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How The Laser Printer Works.

Updated: Oct 31, 2019

The Printerworks With The Help of Collection Of Mysterious Sub Systems

Laser printers are truly amazing machines. Think about it: They accept an input of electrical pulses through a wire with a fancy plug. Seconds, mere seconds, later they eject a perfectly printed document. That document can include any number of fonts, lines, illustrations, and photographs.

The Printerworks With The Help of Collection Of Mysterious Sub Systems
The Printerworks With The Help of Collection Of Mysterious Sub Systems

To the uninitiated, it looks like a miracle Demystifying the miracle is the first step toward becoming a qualified repair technician. While it may not be strictly necessary to understand something to fix it -you fix your headache by taking an aspirin, but you don't understand what's happening in your brain-true confidence and competence in your technical abilities will only come with a thorough understanding of the way the printer operates.

Often on a repair call, I can determine what is wrong and what I'm going to do just by listening to the printer go through its cycle. Invariably, I can form a detailed diagnosis after the machine has printed, or at least tried to print, a page. I can do this because I am intimately familiar with the way the machine operates, intuitively and intellectually.


Xerography has been around and popular for 30 years. The principles of using toner to develop an image are so well understood, it's surprising that laser printing wasn't developed earlier. The first HP LaserJet was developed based on the PC 25 copier.

The Nature of Selenium Xerography became possible when it was discovered that the element selenium had peculiar electrical characteristics. It had the capability of receiving and storing a localized charge when confronted with high voltage. Even more interesting, it was found that light would dissipate the accepted charge. Tests indicated that a charged sheet of selenium-coated aluminum (or any other suitable base metal) would release its charge when struck by light, only in the area exposed to the light and proportionate to the intensity of the exposure, providing there was a ground path.

This created the opportunity to use static charges in the photographic process, as the electrostatic image created by light on the charged field was analogous to a photographic negative.

The Action of Charged Particles and Charged Surfaces.

What was lacking was a method to develop the image and to establish the best way of creating it. Clearly, the electrostatic process could be used in cameras, but that application seemed impractical and inferior compared with processes using existing exposure materials. However, it was tailor-made for copying, a field previously dominated by mimeographs (producing low-quality output), thermal copiers (an undesirable process) and actual photocopiers (expensive to operate). Since one could create an electrostatic image of contrasting charges within a detained field, it became inevitable that a process would be found to develop the image visually using some form of opaque charged particle.

Static electricity had been well understood for some time; it had been used in parlor tricks back in the 1600s. There were, however, few practical applications using static electricity until the Twentieth Century The basis for developing an image on the selenium surface was a simple idea that like-charged particles repel one another and opposite-charged particles attract each other.

Fairly quickly, it was discovered that an image could be formed by shining an intense light on an object or paper and focusing the reflected light on the selenium-coated Organic Photoconductor (OPC). The result was that light areas in the original document were represented by discharged areas on the OPC, while the dark areas maintained their charge.

It was necessary to develop the darkened areas, the areas that still held a charge. In these machines, the initial charge placed on the OPC had one polarity, and the toner was charged with the opposite. This was called a "write white" system because the areas discharged by light, the written area, did not attract toner, while the areas not exposed to the light retained their charge and attracted toner.


The first copiers were large, clumsy, heavy, failure-prone machines that made fair copies but were lousy at handling paper. By the end of the '60s, however, copiers had become common in American offices; the age of xerography had begun. By the late '70s, Canon brought the convenience of quick copying into small offices and homes with the PC copier. As personal computers gained power and popularity, it became apparent that some practical printer was needed that could readily merge text and graphics for office-prepared documents.

The PC copier looked like a fine platform for developing an electrostatic printer. The problem challenging early printer developers was designing an electrostatic imaging process that would take computer data and convert it to an electrostatic image.

Lasers were fairly well understood about this time, although they were far from being the logical choice for producing an image stream in an electrostatic printer In fact, though they were used in the earliest laser printer prototypes, true lasers have been discarded in favor of light-emitting diodes that offer similar properties.

