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FAILURE TO IGNITE
continued
never hold it in longer than two seconds, as you can destroy
the igniter's high voltage transformer.
The reason that lamphouse manufacturers use the auto start
circuit is to assure that the open circuit voltage is at a pre-determined level
prior to bulb ignition. This pre-determined level provides the proper level of
charge for the power supply boost capacitors for bulb ignition. Depending on
the lamphouse manufacturer, this open circuit level is usually between 80 and
110 volts DC and can usually be monitored by depressing the bulb voltage switch
during ignition. If the system does not have a voltage switch, it can also be
measured at the power supply by using a voltmeter.
If the bulb doesn't attempt to strike with the emergency
ignition switch, then the problem is usually in the igniter. In most cases, it
is either a defective high voltage transformer or spark gap. If you can hear
the igniter, it is a goad idea to turn the lights out in the booth and look
through the lamphouse observation port to try and ascertain where the are is
occurring. The lead from the bulb to the igniter should be at least
1/2 inch from any metal, If the path between the lend and
metal has less resistance than that between the electrodes, the RF are will
occur between the lead and metal and not across the bulb.
If an are does go across the electrodes and the bulb does not
start, the problem is not in the igniter. We must now look at the boost circuit
of the supply. A very common reason for this phenomenon is loose or corroded DC
connections either in the lamphouse or the rectifier. These connections should
be checked during periodic lamphouse maintenance and whenever a bulb is
changed. Another possible cause within the rectifier is a defective boost
capacitor. If you have a repeating occurrence of this nature in the same system
with several bulbs, it's a good idea to exchange these capacitors with those
of
Page Six
another system that
does not exhibit the same problem and see if the problem follows the
capacitors. If you change the bulb and the problem is corrected for five or six
hundred hours before it re-occurs, try the capacitor trick and document the
results to see if there is a pattern.
The bulb can also be the cause of ignition problems, The
cathode contains thorium to lower its work function and provide easy ignition.
If the bulb is operated at a level of less than 75% of its rated amperage, the
cathode electrode will not operate hot enough to replenish the surface thoria
and in turn will exhibit starting problems after four or five hundred hours of
operation. Bulb- related ignition problems usually center around the cathode
electrode of the bulb and a good visual examination of the condition of the
cathode is in order if you are encountering ignition problems. A cathode with a
mushroom-looking tip can cause erratic ignition problems.
Bulb fill pressure and are spacing can also be suspect in bulb
ignition problems. While a high xenon fill pressure will provide higher initial
light output on a xenon bulb, it can also lead to poor bulb ignition after the
bulb ages some and contaminates are induced into the xenon from the normal
outgassing of the electrodes during operation. The higher the xenon pressure
is, the higher the resistance of the are in the bulb will be, The spacing of
the are also affects the resistance of the are; as the wider the gap, the
higher the resistance will be. Running a bulb at higher than rated current will
cause excessive burn-back on the cathode electrode and in turn increase the
length of the are.
So what's this wire around the outside of the envelope for?
This wire is usually made of nickel or ni-chrome (short for nickel-chromium)
and is used to set up an "E" field during bulb ignition. You old tube buffs can
picture it as a grid on an electron tube, Without this wire on the bulb, you
will
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more than likely experience ignition problems about halfway
through the bulb's life and especially if the bulb is hot when you try to start
it.
BULB EXPLOSION
A bulb explosion does one thing for the projectionist,
technician, or manager/operator--it makes them respect the warning labels on
the bulb box. The pressure inside of an operating xenon bulb is as much as
three times the non-operating pressure of the bulb, which is typically 60
p.s.i. absolute pressure. When the bulb explodes, it makes an extremely loud
"bang" and in many situations does considerable damage to the lamphouse. Once
you've experienced a bulb explosion, you will become an avid user of the
recommended safety equipment when handling xenon bulbs.
So what causes a bulb explosion? The Number One cause is
excessive strain in the bulb's quartz envelope. Once the envelope has developed
a strain pattern, it is only a matter of time and thermal cycling until the
envelope ruptures or in actuality explodes due to the high internal xenon
pressure. It is a very rare occurrence for a bulb to explode while not in
operation due to the lack of temperature and the reduced xenon pressure of a
non-operating bulb. When the bulb is operating with strain in the envelope, the
internal pressure at the elevated temperatures works against the vitreous
structure of the quartz (Si02) and the strain pattern actually grows
until the integrity of the envelope gives way and you have an explosion.
Strain is usually induced into an envelope during the
fabrication process of the bulb. However, this strain is removed by an
annealing process by the manufacturer and is inspected using a polariscope to
ascertain its removal, The only other manufacturer-induced strain possibility
is during
Page Eight
the filling operation and the following tip-off from the fill
station, Manufacturers do inspect for strain induced during this process and
will reject the bulb if it is severe. The polariscope test for strain around
the fill tube is not as accurate as that of the envelope due to the optical
distortion of the fill tube. This results in a judgement call by the quality
control inspector as to the product's acceptability.
Another cause of bulb explosion is devitrification of the
quartz used in the bulb. The quartz used in xenon bulbs is both a chemically
stable and a refractory material of pure silica. When this material is
subjected to chemical impurities, such as the oil in a human fingerprint, and
then heated, nonreversible damage is done to the material's integrity. The
vitreous structure is changed to a crystalline condition and the materials
strength is greatly reduced, This reduction in strength will eventually reduce
to a point whereby the bulb can no longer hold the pressure and an explosion
occurs.
One last item that has caused a lot of explosions is operating
the bulb with a very dark envelope. While you may save a few dollars in bulb
costs, you'll also have poor screen illumination and eventually lose a show
because you're asking for an explosion. The bulb is transparent for two
reasons: one, to let the light out: two, to let the heat out. The quartz
envelope on a new clear bulb can be up to 600 degrees centigrade and as the
bulb darkens, this temperature goes up, as the heat cannot escape due to the
darkness in the envelope.
It goes without saying that one should always check the
condition of the lamphouse's exhaust fans and blowers. Most manufacturers run
extensive tests in the development of their equipment's bulb cooling system.
Another thing to remember is to clean that dust off the bulb occasionally but
be sure to use a lint and oil-free towel. This will help to prevent external
envelope contamination that leads to devitrification.