Projectors

This page covers data projectors.

Summary

Data projectors are often quite expensive items and so well worth fixing when they go wrong. With care, disassembly and fixing are not generally too difficult, but an understanding of their principles will help.

Safety

Projector lamps get extremely hot. Always leave a projector to cool down for 30 - 45 minutes before working on it, so as to avoid the risk of burns.

Take great care not to touch any of the glass of a projector lamp as finger grease can shorten its life, and in the worst case, cause it to explode.

How projectors work

There are two basic types of projector: DLP and LCD. Except for pocket-size projectors, they almost invariably use a discharge tube as the light source.

DLP projectors

DLP (Digital Light Processing) projectors are probably the commonest. The image is created by a digital micromirror device (DMD) fabricated on a silicon chip. Ech pixel is controlled by a tiny mirror which can be electrically deflected to reflect light either through the lens and on to the screen, or into a "light sink" where it is absorbed. By oscillating between the two states at high speed and spending more or less time in one than the other, the apparent brightness of the pixel can be controlled.

The DMD is essentially a monochrome device. A colour image is produced by a colour wheel in front of the lamp which passes each of the three primary colours in turn. The DMD then creates each of the red, green and blue fractions of the image, one after the other. If you sweep your gaze rapidly across the screen you will see the red, green and blue images separating. Some people find this distracting, and it is sometimes cited as a disadvantage of DLP projectors.

In passing only one colour at a time the colour wheel rejects two thirds of the light. High end home theatre and event projectors sometimes have 3 DMD chips. The light from the lamp is split into red, green and blue beams which are focussed onto the 3 DMD chips before being recombined into a colour image for projection onto the screen.

LCD projectors

These typically have 3 LCD modules, and as in high end DLP projectors, the light from the lamp is split into red, green and blue beams, which are focussed on the 3 LCDs before being recombined. When you take the lid off you may be able to see 3 identical ribbon cables at 90 degrees descending from the main logic board to the 3 LCD modules.

Lamps

In the last few years, a range of pocket projectors has appeared which use LEDs as the light source, but these have a limited brilliance making them unsuitable for projecting more than quite a small image for a very few people. LEDs intrinsically have a very long lifetime, though they are likely to be operating to the limits of their power, which may reduce their life or result in elevated failure rates.

Virtually all other projectors use a xenon or metal halide discharge tube as the light source. These lamps run very hot and have a life of only a few thousand hours. However, the rated life tends to assume that it's used in long stretches at a time, and the actual life may be considerably shorter if the projector is only used for an hour or so at a time.

These lamps take a minute or so to reach operating temperature and full light output. They require a high voltage to initially strike the discharge but thereafter they run at a much lower voltage. However, the striking voltage is significantly higher still if you try and switch them on while still hot from previous use, and in this situation a projector may have to automatically retry several times in order to strike the discharge.

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