What is a drum unit in a laser printer?
The drum unit transfers the toner into an image onto a drum, which the paper is fed through to pick up. Some printers have the drum unit integral with the toner cartridges (as in the HP 5L & 6L). Others require you to buy a separate unit (as in the Dell 3100 series). The more technical description is here: Initially, the drum is given a total positive charge by the charge corona wire, a wire with an electrical current running through it. (Some printers use a charged roller instead of a corona wire, but the principle is the same.) As the drum revolves, the printer shines a tiny laser beam across the surface to discharge certain points. In this way, the laser “draws” the letters and images to be printed as a pattern of electrical charges — an electrostatic image. The system can also work with the charges reversed — that is, a positive electrostatic image on a negative background.
