28Sep Laser Printer

The laser printer power supply unit is the most complex found in any type of printer. It must deliver ac power to the fuser unit. The power supply also delivers a high voltage dc supply (+1000 Vdc) to be toner supply mechanism in the drum area. Finally, the power supply unit must deliver dc operating voltages to the scanning and paper handling motors, as well as the digital electronic circuitry on the control board.From manufacturer to manufacturer, and model to model, the exact arrangement and combinations of components may vary in laser printers. The six stages of operation in a laser printer include:

1. Cleaning
2. Conditioning
3. Writing
4. Developing
5. Transferring
6. Fusing

When data is received from the host computer, the control board generates all of the enabling signals to place the information on the page as directed. The character information is converted into a serial bit stream, which can be applied to the scanning laser. The photosensative drum rotates as the laser beam is scanned across it. The laser creates a copy of the image on the photosensative drum in the form of  a relatively positive charge drawing. This operation is referred to as registration. Before the laser writes on the drum, a set of erase lamps shine on the drum to remove any residual traces of the previous image. This leaves the complete drum with a neutral electrical charge. A high voltage applied to the primary corona wire, creates a highly charged negative field that conditions the drum to be written on by applying a uniform negative charge (-1000 V) to it.  As the drum is written on by the laser, it turns through the toner powder, which is attracted to the more positively charged image on the drum.

The developer roller, in the toner cartridge, turns as the drum turns,  and expels a measured amount of toner past a restricting blade. A regulating ac voltage assists the toner in leaving the cartridge, but also pulls back some excess toner from the drum. Excess toner is recycled within the toner cartridge so that it can be used again. After the image has been transferred to the paper, a pair of compression roller in the fusing unit act to press the toner particles into the paper, while they melt them to it.

Hewlett Packard printers, the main portion of the printing of the printing system is contained in the elecphotographic cartridge. This cartridge conatains the toner supply, the corona wire, the drum assembly and the developing roller.In other laser printers, the basic components are combioned so thatthe printer consists of a developer unit, a toner cartridge, a drum unit,  a fuser unit,  and a cleaning pad. While it is less expensive to change toner attention must be paid to how much the other units are wearing. Notice that the photosensative drum is also a separate component.

Laser printers are very mechanical in nature. The paper handling motor and the gear train assembly perform a tremendous number of operation to process a single sheet of paper. The paper transport mechanics must pick up a page from the paper tray and move it into the paper’s registration area. After the drum has been written with the image, the paper handling mechanism  moves the paper into registration. A roller system moves the page past the drum, and into the fusing unit. When the page exists through the fusing rollers, the printer senses that the page has excited, and resets itself to wait for anopther page to print.

Paper weight-Paper is specified in terms of its weight per 500 sheets at 22”x17”(i.e., 500 sheets of 22″x17″,21-pound bond paper weighs 21 pounds).