Because we're talking about laser printers, however, we'll continue to call the light source a laser beam.

Since one could create an electrostatic image of contrasting charges within a defined field, it became inevitable that a process would be found to develop the image visually using some form of an opaque charged particle.

Static electricity had been well understood for some time; it had been used in parlor tricks back in the 1600s. There were, however, few practical applications using static electricity until the Twentieth Century.

The basis for developing an image on the selenium surface was a simple idea that like-charged particles repel one another and opposite-charged particles attract each other. Fairly quickly, it was discovered that an image could be formed by shining an intense light on an object or paper and focusing the reflected light on the selenium-coated Organic Photoconductor (OPC). The result was that light areas in the original document were represented by discharged areas on the OPC, while the dark areas maintained their charge.

While a specific area of the OPC or the paper for the final print is being subjected to only one part of the total process at a time, all six process steps occur simultaneously at different positions, once the paper feed is initiated. While one area of the OPC is being cleaned, another is being conditioned, while a third area is being written upon, and so on.

  1. Cleaning - In older printers, cleaning required two steps: (1) Toner left on the drum from the last transference was wiped off by a Cleaning Blade; and (2), the remaining electrostatic image had to be erased. Erasure was accomplished by focusing incandescent light through red filters onto the drum in the area just past the Cleaning Blade. This effectively grounded the drum surface in that area, discharging it uniformly and destroying any latent image. Later development introduced Primary Charge Rollers (PCRS) that combine electrostatic cleaning with conditioning and eliminate the need for Erase Lamps.

  2. Conditioning - Before any image can be produced, the OPC must be conditioned so that a uniform negative charge can be given to the drum. In older printers, this was done by a Primary Corona Wire. In newer printers, it is accomplished by a Primary Charge Roller. Corona Wires work by ionizing the air adjacent to them. Minus six-thousand volts DC (-6KVDC) are sent through the wire, which has no ground. This ionizes the air around the wire, creating a corona effect. Separating the wire from the drum is a varistor grid drum. The corona results in a uniform minus six-thousand DC volt charge on the screen that smooths the resultant charge before it reaches the drum surface (-6KVDC). The problem with using Corona Wires, particularly negative DC Corona Wires, is that they create a dangerous gas called ozone (a negative DC current twenty times the ozone produces produced by a positive current). Primary Charge Rollers, on the other hand, produce only trace amounts of ozone and eliminate the need for Erase Lamps, as well. PCRS is coated with a conductive rubber supplied with an AC bias that is applied to erase any residual charges on the drum and to maintain a constant drum surface potential. This is what eliminates the need to Erase Lamps. Additionally, the uniform negative potential bias applied by the Charge Roller.

  3. Writing - Think of the OPC as a child's "magic slate" toy. When you pick up the film on the Slate, separating it from the soft, dark background, you erase the existing image (cleaning). Lay it back down against the backing and it's ready to be written on gain (conditioned). That's the equivalent of what you have once the OPC is charged. Writing, then, becomes a matter of control, focus and timing. In HP printers, the light is produced by a small modulated Laser Diode. The diode is mounted on a board that regulates it and receives power to fire it on the Laser/Scanner Assembly. The diode is precisely aimed at one point within the assembly. The diode itself doesn't move but fires directly at a rotating polygonal mirror. Each mirror facet determines one "sweep" from left to right, also called a "scanning line." As the Scanner Motor rotates the polygonal mirror, successive facets are exposed to the laser, producing successive sweeps. In older printers, the speed of the Scanner Motor's rotation determined that there would be 300 "sweeps per inch of drum rotation, corresponding to one inch of paper feed (thus 300 dots per inch). I did the math on this, and it's pretty impressive. The maximum engine speed of an M601 is 55 pages per minute. That means, in one minute, 605 inches of paper pass through the machine. If there are 600 scanning lines per inch, and each scanning line involves 1/6 of a revolution of the Scanner Mirror, then, in One minute, the mirror makes 13,200 rotations or 220 rotations per second. Since a scan is 1/6 of a rotation, that means a scanning line is placed in 1/1,320 of a second. It gets more amazing than that. If the Laser Diode were left on continuously, it would make a solid sweep or scanning line; something better-defined is usually called for. Most documents require the Laser Diode to be turned off and on quite frequently to produce alternating areas of light and dark (characteristics of the desired image). All differentiation of on/off is made during a single sweep. In older HP laser printers, the laser can turn on and off in 1/600 of a sweep. In other words, the switching speed is 1/600 of 1/1,320 of a second, or 1/792,000 of a second. The printer can turn the Laser Diode on or off approximately 800,000 times in a second. This is what gives new laser documents their incredible graphic fidelity. Each point where the laser's light strikes the OPC is rendered conductive from a conductive state, allowing the charge at that point, and that point only, to find the ground. Once the subject area of the OPC passes this point, it possesses a completed electrostatic image formed of negative and effective positive charges. The background of this invisible latent image is the residual uniform negative potential placed by the Primary Charge Roller. Those portions exposed to the laser form the area that will attract toner and produce an image.

