[GreenKeys] Pulling vs Holding selector magnets

[Harold Hallikainen]

As mentioned by others, a holding magnet has a cam that pushes the
armature up to the magnet pole pieces during each bit. The cam releases
the am=armature slightly before the center of each bit time. The bit is
sampled slightly later. If the armature stayed stuck to the pole pieces,
the bit is a mark. If it falls away, it is a space.

To me, the sure method of determining whether you have holding of pulling
magnets is to watch or feel the armature when the machine is running open.
The armature will move back and forth if you have holding magnets. It will
not if you have pulling magnets.

For pulling magnets, the coils are in series and require 60 mA. For
holding magnets, the coils can be put in parallel for 60 mA loop (putting
30 mA through each coil). If you put the coils in series, you can operate
with a 20 mA loop.

I really like a 60 mA loop since everything can be used on that.

I only recently heard of the use of holding magnets with a lower loop
voltage. That is a clever idea! I THINK that would only work if the coils
are in parallel (60 mA loop). Changing the coils to series would increase
the inductance to four times the original inductance (double the number of
turns). Note that when they are in parallel, the inductance is not half
that of a single coils since they are mutually coupled. Here, the parallel
winding is the same number of turns with thicker wire.

Anyway, since the Time Constant T = L/R, having the holding magnets in
series for a 20 mA loop results in a longer time constant than when in
parallel. So, I SUSPECT the lower voltage loop with holding magnets was
only on a 60 mA loop, not a 20 mA loop.

Harold
https://w6iwi.org