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How HP Laser Printers Work- Operating Principals

Updated: May 28, 2019

P Laser printer theory of operation, subsystem functionality and how the modules work together to solve problems

When you don't know much about them, laser printers seem like magical contraptions. You get to know quite a bit about them just by using them every day. You understand when they are not "feeling well" and learn to live with their idiosyncrasies as you would your car or your computer.

The term magic seems appropriate when describing how HP laser printers work. (Even more so when you know how they work!) True, you don't have to know how they work to be happy working with a laser printer. But, if you could understand enough to be able to fix the simpler problems, you develop a sense of pride and relief when you do.

Overview of how HP Laser printer subsystems function and how the modules working together to solve problems
Overview of how HP Laser printer subsystems function and how the modules working together to solve problems

When you want to learn how something works you start with the outside before you look inside. You look at what goes in and what comes out. You analyze how it runs when nothing is wrong with it. You learn the controls and how to manipulate them.

Our experience tells us that all models of HP laser printers are more alike than they are different. All laser printers contain more or less self-contained subassemblies called modules. With very few exceptions, all contain the same types of modules, however individualized and packaged within the bounds of their cases.

These modules quite often look similar between different machine models. Late model HP laser printers quite often combine several modules into one package. But, the functions of those modules still exist within the machine and can be identified.

Our philosophy in training technicians how to fix them continues to be, "Learn one laser printer well, and you can apply that knowledge to other models quickly and easily." With this in mind, we will now examine a simple model of a HP laser printer and describe the basics of how it works.

We all know what a HP laser printer does: It produces a printout, exactly how we envisioned it to be. (Maybe it didn't come out right the first time, but we eventually constructed the page we wanted!) From the perspective of a novice, how the printer makes the final printout is hard to imagine. What comes out of the printer are outputs.

To get outputs from a laser printer, we must first introduce inputs Let's look closer at the inputs, returning later to the outputs.


HP Laser printers are electromechanical machines. In the U.S.A., and most other countries, they consume wall-socket power of 115 Volts AC (alternating current). Most printer electrical circuits, however, operate on DC or direct current. Therefore, inside every laser printer there must be a way of converting AC to DC power at the appropriate levels for the machine to operate.

Computer Data

All laser printers react to digital, coded communication from the computer or network connected to them. The data sent to them contains instructions for how the page is to be constructed. In this respect, the signal is like a toggle on a light switch-the control input. Without this input, the printer would just sit there, ready to run.

Paper Often we forget that paper is key to forming the perfect print out. HP laser printers are made to operate within a narrow range of paper available to us. They were not made to pass cardboard or sheet metal!(We all know some creative graphic artists who insist on trying some bizarre paper through printers.) Using only the prescribed paper in laser printers helps ensure of good performance.

Toner Toner, in one form or another, must be added to laser printers regularly as consumable material. For most laser printer substituting a fresh toner cartridge containing not only the toner powder, but the image drum, development apparatus, and waste bin, refreshes the heart as well as the life's blood of the printer.

As we said, the most obvious output of the laser printer is, of course, a perfect page bearing the exact image we expect out of it. Beyond that, other outputs, including sounds, displays, movements of rollers, air blowing from fans, indicate how the machine is operating.

As long as you are satisfied with the printed page, and there are no indications of machine problems, you probably won't need service. Don't forget though, routine cleanings help keep your printer running and prevent premature wear of certain parts.


To better grasp how HP laser printers work and develop problems, we can divide a typical printer into functional subsystems. Each subsystem contains modules linked together to perform certain tasks Laser printers house four functional subsystems:

  1. Power

  2. Paper Handling

  3. Image Formation

  4. Control

By dividing the HP printer into four functional subsystems, we can concentrate on individual modules within each subsystem. Each module has inputs and outputs of its own. Within each functional subsystem, modules are linked together. The output of the first module becomes the input for the next and so on.

SUMMARIZING THE FUNCTIONAL SUBSYSTEMS The following information shows the modules of each functional subsystem and how they form functional chains.


The power subsystem consists of three modules:

  • AC Power Module

  • DC Power Supply

  • High Voltage Power Supply

HP Laser printers require power to operate. Alternating current (AC) from the wall plug illuminates the fusing lamp and feeds the direct current (DC) power module. The module which distributes the AC power to these devices inside the machine is the AC Power Supply.

Overview of how HP Laser printer subsystems function and how the modules working together to solve problems