  4. Developing - Developing is where the invisible is made visible. Think of the latent image as being adhesive and the toner as dust pushed against it. The result is an image formed backward, so it will transfer meaningfully. Of course, it isn't quite so simple. Charges are used instead of adhesives, allowing us to move the toner more than once. Toner is basically black plastic bound to iron-infinitesimal particles of plastic and iron-bound together in a powdery mix so fine it behaves as much like a liquid aggregation of solids. The individual toner particles are as small as four microns, about the size of a particle of smoke! Toner is stored in a bin within the cartridge and is kept next to a Magnetic Developing Cylinder. The cylinder is connected to a supply of negative, high voltage DC power. As the cylinder rotates and rubs against the adjacent toner, the toner acquires a negative surface charge from the cylinder. Before the toner is Drought into opposition with the OPC, a properly positioned rubber blade brushes" the toner to a uniform thickness. In the SX engine, this is accomplished by a metal Doctor Blade. Once the toner is brought into opposition with the OPC, it is attracted to the is charged areas, and repelled by the areas still negatively charged. To help the toner leap from developer to OPC, the Developing Cylinder is given an AC potential. (The AC potential improves density and contrast.) The density setting changes the DC bias of the Developing Cylinder, thus changing the relative attraction between the toner and drum. The DC bias of the Primary Charge Roller is also affected when the density setting is changed. As the Developing Cylinder rotates, toner migrates from cylinder to drum because of the charges on the drum, the charges on the toner, and the AC and DC bias applied to the cylinder to facilitate development of the latent image into visible one.

  5. Transferring - If you stopped the machine in mid-print, removed the toner cartridge and opened the drum shutter, you would see the developed image You would also notice a line from one of the OPC formed in the toner. end to the other. It isn't a real line; it's just the separation between the transferred image and non-transferred image. On one side is the richly developed image, on the other, the faint ghosted one formed by the toner residue that didn't transfer. To get this image from the OPC to paper, the paper is given a positive charge by a Transfer Charge Roller (in newer machines) or a Transfer Corona Wire (in older printers). The positively charged paper attracts the image formed on the drum by negative toner. This works very well, but the paper is also attracted to the negatively charged OPC. If that charge were left in place, thin paper might wrap itself around the drum. To preclude that from happening, immediately after the image (toner) is transferred to the paper, a Static Charge Eliminator bleeds off the charge to ground. The toner on the paper is held in place by a very mild residual charge and by gravity.

  6. Fixing - The Fusing Assembly, which "fixes" the toner to paper, is something like an old washing machine wringer. In the Fusing Assembly, two rollers are held tightly together. The top one is a hollow, Teflon-coated aluminum tube with a halogen lamp running through its center, providing heat. The lower roller, called the Pressure Roller, is made of a rubber-like resilient material; it maintains continuous pressure against the Upper Heat Roller. The Upper Roller is heated up in excess of 183° C during fusing. Heat applied to the paper and toner, combined with pressure from the Lower Roller, melts the toner to a high-viscous molten state and forces it into the paper fibers, locking the image in place. The temperature is registered by an on-board thermostat called a Thermistor, a variable resistor whose values change relative to heat sensed. The Thermistor reports its readings to the machine's DC Controller, which turns the Halogen Lamp off and on to regulate the temperature.

