[GreenKeys] M19 Selector Magnet - magic smoke?

[Harold Hallikainen]

Back in high school (a little over 50 years ago), I discovered T=L/R. I
had a couple mile long teletype loop to a friend's house. I could get 60
mA with about 12V, but the printout was garbage. With the low loop
resistance, it took too long for the magnetic field to build up in the
selector magnets. So, I went to a high voltage supply with series
resistor. The high R (10k or so) made the time constant short enough to
give reliable copy.

As shown at https://w6iwi.org/rtty/, I have the series resistor mounted on
the 152950 loop supply. I did add a fuse holder on the AC line side of the
transformer. But, I think it would be quite rare for the resistor to fail
to a short (it is a large wire wound resistor), so I don't think a loop
fuse is required.

The TU-170, however, uses a bipolar transistor in an active current sink
circuit (
https://w6iwi.org/rtty/tu170/Flesher%20TU-170%20Terminal%20Unit.pdf#page=18
). A common failure of a BJT is an emitter to collector short, so a fuse
here makes sense (and there is one, the 0.125A fuse at terminal U).

I have a bunch of scope photos at https://w6iwi.org/rtty/tu170/ showing
the varying effects of the selector magnet inductance based on the
armature position. The inductance is a lot higher when the armature is
against the selector magnet pole pieces. And, for a holding magnet, it is
always against the pole pieces on a mark to space transition.

Harold




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