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

The rest of the machine operates on direct current (DC) power The DC power supply converts AC power into DC power typically at two levels: +24 and +5 VDC.

One branch of +24 VDC passes as an output from the DC power supply becoming an input to the third power supply in the chain, the High Voltage Power Supply (HVPS).

Its job is to convert +24 volts DC into much higher static electrical charges used in forming the image on the page.

These three power supplies are interconnected, supplying the required power to other devices in the printer. In later printers, these power circuits are combined together on the same circuit board, but their individual circuits are usually segregated into recognizable locations.


Laser Printer Power Systems

  • AC Power

  • DC Power

  • HV Power

Laser Printer Paper Handling

  • Cassette or Paper Tray

  • Pickup Assembly

  • Registration

  • Drum/Transfer Roller

  • Fuser

  • Delivery Assembly

Laser Printer Image Formation

  • Logic Board

  • DC Controller

  • Laser Unit

  • Scanner

  • Toner Cartridge

  • Transfer Assembly

  • Fuser

Laser Printer Control

  • Heat Sensor

  • Paper Sensors

  • DC Controller

  • Motors

  • Feed Solenoids


The mechanisms feeding paper through the HP laser printer comprise paper the paper handling subsystem. The Handling modules of this subsystem are chained together in a common task: to pass one page at a time from the paper cassette to the exit of the ma- chine.

In this subsystem we load paper into the paper cassette. At print time, the pickup assembly pushes the top page into the registration assembly. In older printers the pickup roller purposely pushes the page ahead far enough to form a buckle in the page. The buckle forms because the leading edge of the page is blocked by the registration rollers. This buckle-key to continued paper movement-is maintained until the registration rollers are allowed to turn.

Stages of paper handling

Paper moves through the laser printer in three stages common to all laser printers:

  • paper pickup and separation

  • registration and feed

  • fusing and delivery

Most paper jams result from worn paper feed modules in the pickup/separation stage of paper handling.

Paper Pickup/Separation lifts the top page from the paper cassette and delivers it to the next stage. Separation mechanisms keep pages below the top sheet from entering the paper path.

The Registration/Feed stage precisely coordinates paper feed with the rotating image on the cartridge drum to assure proper vertical alignment of the image on the page.

After the image transfers to the page, the Fusing/Delivery stage fixes the image onto the page and passes it to the exit of the paper path.

The paper handling subsystem typically consists of six modules:

  1. Paper Cassette (or Input Tray) holds a stack of paper aligned and ready for paper feed.

  2. Pickup Assembly lifts top page in paper cassette or input tray and moves the page into the printer.

  3. Registration precisely positions the paper under the image.

  4. Cartridge Drum/Transfer Roller.

  5. Fuser feeds the page (as well as melting the toner image into the fibers of the page) to the delivery rollers.

  6. Delivery Assembly feeds the page to the appropriate exit tray.

Points to remember about the paper handling subsystem...

  • Most paper handling problems result from worn pickup assembly parts.

  • Pickup rollers are textured for positive grip. After the texture wears off the roller needs to be replaced.

  • All paper handling modules take paper in and deliver paper out. Another input of most paper handling modules is mechanical drive.

  • Corner separators, separation pads,and separation rollers peal the top page from the stack in the paper cassette.

  • Registration rollers, plates, or sensors coordinate the page movement with the image to be transferred.

  • Some printers develop worn or broken fuser gears, resulting in paper jams.

  • Worn HP delivery rollers produce aggravating noises. Replacing upper output roller cures these noises.

THE IMAGE FORMATION SUBSYSTEM The image formation subsystem of a typical laser printer consists of modules linked together for a Image formation single purpose to produce an image on the page.

HP laser printer's image formation subsystem consists of seven modules:

  1. Logic Board [or motherboard, interface board, I/O board, formatter translates. computer instructions into a dot-by-dot layout (bitmap) of the image to be formed on the page.

  2. DC Controller runs all printer activities including flashing the laser unit according to the bitmap signal supplied by the logic board.

  3. Laser Unit emits laser light corresponding to the image to be formed.

  4. Scanner sweeps laser pulses through lenses and mirrors onto the cartridge drum.

  5. Toner Cartridge forms an electrostatic image, then a real toner image on its drum. surface.

  6. Transfer Assembly draws the toner image from cartridge drum onto paper.

  7. Printer fuser melts the toner image onto the page.

No HP laser printer operates in a world by itself. Each is connected to at least one computer from which it gets its instructions for printing. Ultimately, of course, the information to be printed out comes from humans. So the computer puts the human input into a form the printer can understand.

When the user commands the computer to print a document the computer responds by sending coded signals over a cable to initiate printing.

The vast majority of computers communicating with laser printers are PC compatible. Most often, simple documents are printed with predominantly text and few graphic images.

The laser printer receives coded electronic print instructions from a computer. This is the control input for the image formation process. As these instructions enter the laser printer, the image formation modules process them and form the corresponding image on the page.

The laser printer's DC Controller pulses the laser light corresponding to the image sent to it by the formatter board. When the light illuminates, a black dot will appear on the page. The Scanner sweeps the pulsing laser beam along a line from one end of the cartridge drum to the other.

Meanwhile the cartridge drum turns so that the next scan ends up one line down on the drum surface.

In HP printers, the toner cartridge, containing the heart of the image formation subsystem, can be easily changed. Certain other engine makers mount the photosensitive drum, hopper, and cleaning stations separately inside the machine, making it more difficult to solve image problems.

Since many steps of the complex image formation take place within the cartridge, a simple way to diagnose a cartridge problem is to simply try another cartridge in your printer. We recommend that you always keep an extra cartridge on hand.