HP Laser Printers- How The Printerworks
HP Laser Printers- How The Printerworks

Should the temperature get above 230° C, a thermal switch in the fusing unit's AC circuit sometimes called a Thermoprotector, opens to turn off the lamp. This Will result in a 50 SERVICE message being displayed, and a repair being required (the thermoswitch is not a user-changeable component). All these processes occur simultaneously during printing, but to separate areas of of the OPC or paper. Sequentially, they translate the image on the computer screen to the printed page.


Historically, there are two customers for office machines at larger companies Office managers have traditionally been the custodians of photocopy machines. Typically, these people are responsible for keeping the busy work of the office going, ordering the paper-clips and staples, hiring and discharging clerical help and keeping the office copiers working. Printers usually have a different employer.

Because they are connected to computers, printers are normally purchased and managed by the MIS (Management Information Systems) department, the people who run the computers. Usually, the MIS director outranks the office manager, although one seldom reports to the other. Historically, the copier salesman has approached the office manager, as have people selling supplies and services for copiers. Similarly, the printer salesman has targeted the MIS director, as have the follow-on salespeople.

The "multipurpose" or "merged" machines blurred the dividing line and are causing particular concern in the photocopier industry, which is threatened by the potential disappearance of their customers. Because the merged machines are attached to computers, MIS directors need to manage them. As more and more "copies" are generated directly from printers, the total share of the office-production market for the copier industry will shrink, while the printer industry swells. As a result, many office copier dealers entered the printer market.

Let's then go back to our original, logical question. Why didn't they start out making combined machines? The answer is a technical one.

You will remember that copiers are what we called "write white" analog machines, in which toner is attracted to the area that is not exposed to light. Printers are "write black" digital machines, in which toner is attracted to the discharged area struck by the laser's light. Thus, copiers give toner a positive charge, while printers use negatively charged toner.

Resolving these different processes is difficult. Rather than try to get the disparate functions to work with similarly charged toner, effort has been expended to unify the imaging process. Since there is no an analog signal, the choice was made to equip copiers with a digital scanner.

Since digital scanners, even color digital scanners, have been around for a good while now, this pricing problem is difficult to understand. It becomes more difficult when you consider that the first generation of "Swiss army printers," in personalized desktop formats, are competitively priced.

The future isn't easy to predict.

On one hand, the convenience of the all-in-one machine is attractive, and it offers the prospect of greater capability to the average printer user. On the other hand, maintenance is complicated. The user runs the risk of a small problem disabling his copier, scanner, fax and printer, all at one time. Whichever machine format emerges, it will require servicing. A digital copier/ printer would essentially be a laser printer with a digital flatbed scanner mounted on top. We've already explained the imaging system that machine would use. Now we'll explain how the rest of the machine works.


My approach to printers and printer repair is organic. I relate to the printer as if it were a living organism, comparing its systems with human organic systems. This not only helps novice students feel more comfortable with a machine that seems very mysterious, but it is also, in truth, the way I intuitively feel about printers.

Printers may not be alive, but they do mimic life in a number of ways. They feed, produce waste and react with their environment. They age, deteriorate, and even die-early, if they aren't cared for during heir working lives. They lack mobility, but not motion. They are fixed to one place, taking their inputs and ejecting their outputs, much as a plant, an oyster or mussel.

The machine's assemblies combine in action much like an organic system. This may sound like mystical mumbo-jumbo to you, but it's a very real part of my troubleshooting method. I listen to the machine's sounds, feel its vibrations, watch its lights, and even catch the air for a smell that doesn't belong there. If something is wrong, I can frequently tell without touching the machine, much as a doctor can diagnose a very ill patient just by looking at him and listening to him.


