common form of fluorescent light used in Australia (240V supply) is a 4
ft tube and it is associated with a glow bottle or electronic starter
and a simple iron ballast inductor that limits the current drawn by the
tube. Tradesmen who change a lot of fluorescent tubes tell me that
shorted ballasts are a common occurrence and when confronted by a non
working fitting they suspect a shorted ballast and first try an
old tube. When a ballast is shorted there is nothing to limit the
current and thus a new tube will be instantly destroyed together with
the starter. This destruction can be dramatic and may cause the
circuit breaker to trip and smoke to be produced.
below is a tester that may enable shorted ballasts to be detected. The
description is only of the CONCEPT. A WORKING prototype HAS NOT been
made and HAS NOT been used practically. The construction of mains
equipment should only be undertaken by qualified persons.
arrangement of 6 x 3.3K in series with the 12V 50ma globe produces a
bright light at 240V and a much dimmer light at lower voltages.
OKO6 electronic starter in common with similar starters provides a
short circuit for the first 2 seconds after power is applied then a
virtual open circuit from then on until the power is removed.
During this 2 second period a significant load is drawn through the
arrangement of 6 x 330R resistors the ballast and the external
(dummy shorted ) starter. If the ballast is shorted then the full mains
voltage will be available to light the globe. If the ballast is not
shorted it will reduce the voltage to about 170V and thus the light
will be dim. The 6 x 330R resistors have to dissipate about 100W
with a shorted ballast but the time is only about 2 seconds so the
temperature rise is only about 3 degrees. The thermal fuse protects the
device if there is some fault condition. The resistors in series
with the light globe dissipate about 11W. It is thought that this would
not lead to a excess temperature rise but thus has not been confirmed.
The addition of a second thermal fuse positioned near these resistors
is recommended. The resistors have to dissipate high power for
short periods and a wire wound type that is specified at "five times
the power for 5 seconds" should be used. When the OKO6 turns off the
external ballast being an inductor will attempt to keep the current
flowing and thus produce a very high voltage spike. This spike is
limited by the MOV.Concept testing.
A circuit board (see pdf)
was designed to hold the components and to fit inside a polycarbonate
tube of 28.6mm diameter. The external diameter of such a tube was
31.8mm and it was hoped that this diameter would be small enough to allow
the dummy tube to be inserted in common light fittings although this
has not been confirmed.
is the prototype set up on a breadboard. The dummy starter (short
circuit) has a tag on it so it so it will not be forgotten after a test.
The resistors were mounted on both sides of the
board as shown below before soldering.
Below shows the OKO6 starter circuit board mounting and the MOV.
Below left shows the relative brightness of the light for 2 seconds on a good ballast. A shorted ballast shows bright only.
some company could produce a properly engineered safely
housed version of this circuit or something similar it would be
useful to those involved in the bulk replacement of fluorescent tubes.