HP LaserJet 4 Service Manual - Page 129

Photosensitive Drum The photosensitive drum is the "heart" of the Image Formation System. The special properties of this drum allow an image to be formed on the drum's surface and then transferred to paper. The drum is an extruded aluminum cylinder. The outside of the cylinder is coated with a layer of organic-photoconductive material (OPC) which is non-toxic. The aluminum base of the photosensitive drum is electrically connected to ground potential (see Figure 5-7). Figure 5-7 Photosensitive Drum Drum Sensitivity The OPC material has properties similar to a photo-resistor. It becomes electrically conductive when exposed to light. When the photosensitive drum is exposed to light, the negative charges deposited on it are conducted to the ground potential of the drum's base. Areas not exposed to light remain nonconductive and maintain their negative charge. Caution Do not expose the Toner Cartridge to any bright light source. Too much light permanently damages the drum. Functional Overview 5-17