If the printer is an organism, then almost the entire universe of its sensory data is the computer. All the information processed by the printer comes from the computer, or from process and maintenance back-flow from its own engine.

Unlike people, printers and computers behave with monotonous predictability. They are designed and built to do the same things over and over again, with high reliability. Only infrequently will they fail in their mission. The problems emerge when the rules change and humans get involved.

Ideally, the computer generates a signal, usually in some applications package and transmits applications package, to the printer. Various computer applications are written to accomplish various tasks that need hard-copy output from a printer.

Because there are many and various printers that the computer and its applications could be connected to, most application packages provide what are called printer drivers, small subprograms that enable the particular printer being used to recognize and process the computer's commands.


By now, you understand how the image is formed, but image formation is only a small part of the total process, First, a control system is necessary to regulate the timing and sequencing A higher-brain function is received and convert it to a signal the control system can understand.

Twin circulatory systems are required to initiate the movement of paper and toner. And the control system needs its own self-maintenance mechanisms, just like our body needs a liver and kidneys.

In addition to the Imaging System (already described), printers require and contain the following:

  • Interface/Formatting System

  • Control System

  • Power System

  • Paper Movement System

Laser Printer Interface and Formatting Systems
Laser Printer Interface and Formatting Systems

Laser Printer Interface and Formatting Systems

Interfaces - Data enters the printer through an interface, a connector receiving the flow of data from the computer. There are multiple types of interfaces in most printers.


The USB cable used for printers is called a USB AB cable, named for the plugs on each end. The USB-A end is a flat, rectangular plug; the USB-B end is a square plug with two curved edges, which goes into the printer.


USB 1 is the default printer port for USB printers and is the first port Windows selects when connecting a printer via the operating system's Devices and Printers "Add a Printer" utility. The printer installation process is done via the utility’s wizard, which walks you through selecting a port, installing the proper driver for the printer and printing a test page. Once the installation process is complete, the printer is assigned to the USB 1 port until you uninstall it, if desired.


USB 2 is also a Virtual Printer Port that appears if USB 1 is already in use with another USB printer. The port works the same as USB 1 in allowing you to select it for your USB printer and completing the installation process. Windows' Devices and Printers "Add a Printer" utility also walks you through your printer's installation process after selecting USB 2.

Wireless Printers

Wireless printing is when the device is not physically connected to a computer or network by a traditional computer or Ethernet cable. Instead, all printing information is sent to the printer via a wireless connection.

Network Connection Laser printers can have external or internal network cards that allow for a physical connection to networks. Newer LaserJet printers may have the network board built on the formatter or logic board.

Printer Format Languages: PostScript and PCL 5-6

Just as Mexicans speak Spanish and Irishmen speak English, a printer requires a language of its own in common with the computer. This is called the Printer Format Language (PFL). It defines the printer's personality and is resident in the printer's Read-Only Memory (ROM). A printer's PFL transforms the ASCII characters sent from the computer into instructions for the control system to turn the laser off and on.

Early in the history of laser printing, HP machines used a form of PCL (Printer Control Language) and Apple machines used Adobe's PostScript. PostScript had been developed previously for the typesetting and publishing industry; it was logical to adapt it to laser printers.

The primary difference between the early versions of PCL and PostScript had to do with how printers would handle fonts. PCL treated each character as a separate graphic entity. It required that the character be bitmapped into memory on a dot-by-dot basis. If the font was to be present in different sizes, each size had to be described as if it were a separate and distinct character.

For a font to be readily available for the printer to process, its bitmapped images needed to be stored in the printer's random access memory (RAM). The HP Laserjet and LaserJet Plus had limited complements of memory, and could only accept special fonts through font cartridges, cartridges that essentially eliminated the need for extra RAM for font storage.

PostScript handled fonts more efficiently. Rather than treat each character, and each different point size of the same character, as a separate graphic entity that had to be fully bitmapped and transferred to RAM, PostScript described only the outline of each character. Beyond that, it provided instructional algorithms for expanding it or contracting it, and "coloring" the fill area, making it unnecessary to have huge reservoirs of RAM available for individual character storage.

