Lamp failure occurs due to a mechanical break in
the tungsten filament winding. As the lamp operates the tungsten filament becomes thinner
due to evaporation and it eventually breaks. Failure may be accelerated by factors, such
as, mechanical shock, vibration, high line voltage and temperature.
Shock and Vibration
Lamp failure in an overhead projector can be due to
mechanical shock. This occurs when using the lamp changer or bumping the unit while the
lamp is operating. A hot lamp filament is very fragile. You should always make sure the
unit is turned off and the lamp is cool before moving the lamp changer or the unit.
Switching Projector On and Off
Each time you turn the unit on there is an inrush current
that applies stress to the lamp filament.
The more often this stress is applied the sooner the lamp
will fail. For this reason turning the overhead projector "on" and
"off" frequently during presentations is not recommended. Some projectors have a
"soft start" power supply that provides a gradual voltage increase which limits
the inrush current shock. This results in extended lamp life.
Cooling
Improper cooling can cause lamp envelope seal failure or
lamp capsule swelling. Always make sure the fan is operating and the intake and exhaust
vents are not blocked.
Power Line Voltage
High power line voltage is a major cause of short lamp
life. Using a voltage less than the rated lamp voltage will increase lamp life but, reduce
light output. Higher line voltages will reduce lamp life but, increase light output. The
table below shows an example where a lamp at normal 120VAC line voltage would have a 75
hour life, however, if the line voltage increases to 130 VAC it will only last 28 hours.
Power Line Voltage vs. Light Output and Lamp Life
| Line Voltage |
% of Light Output |
Lamp Life, Hours |
|
| 115 |
87 |
130 |
|
| 120 |
100 |
75 |
Rated
Life |
| 125 |
115 |
45 |
|
| 130 |
130 |
28 |
|
Some lamps have a version for use where the power line is
consistently higher than 120 volts.
If your projector is equipped with a" high/low
lamp" switch you can extend your average lamp life by using the " low lamp"
position whenever possible. Also, some projectors are equipped with regulated power
supplies that deliver the correct voltage to the lamp. This also extends the lamp life.
When installing lamps, be sure the lamps are seated
completely. The tendency is to stop at the first sign of resistance. Continue to apply
force at the base of the lamp until you are sure the lamp is secure.
Summary
To extend your projectors lamp life; run at the lower lamp
setting, be sure vents are open, minimize the "on" and "off" cycles,
assure lamps are completely seated, and don’t bump an operating unit. Also, for high
line voltages consider the use of special lamps.
