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of its most obvious activity: supplying the toner that creates the image on the page (Figure 22-10). To reduce maintenance costs, however, many other laser printer parts, especially those that suffer the most wear and tear, have been incorporated into the toner cartridge. Although this makes replacement of individual parts nearly impossible, it greatly reduces the need for replacement; those parts that are most likely to break are replaced every time you replace the toner cartridge.

Figure 22-10 Laser printer toner cartridge

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NOTE Color laser printers have four toner cartridges: black, cyan, magenta, and yellow.

Photosensitive Drum The photosensitive drum is an aluminum cylinder coated with particles of photosensitive compounds. The drum itself is grounded to the power supply, but the coating is not. When light hits these particles, whatever electrical charge they may have “drains” out through the grounded cylinder.

Erase Lamp The erase lamp exposes the entire surface of the photosensitive drum to light, making the photosensitive coating conductive. Any electrical charge present in the particles bleeds away into the grounded drum, leaving the surface particles electrically neutral.

Primary Corona The primary corona wire, located close to the photosensitive drum, never touches the drum. When the primary corona is charged with an extremely high voltage, an electric field (or corona) forms, enabling voltage to pass to the drum and charge the photosensitive particles on its surface. The primary grid regulates the transfer of voltage, ensuring that the surface of the drum receives a uniform negative voltage of between ~600 and ~1000 volts.

Laser The laser acts as the writing mechanism of the printer. Any particle on the drum struck by the laser becomes conductive and its charge is drained away into the grounded core of the drum. The entire surface of the drum has a uniform negative charge of between ~600 and ~1000 volts following its charging by the primary corona wire. When particles are struck by the laser, they are discharged and left with a ~100 volt negative charge. Using the laser, we can “write” an image onto the drum. Note that the laser writes a positive image to the drum.

Toner The toner in a laser printer is a fine powder made up of plastic particles bonded to iron particles. The toner cylinder charges the toner with a negative charge of between ~200 and ~500 volts. Because that charge falls between the original uniform negative charge of the photosensitive drum (~600 to ~1000 volts) and the charge of the particles on the drum’s surface hit by the laser (~100 volts), particles of toner are attracted to the areas of the photosensitive drum that have been hit by the laser (that is, areas that have a relatively positive charge with reference to the toner particles).

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EXAM TIP The toner used in laser printers is typically carbon melt mixed with a polyester resin.

Transfer Corona To transfer the image from the photosensitive drum to the paper, the paper must be given a charge that will attract the toner particles off of the drum and onto the paper. The transfer corona is a thin wire, usually protected by other thin wires, that applies a positive charge to the paper, drawing the negatively charged toner particles to the paper. The paper, with its positive charge, is also attracted to the negatively charged drum. To prevent the paper from wrapping around the drum, a static charge eliminator removes the charge from the paper.

In most laser printers, the transfer corona is outside the toner cartridge, especially in large commercial grade machines. The transfer corona is prone to a build-up of dirt, toner, and debris through electrostatic attraction, and it must be cleaned. It is also quite fragile—usually finer than a human hair. Most printers with an exposed transfer corona provide a special tool to clean it, but you can also—very delicately—use a cotton swab soaked in 90 percent denatured alcohol (don’t use rubbing alcohol because it contains emollients). As always, never service any