Unfortunately, the PostScript language itself is a RAM hog, invariably requiring more RAM for a given document than PCL. And PostScript costs more than PCL, as well. Over the years, successive generations of PCL have been developed so that PCL is now capable of scaling fonts on the fly, eroding the primary motivation for getting a PostScript printer in the first place.

PostScript still is graphically superior to PCL, make no mistake about it. However, it is questionable if these marginally better features are important enough to the average current user to justify the additional cost.

Laser Printer Formatters: Taking the Signal and Translating It

Continuing the organic analogy, the Formatter is the machine's higher brain, the area of thoughts and concepts. In some machines, the Formatter is part of the Motherboard. In others, it is a separate board with no other function. Most commonly, the functions of document formatting and interface with the host system are combined on the same board. The formatting system follows the instructions for describing the document and arranging the print on the page. It utilizes a personality known to the computer and lays the document out as the user intended.

All the HP printers use what is called PJL in conjunction with their standard PCL (Printer Control Language). The PJL , or Printer Job Language, establishes and maintains the control functions of the Formatter. Among these are:

  1. Allowing the printer to talk to the host computer. This is done through a bidirectional parallel connection. The printer can tell the host such things as control panel settings and allow the control panel settings to be set from the host computer.

  2. The same sort of seamless operation pertains to the personalities (PCL and PostScript). This is accomplished through what is called context-sensitive Switching. The Formatter knows which personality is operative from the host system, much as a bilingual person can tell whether he's being spoken to in English or Spanish.

  3. Each job is formatted by its accompanying instruction set in the newer printers. In older printers, users are frequently frustrated by print commands that incorrectly format their documents because previous commands have changed the printer's preset defaults.

  4. PIL can act as a spooler to print only sections of a print job as defined from the software.

Laser Printer Control System

If the Formatter is the machine's

higher brain, then the DC Controller is the machine's limbic cortex-its autonomic nervous system. The Formatter may do all the thinking, but everything else in the printer gets its marching orders from the DC Controller.

The DC Controller is a small, printed circuit-board assembly that controls the timing of all the assemblies within the printer. All the sensors in the machine report data to it. All accessories are required to ask it for power. It powers the Formatter as well. Below is a list of DC Controller systems and functions:

  • Paper Motion

  • Laser and Scanner Drive

  • Timing

  • Machine Status

  • Paper Size and Availability

  • High Voltage System

  • Fusing Temperature

  • Main Motor Drive

  • DC Power Supply

  • Toner Level

On some printers, the manufacturer has incorporated a method to test componentry, independent of whatever Mother Board a secondary manufacturer may have placed on the machine. This is the Engine Test, a pin-striped pattern that tests the ability of the engine to print out a page. On earlier models, the switch for this function is installed on the DC Controller. On newer models, it is on a Paper Control PCA (Printed Circuit Assembly).

Laser Printer Power System

Laser printers all incorporate a group of smaller appliances that act as sub assemblies. Each of these sub-assemblies requires power. Most run off low DC voltages provided by a Low Voltage Power Supply. The Fusing Assembly requires AC power to drive its Heat Lamp, but everything else runs off DC Power.

Laser Printer Paper Movement System

It is central to every laser printer or photocopier that paper movement and imaging be inextricably linked. It is of critical importance that the paper travels through the machine at exactly the same speed as the OPC drum rotates. This allows the toner to be deposited properly on the paper. Typically, this is accomplished by integrating the OPC's drum gear with the paper drive gear train.

The paper feed system has six jobs:

  1. Extracting the paper from the storage tray.

  2. Aligning the paper so that the printing is parallel to the paper's edges, every time.

  3. Propelling the paper under the OPC at the same speed the OPC drum rotates.

  4. Drawing the paper through the Fuser at the same speed while the toner is bonded to the paper.

  5. Ejecting the paper into a designated storage area when complete.

  6. Keeping the DC Controller constantly advised of the status of the system.

This process is handled differently by various printers, but long experience has given us the basis for some interesting generalizations.