THE CONTROL SUBSYSTEM The Control Subsystem is the central nervous system of the laser printer. The brain of the control subsystem is the DC Controller PCA. It receives signals from several sensors and outputs commands to all the major modules of the machine. Fortunately, the control subsystem does not usually present problems.

The control subsystem consists of the following devices:

  • DC Controller orchestrating all printer activities.

  • Thermistor sensing fuser temperature.

  • Photo sensors sensing paper presence and movement.

  • Motors driving gears, rollers and scanner mirror.

  • Feed Solenoids starting paper feed.

  • Control Panel for configuring and testing the printer.

Points to remember about the control subsystem:

  • The DC Controller is the heart of the control subsystem.

  • By pressing a test button, you can run an engine test on Canon based machines.

  • Among the many outputs of the DC Controller are signals which operate feed solenoids, triggering paper feed.

  • A laser printer DC Controller "feels" the temperature in the fuser using a thermistor.

  • A DC Controller senses presence and movement of paper in the machine using photo sensors in the paper path.

NORMAL MACHINE START-UP INDICATIONS Certain sounds, sights, and other sensations indicate proper laser printer operation. Becoming familiar with these sensations increases your awareness of interruptions in the sequence of these events and their significance when malfunctions occur.

For example, on some of the older HP laser printers start up, one of the first events is an audible "click." This reports closure of an electrical relay inside the AC power supply.

Next, check the control panel. Since the control panel operates on +5 VDC, the DC power supply +5 VDC output must be working. Since the DC power supply gets its input from the AC, it must also be working. And, the click indicates that the AC branch feeding fuser power is not short circuited.

Three noises follow at this point in the start-up sequence. The loudest, most obvious whining sound is the main motor starting up. Since the main motor powers all gears and rollers of the drive train, the rotation of the exit delivery rollers at the paper exit at the top of the printer corresponds to this sound.

COMPLETING THE START-UP SEQUENCE Then the message "02 WARM UP" appears, replacing the previous "05 SELF TEST. After a minute or two, the warm-up sequence ends when the message "00 READY" appears and the "Ready" and "On-line" LEDs turn on.

RUNNING PRINT TESTS AFTER START-UP After the start-up sequence concludes, it appears that the basic operations of engine and logic circuits are working properly. To prove it, try running a self test.

A self test page will soon emerge, showing the menus and selections for the printer.

Proving that the machine fully works requires connecting up and running a test from a computer. Refer to the operators manual for the computer or printer for how to do this.

USING YOUR KNOWLEDGE OF LASER PRINTER OPERATION TO SOLVE PRINTER PROBLEMS Throughout your life you have learned by experience that certain breakdowns are likely to occur. For example, in your home, you have undoubtedly changed light bulbs. The light circuit of a light circuit includes a switch, wiring, power source and the light bulb linked together to form an electrical circuit.

Why you went to the light bulb first was because from your experience you know that a light bulb is much more likely to fail than wiring, power source, or the light switch.

Studying laser printers we have noticed that some modules fail more often than others. Fusers, toner cartridges, and pickup rollers top the list. Use this knowledge to your benefit. For example, when you have a problem mark or line on the page, you can eliminate all subsystems except image formation.

Since the cartridge and fuser are the most troublesome modules within image formation, these are the ones to look at first. It takes only a minute to change a cartridge. Do the easiest and most likely checks first.

If the cartridge did not cure the problem, stop the machine in mid-print cycle. Remove the toner cartridge to see if the image defect is on the drum. If not, does it appear on the page before the fuser? In so doing, you are checking inputs and outputs of the respective modules.

What if the page emerges from the printer with an unexpected image? Or, perhaps, no page came out at all? What if the page has poor print quality? Or, the paper is creased? Perhaps the page is fine but the machine is noisy and irritating. Where do you start to isolate these problems?

"If you have a problem with the output, first check that all inputs are present and correct before concluding that the problem lies inside the printer."

For example, if the printer does not react when the switch is turned on, is it plugged in? Or, if a page comes out creased on wrinkled, is the paper in the input cassette crooked, damp, or wrinkled? Does the paper meet the specifications for printer use as published in the owners manual?

If all the printer inputs are proper and there is still a problem, the problem is inside the printer.

OFTEN A LASER PRINTER WILL TELL YOU WHAT'S WRONG WITH IT Each laser printer has a control panel or display which reports it status. Usually this is accomplished using indicator lights or LED light emitting diodes), LED messages, or coded signals fed to to computer. As such, they are very useful outputs which help us determine printer problems.

When all is well, the machine reports a "READY" message some form to indicate it waits to accept instructions from the computer. In some machines the ready condition is indicated by an LED (usually green).

When something goes wrong, many laser printer display messages require translation. For example, you must look in a list codes to tell that "50 SERVICE" means trouble in the fuser heat circuit. In this case you may need to call a technician to determine the cause.

Error messages are symptoms. Other symptoms may occur which machines do not "sense." You are usually the first to notice symptoms, such as squeaks, groans, grinding, and other noises. Blank pages, print defects, and bad smells are other common symptoms.

To solve problems, observe the printer, analyze unusual symptoms and decide a course of action for repair. Understanding inputs, outputs and subsystems helps you decide your course of action.

Other Helpful Resources

Metrofuser is a leading global innovator, manufacturer of printer parts, equipment, diagnostics, repair information and systems solutions for professional users performing critical tasks. Products and services include hp printer parts, printers and printer repair training. Parts include hp printer parts such as printer fusers, printer maintenance kits and other hp printer replacement parts. 

The company's, customers include office equipment dealerships, online retailers, repair centers and MPS service providers nationwide. Metrofuser has been named to Inc. Magazine’s fastest growing companies five consecutive years. 


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