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A printer is an electromechanical output device. It is used to transfer a computer document to images or text on paper. The paper document can then be used for later reading or handing in. The main types of printers are impact printers, bubble-jet printers, and laser printers.

Impact printers operate similar to typewriters. An impact head hits an inked ribbon, which presses a character or dot onto a paper. A daisy-wheel printer has a limited number of characters it can print. An electromechanical hammer (solenoid) hits a "petal" into the ribbon, transferring the character to the paper. A daisy-wheel printer can do multi-part forms and it has good quality, but it is quite noisy and it can only do a select number of characters. A dot matrix printer uses pins to print dots, which make up characters or graphics. A dot matrix printer has poorer quality than a daisy-wheel printer does, but it is faster and can do different characters.

Bubble-jet printers, or inkjets, have cartridges with ink in them. The ink shoots out through a microscopic pinhole onto the paper, forming text and images. Bubble-jets give much better resolution than a dot matrix, and are commonly used with computers at home.

Laser printers are also called page printers because they receive instructions one page at a time. There are three types of laser printer processes- Electrophotographic process (EP), Hewlet-Packard process (HP), and the Light Emitting Diode (LED) print process. The EP and HP processes are very similar. Initially, the charging corona charges the whole electrophotographic drum to –600 Vdc. The drum’s surface is specialized so that the parts of it exposed to light lose voltage. The laser (or LED, in LED printers) exposes the proper drum areas, "drawing" a picture onto the drum. The laser discharges the areas to –100 Vdc (EP) or 0 Vdc (HP). The toner (special ink) is charged to –600 Vdc. The toner is attracted to the discharged points on the drum, and sticks to it. A corona wire or roller is at 600 Vdc. The paper is moved between the wire and the electrophotographic drum. The toner on the drum is attracted to the wire and is transferred to the paper. A static eliminator strip removes the high voltage from the paper, so it won’t curl around the drum. Now the paper is holding the image loosely. The paper now rolls through the fuser, which fuses the image permanently to the paper by pressing and heating it. Finally the page comes out with the document on it. The printer then cleans the drum.


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