[TowerTalk] TowerTalk] Vacuum Variable Over Torque Protection?

Roger Parsons ve3zi at yahoo.com
Sun Sep 14 20:01:15 EDT 2008

I had a similar requirement for a motor operated vacuum variable in
the auto-tuner at my remote station, although my motor only produces
just enough torque to turn the capacitor. After lots of thought I
could not think of an easy mechanical solution (eg limit switches).
What I did in the end was to notice that the motor current rose 
sharply under stall conditions. I then put an over-current detector in 
the power supply which tripped it if that happened - of course it 
never should. I later found that it did happen when the tower was 
struck by lightning (quite frequently, it's tall) as it would confuse 
the auto-tuner and drive it to one end or the other. I cured that 
by making the tuner only operative when the transmitter ptt is 
activated. Seems OK.

A better solution might be to somehow gang a multi-turn pot to the 
shaft of the capacitor and use electronic trip points.

73 Roger


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