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XENON BULB FAILURE ANALYSIS BY: RAY F. BOEGNER -
STRONG INTERNATIONAL
There are two things that are always true regarding xenon
bulbs; they are expensive, and they will eventually require replacement.
Hopefully, not on a Saturday night with a full house There are various reasons
why a bulb requires replacement: It doesn't operate, it operates poorly, or it
just plain gets old, Let's explore these reasons in further detail in hopes of
understanding that expensive bulb's moodiness.
The bulb doesn't work anymore! That's a broad statement
defining a problem. It's kind of like saying your car doesn't run! Is it out of
gas? Are the tires flat? Did somebody steal your engine? One must first be more
definitive when describing the problem. This will save a lot of time and
eliminate unnecessary and costly mistakes. The easiest thing in the world for a
bulb or lamphouse manufacturer to do is blame the other guy. Both of these
manufacturers believe they make the perfect product, and if there's a problem,
it can't be their fault, They are both correct to an extent. There are such
things as bad xenon bulbs, and there are also bad lamphouses and power
supplies, Obviously they are not consistently bad, or the manufacturers would
be out of business.
Let's start by defining the common bulb failure modes, Then
we'll get into what can cause them -- both within the bulb itself and in the
lamphouse/rectifier system. Once we've analyzed the symptoms, it's much easier
to find the problem.
Contaminated Bulb Envelope. This is
indicated by a blackened or dark envelope. It is usually most prominent around
the anode side of the envelope.
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Failure to Ignite. This is indicated by the bulb's refusal
to establish a maintained are either off the automatic or manual ignition
system of the lamphouse.
Explosion. Or as
bulb manufacturers call it, "A catastrophic failure," requires little
explanation once you've encountered your first one.
Leaker. This is indicated by a
bluish-white color of the envelope and are when the bulb is running. The real
"telltale" to a leaker is high amperage and low voltage during bulb
operation.
Low Light Output. Usually occurring
on older bulbs and is indicated by poor screen foot lambert readings.
Unstable Arc or Flicker. Indicated by
the bright spot on the screen jumping around and is especially noticeable on
bright sky scenes.
CONTAMINATED BULB
The contaminated bulb is one of the harder ones for a bulb
manufacturer to put off on the lamphouse manufacturer. It is usually caused by
poor quality tungsten and/or tungsten processing. In a lot of cases it is
evident by excessive pitting on the face of the anode electrode. It can also be
caused by an inadequate vacuum prior to filling the bulb with xenon. Sometimes
the bulb manufacturer will blame the rectifier saying that it has excessive
current ripple, so be certain to examine both electrodes prior to returning the
bulb. Excessive current ripple destroys cathodes much sooner than anodes, and
this would be evident by excessive cathode disconfiguration with lengthwise
slits in the cathode head.
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If the power supply is suspected of having excessive current
ripple, don't run out and call your local electrician, He probably won't know
how to read it anyway! Talk to your dealer about the problem, and if you've
done what all bulb and lamphouse manufacturers recommend and have kept good
records of the individual system's bulb life, you'll be able to tell right away
if you've experienced short bulb life in a specific system. If that's the case,
then attack the problem and have your dealer contact the manufacturer or a
trained serviceman.
FAILURE TO IGNITE
Failure to ignite is far and away the most complex problem to
diagnose, However, as with all bulb problems, if you've kept good records, the
solution is much easier to establish. First of all, one must know what it takes
to start a xenon bulb and the components utilized to accomplish that desired
result.
First, the lamphouse has an igniter that is similar to a coil
on an older car. This device takes either 115 volt or 220 volt AC input and
converts it into 40,000 volts RF, which is used to break down the are or space
between the anode and cathode electrodes. This is accomplished by means of a
high voltage transformer that takes the 115 or 220 volt input up to
approximately 5000 volts; a spark gap which breaks down and passes current at
approximately 5000 volts; a few door-knob-type capacitors, and an RF
transformer which takes that 5000 volts and steps it up to approximately 40,000
volts and induces it into the DC line going to the bulb.
The igniter has one function: That is to break down the are or
space between the electrodes of the bulb. After the are is broken down, the
power supply takes over in two steps. First there are a couple of large
capacitors in the power supply that discharge across the are directly following
the breakdown by the igniter. This phase of the ignition is
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called the boost current and is typically two to three times
the current that the lamp normally operates at when in normal operation, This
boost current portion, or phase, of the ignition process lasts for no more than
250 milliseconds or 1/4 of a second. This is also the most detrimental phase of
the ignition cycle to a bulb's life. Excessive boost current of higher than
three times the operating current will help to start hard-starting bulbs, but
it in turn destroys cathode electrodes. In a study made by the military in the
late 1960's, it was determined -that each bulb ignition decreased the expected
bulb life by thirty minutes because of the effect of boost current.
The last portion of the ignition cycle is the running DC
voltage supplied by the power supply rectifier. Most theatre xenon bulbs
operate between 22 and 33 volts DC, depending on the size and wattage. This
running voltage along with the corresponding current follows the boost current
in the ignition process.
Most theatres contain automation systems to light the lamp and
operate the projector. This is the easiest item to eliminate in an ignition
problem. See if the bulb lights without the automation. If it does, then the
ignition problem is in the automation system and you should refer to that
specific system's manual or drawings to correct the problem.
Another item that is easy to eliminate is the lamphouse or
console auto strike circuit. This circuit typically consists of a relay and a
zener diode that senses that the open circuit voltage is high enough and then
provides a closure to the high voltage transformer of the igniter. If the bulb
starts using the manual or emergency ignition switch, but does not start
otherwise, the problem is in this circuit. In certain power supply models, a
shorted blocking diode will also cause this problem, When using the emergency
or manual ignition switch,