Printer Faults - High Voltage Power Supply

In a laser printer most of the work is done by a succession of static electrical charges. Although static electricity is a commonplace phenomenon, laser printers are one of the few machines to make use of it for a mechanical effect. Laser printers use static electric fields to manipulate an artificial dust called "toner".

The laser actually plays quite a small part in the print process and can be replaced by a row of LEDs, or indeed older analogue photocopiers just use bright light.

The power for the actual process of moving toner is mainly in the form of high voltages provided by a high voltage power supply. Some charges also come from triboelectric effects (i.e. the rubbing action of toner particles on one another and on cartridge surfaces).

High Voltage Power Supplies sound scary and do need to be treated with caution. The charges are intended to be at low currents, but the little transformers that do the job could deliver a heart-stopping high current momentarily. A similar issue is experienced as the CCFTs in LCD screens or the final anodes in TVs: beware of high voltages.

Recent printers only turn parts of the HVPS on when they need them. If a machine cover is open, the interlock switch cuts them off and resistors discharge the circuit so it is unlikely that an ordinary user will get a shock.

It is possible that a static discharge from a user might damage printer circuits. Static discharges from vacuum cleaner hoses and polyester fleeces definitely do damage equipment. The static discharged from clothing and plastics can exceed the breakdown voltages of the coils, diodes and capacitors that make up the HVPS. For instance a coil exposed to excessive static will have a small breakdown in its insulation.

Outline of the Mechanism.

Photoconductor.

The heart of the printer is a photoconductive material. A photoconductor is a material that acts as an insulator in the dark but becomes conductive in the light. TV cameras and streetlight controllers are based on photoconductors - there are several different types of these.

The photoconductors used in photocopiers and laser printers can be selenium, silicon, cadmium or organic polymer based. Selenium and cadmium have health concerns. Amorphous silicon is used but requires heating to make it sensitive. The most popular photoconductors in printers are organic polymers called organic photoconductors or OPCs. The polymer substance is normally coated onto drums because that is the most suitable shape for a continual process.

Charges

Similar electrical charges repel one another, opposite charges attract. The same holds for small charged particles. The magnitude of the effect on a particle depends on its size and on the voltage. In common experience the charges across cell membranes and skin are fractions of a volt but take off a polyester fleece and it generates several thousand volts - the effect can be heard and occasionally felt as static.

The mechanical effect of static electric fields is not very great, as several hundred square millimetres of paper charged to a thousand volts exert just a few grammes of force. It takes a considerable voltage to have much effect over a short time. In older photocopiers the particles were about 8-10 microns across, the charges were several thousand volts and delivered to the photoconductor by corotron wires. In recent printers the particles are about 5 microns across, the charges are several hundred to a couple of thousand volts and delivered by conductive rollers.

In principle the voltages could be generated by triboelectric, piezoelectric or even pyroelectric means (rubbing, pressing or heating materials). Most of the charges used in a printer come from the special high voltage power supply and this is transferred to the toner and made to control toner movement using metal and conductive plastic rollers.

Print Process.

In the dark a photoconductor surface is given a charge; to have the desired effect on toner particles a typical drum charge is -600 volts. In the absence of light the drum just holds that charge and turns toward the next process.

The photoconductor is next exposed to a light source. In a copier this is a narrow strip of light focused from the document by a mirror. In a laser printer it is raster scanned. The laser generates pixels and a polygon mirror scans the beam as the drum turns. Where it is exposed to light the photoconductor loses charge. The photoconductor surface then holds a "latent image" in varying electrical charges. There is nothing to see, the image could be detected with an electrometer.

A developer makes the image visible by presenting a layer of toner powder close to the photoconductor. They usually get close but don't actually touch. A typical developer has a roller which attracts toner powder (it might use a magnet or static to do this) and then carries it under a doctor blade. The doctor blade leaves a coat of toner of sufficient thickness.

Voltages.

A succession of voltages are applied around the photoconductive drum.

The sequence goes something like this:

• Precharge - the photoconductor is charged to a high voltage, around 600 volts negative in modern printers.
• Illumination - the optical source (laser) discharges some areas. Most systems are write-black, the laser discharges areas that are to carry toner powder. In a write-white system the laser discharges areas that will not carry toner.
• The OPC now carries a "latent image" in varying static charges.
• Developer - The developer unit brings electrically charged toner powder in close proximity with the OPC. There are several kinds of developer.

• In one type the toner is mixed with iron filings and rubbing against them creates charge, then the filings are brushed near to the OPC.
• in another type the toner powder is simply stirred and that gives it a charge (the polyester pullover effect). The contains some magnetite and that is attracted to the developer magnet.Most developers carry toner around a roller over which is hung a doctor blade. The doctor blade is set fractionally above the roller so that only a thin layer of toner or toner/filings can pass through. Sometimes friction with the doctor blade also imparts charge to the toner.
• Whatever the developer mechanism, its purpose is to present a thin but opaque layer of toner to the OPC and have sufficient adhere in places where charge differs. Several voltages might be involved at this point. The OPC drum will be grounded so it carries charges ranging from zero in discharged areas down to -600. The developer might be carrying toner with a surface charge of -600 that will be repelled by the OPC in areas remaining charged but attracted to areas with no charge. This is called the DC bias and is usually somewhat adjustable, controlling exactly how much toner moves. There is usually also an AC bias without which the toner is not very inclined to actually move.
• The image is now real and visible on the drum.
• Transfer is the next step. The drum turns over the transfer station which is usually a conductive rubber roller carrying a positive charge. Paper takes up the charge. Toner is attracted to the positive charge on the paper and most of it adheres. (why there is no AC bias I don't yet know)
• Static Eliminator - Finally the paper usually travels across a charge eliminator, which removes all the charge before the paper gets near the fuser rollers. Some recent printers do this differently, actually putting a charge on the fuser rollers to prevent the toner shifting.

