently not
patented, used high voltage and high resistance to pull the reeds up
fast, but then switched to a much lower voltage to hold them up.

Look at that patent to get a good idea of how it all worked.
www.pat2pdf.org



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