🚀 go-pugleaf

     We recently had a fluorescent lamp ballast go bad at our
house which got me wondering about what actually happened.
The lamp in question is an instant start type, using a
12T/20CW tube.  This tube has two pins at each end and can
operate in either the instant start-type fixtures or the old
style ones that use the starter.
     The trouble started innocently enough.  The lamp didn't
light when switched on.  One could see a flash in the tube
when turning it off, but there was no other light at all.  I
had a spare tube which had, at one time, been in an old
starter-type fixture so I decided to use that one until we
could go out and buy a new one.
     The light came on with the installation of the spare
tube and we went on about other things.  A few minutes later,
my wife said that she had seen a flash.  We weren't really
sure, though, so we didn't take any action.
     About five minutes later, it was obvious that something
was terribly wrong.  The ballast was sizzling and filling the
house with the aroma of melted pitch and burning paper.  We
are, now in the market for a new fixture.
     Just for fun, I did an autopsy on the original
fluorescent tube and the spare.  The original one shows that
both cathodes are open, (infinite resistance at each end.)
The spare tube that had bin in the ballast when it smoked
shows one open cathode and one which is still good, (about
4.5 Ohms.)
I am not really sure how the instant start ballasts work, so
I am wondering if the presence of one open cathode might have
actually been what smoked the ballast since it hadn't smoked
until I installed the spare tube.  One open cathode would
cause the load to be half-wave rectified DC instead of the
full-wave load which normally occurs.
Since the ballast got severely overheated, it must be
replaced, but I would like to know what really happened.
Maybe saving the old tube turned out to be false economy.

Martin McCormick WB5AGZ   Stillwater, OK
O.S.U. Computer Center Data Communications Group