[GreenKeys] Model 15 Selector Magnets

[Harold Hallikainen]

My thoughts on this...

For holding magnets, the armature is pushed to the pole pieces by a cam
before the loop current is sampled. The cam then releases the armature. If
it falls away, this is a space. If not, this is a mark.

It takes considerably less current for the selector magnet to hold the
armature in place than it takes to pull the armature in. So, with pulling
magnets, you pace the coils in series for use with a 60 mA loop.

With holding magnets, fewer ampere turns are required to hold the
armature, so the coils are put in parallel for a 60 mA loop (30 mA through
each coil) or series for a 20 mA loop (20 mA through each coil).

The hold versus pull-in current is interesting. It is a form of
hysteresis. You have to increase the current above some amount to pull in
the armature, but reduce far below that to release the armature. I used
this years ago to make latching relays in a broadcast transmitter control
system. I put two resistors in series with a relay coil, one between
positive supply and the coil, the other between the other side of the coil
and ground. The current with both resistors in place was below the pull in
current and above the release current. If I shorted the resistor to
ground, the current would go above the pull in current, and the relay
would pull in. Removing the short returned the current to the hold
current, but above the drop out current, so the relay stayed pulled in.
Shorting the other end of the coil to ground recued the coil current to
zero, so the relay dropped out. We ended up with a latching relay where
one closure to ground would latch it, and the other closure to ground
would unlatch it.

Another interesting thing about selector magnets is how the inductance
changes dramatically based on whether the armature is contact with the
pole pieces or not. This is evident in the scope photos at
https://w6iwi.org/rtty/tu170/ .

Harold
https://w6iwi.org



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