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 problems.

My clients often say to me You think my Overhead Projector has a bad diode, whats 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.