My Overhead Projector Has What?

Over the past 25 years I have had the unique opportunity to talk directly
with many of the professionals and instructors who use Overhead Projectors
as an integral part of their profession. Through these interactions I have
accumulated notes and information that has inspired me to write these
articles that pertain to some of the most common problems experienced by
owners of today’s and yesterday’s Overhead Projectors. This is the fifth
article in a series of articles that will be written from a professional Electronics Technicians
point of view in regards to some of today’s most common Overhead Projector

My clients often say to me “You think my Overhead Projector has a bad
diode, what’s a diode?”

Let’s start off with quick lesson about diodes, sometimes referred to as
rectifiers. Diodes are semiconductor devices which might be described as
passing current in one direction only. You can even find diodes used in
some of today’s LED (light emitting diode) Flashlights. In the case of
Overhead Projectors, diodes are used to regulate voltage.

With the advent of today’s halogen projection lamps used in most Overhead
Projectors today, a special operating voltage is required to operate these
lamps. Generally the voltage is around 82 volts. Because the voltage at the
AC outlet where you plug your Overhead Projector into is 120V AC, a diode
(rectifier) is used to regulate this line voltage to the necessary 82 volts.

Unfortunately for the end user, when the diode in their Overhead Projector
fails it can be quite costly. Most end users don’t realize that this
component even exists. When it fails, in most instances the diode becomes a
short circuit. This short circuit allows the line voltage of 120 volts to
go directly to the projection lamp which only requires 82 volts to operate.
In our repair shop we refer to this as scorching. The end user assumes that
they have a defective projection lamp, so another lamp is placed into the
projector. This process sometimes continues on with several lamps being
replaced before the end user realizes that it is not a defective projector
lamp causing the problem.

The diode is a fairly inexpensive electronic component to replace and in
most cases is easily accessible. The only caution I must offer here is that
diodes are directional and must be placed back into the electrical circuit
a particular way. If you are not familiar with these types of components
the repair should be left to a professional electronic technician.

Mark Boehm is the president of M-B Electronics
[]. He has over 25 years of experience in the
Audio Visual and Electronics Industry. You can contact M-B Electronics
[] at 800-872-9456 or at:

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