Photocopier is an electromechanical device having a photoconductive cylindrical drum made of cadmium, selenium or an organic photoconductive material. (A photoconductive material changes its conductivity under light).
Upon switching on the machine, the drum is positively charged and the heating section, at the exit of the copier, with a Teflon coated roller is heated to 230 to 320 degrees Celsius by a heating lamp. This temperature is maintained by means of a feedback control loop having a thermistor and an on-off control switch.
The original document is placed on a glass plate and held flat. The paper fed through the feed section is pulled in by contact switches, rollers and a gear mechanism. As the paper touches a registration switch, an optical system equipped with a scanning lamp of 800-900 W is energized and driven by a motor, for scanning the document. An image of the document is reflected by various mirrors and lenses to the drum. Depending on the intensity of light received the photoconductive material loses its resistance at varying levels on its surface. That is, the positive charges on the drum are neutralized except in those areas representing the image). This results in a latent ‘charge image’ on the drum. A toner (negatively charged) is then pumped to the drum. Based on the ‘charge map’ the toner gets deposited and forms a ‘toner image’ on the drum. The drum then rotates and prints this image on the moving paper and ‘fixes’ it at a high temperature. Enlargement and reduction is achieved by adjusting the position of the lens (by varying the focal length) of the optical system.