As the foregoing suggests a typical printer is making precharge, developer bias, AC bias and transfer voltages. There may be others as well. Lexmark use a voltage and a toner feed roller to adhere toner to the developer rollers.

There is considerable variation. Older printers tend to use corona wires and much greater voltages. In some printer designs the initial charge can be positive.

Most printer designs follow similar lines. However there are something like 20 engine manufacturers and they have a battle of wits regarding patents and proprietary secrets. Toners are mixed with charge enhancers, various plastics are formulated to preserve or eliminate charges and some drum technologies like amorphous silicon turn parts of the conventional method on their heads. Manufacturers get terribly concerned about the details.

If the actual voltage is important, use the service manual to find out what it is supposed to be and measure it. However the average service technician doesn't seem to measure voltages very often because it's an inconvenient thing to do. If the precharge voltage is -600 Volts that is at the limit of tolerance for most ordinary multimeters, so it would be unwise to use one because the voltage could be too high and damage the meter, the user or both.

HVPS Techniques

High voltage power supplies typically use a transistor inverter, a small transformer and often have a diode / capacitor ladder to follow.

The HVPS doesn't usually step up a mains voltage. Instead it tends to work from the 24 to 36 Volt DC supply used for motors. This provides a better regulated supply that can be turned on and off by simple transistor circuits and that can run at a high frequency - perhaps a hundred kilohertz. The simplified circuit here suggests how it might work; the transistor starts a condition, creating a magnetic field in the transformer core. The rise of the field shuts the transistor down and the field collapses restarting the process. The other winding of the transformer core has many turns so the alternating field from this side is at a high voltage. This AC is rectified back to DC with a diode and capacitor.

High voltage transformers have to be carefully wound and typically have insulating layers separating the secondary winding at intervals to avoid the arcing that would otherwise be a problem. A transformer with 100 turns on the 20 volt input side needs 10,000 on the 2,000 volt output side. Insulating layers can get quite bulky so the line output transformer in CRT based monitors is typically fist sized. Older laser printers have chunky high voltage transformers. The transformers driving modern laserprinters and notebook cold-cathode tubes are smaller still.

Voltage multipliers are used to double or quadruple the output, allowing the transformer to be considerably smaller. This latter part is also known as a Cockroft Walton generator or a Greinacher Voltage Multiplier after its early 20th Century inventors. The diodes charge the capacitors in parallel but they add up in series making the higher voltage.

Colour laser printers can need three or four voltages for each colour, a further charge for the ETB attachment and separation and yet another for the fuser polarization. Many recent printers have a whole side of the machine dedicated to the HVPS board.

High Voltage Supplies are full of rather fragile looking components with weird air-gaps in the circuit boards to minimise the risk of arcing causing malfunctions. Most HVPS boards cause very little trouble. If there is a high voltage fault in a printer it is much more likely to be due to dirty, tarnished or badly connecting contacts than an electrical failure on the HVPS circuit board.

HVPS outputs can be a bit difficult to measure. Don't connect an ordinary meter - it is likely to become damaged.

A modern printer will typically turn various HVPS stages on when they are needed, so they will only be present for a couple of seconds. Typically there will be a second or so of two or three voltages as the rollers are prepared, used and then cleaned.

Measuring Voltages

To measure voltages which might exceed 600 a high voltage probe is needed. A typical handheld test meter has a maximum input voltage of 600 and anything greater is quite likely to damage the meter. A few meters have a maximum of 1,000 volts but laser printers typically produce more than that - up to 3,000 volts is quite normal and higher voltages perfectly possible. These voltages are much greater than test meters are intended for and could possibly break down the insulation of the meter risking the user's life.

High Voltage Probes are the way to pre-scale the voltage to fit a meter range. A high voltage probe is just a couple of resistors making a voltage divider to pre-scale the signal (say 95 megaohms in series with a 1 megaohm resistor to ground. Together with the meters typically 10 megaohm resistance this gives a 100 to 1 pre-scaling. High voltage resistors need to be used - ordinary low voltage resistors could break down and arc, at the least damaging the meter and at worst proving fatal.

Grounding of the test probe needs to be done properly as well. It's typically a crocodile clip, so not very substantial. If the ground came off there would be about 95 megaohms between the meter user and perhaps 5,000 volts. Happily the very high resistance should limit the current to under 0.2 milliamps - well under the few milliamps needed to make a shock fatal - but still a risk to avoid.

Some laser printers have readily accessible test points but many do not. Testing an inaccessible HVPS is not something technicians want to do in the field where an accident could too easily involve a customer.

The main test for HVPS boards is probably to swap a known good one into a suspect printer after inspecting the connections to make sure it won't be damaged in turn. See if it clears the fault.

At the moment most laser printers seem to get the high voltages primarily from the HVPS however that might not always be true. It would be possible to have mechanisms that generate all their forces by moving belts like a Van de Graaf generator or piezoelectrically.