You will fmd a wide variety of electronic components contained within your printer. Most circuits contain both active and passive components working together. Passive components include resistors, capacitors, and inductors. They are called passive because their only purpose is to store or dissipate a circuit's energy. Active components make up a broader group of semiconductor-based parts such as diodes, transistors, and all types of integrated circuits. They are referred to as active because each component uses a circuit's energy to perform a specific set of functions; they all do something. It might be as simple as a rectifier, or as complex as a microprocessor, but active parts are the key elements in modern electronics. This section is intended to familiarize you with each general type of component, how they work, how to read their markings, and how they fail.
So, Following are some Electrical Parts used in common Printers which are discussed as follows :
Resistors
All resistors ever made serve a single purpose: to dissipate power. Although resistors appear in many circuits, they are used primarily for such things as voltage division, current limiting, volume adjustment, etc. Resistors dissipate power by presenting a resistance to the flow of current. Wasted energy is then shed as heat. Resistance is measured in ohms using the Greek symbol Omega (Q). In a typical carbon-composition resistor, two component leads are insulated by a packed carbon filling. It is much harder for electrons to pass through carbon than copper, so the flow of current is limited. The material composition of carbon filling can be altered in manufacturing to provide many different levels of resistance. Carbon-composition resistors are rarely used in modern electronics because they are large, and they lack the precision needed to support contemporary circuits. The carbon-composition resistor has been replaced by the carbon-film type, as shown in 2-15. This kind of resistor uses a carbon film deposited onto a ceramic or glass core. Metal caps on both ends provide the electrical connections. The entire assembly is encapsulated in a hard epoxy material. Carbon film resistors are far more accurate than carbon composition devices, because films are applied with more precision and control. Resistors are also rated in terms of the power that they can handle. Common values are ~ W, l4 W, !..f W, 1 W, and 2 W. As long as the power being dissipated by a resistor is less than its rating, the resistor should perform as expected and last indefinitely. However, if a resistor is forced to exceed its power rating, it cannot shed heat fast enough to maintain a stable temperature. U1timately, the resistor will overheat and burn out (often damaging the printed wiring board and printed copper traces as well). In all cases, a burned out resistor forms an open circuit. A faulty resistor might appear only slightly discolored, or it might look burned and cracked. It really depends on the severity and dw-ation of its overheating. Replace any faulty resistors wherever you might find them. If the printed wiring board is also damaged, the entire circuit assembly should be replaced to maintain peak reliability. Adjustable resistors (called potentiometers or rheostats), as shown in 2-16, employ a movable metal wiper blade resting along a ring of resistive film. While total resistance of the film remains constant, resistance between the wiper and either end can be varied by turning a knob. Failures among potentiometers usually take the form of intermittent connections between the wiper blade and resistive film. Remember that film will wear away as the wiper moves across it. Over time, enough film might wear away at certain points that the wiper might not make good contact there. This can result in all types of erratic or intermittent circuit operation. Replace any intermittent potentiometers or rheostats.
Capacitors
The capacitor is used to store an electrical charge. This might sound simple enough, but it has impOitant implications when combined with other components in fllters, resonant circuits, or timing circuits. Capacitance is measured in farads (F). In reality, a farad is a very large value of capacitance. Practical capacitors are normally found in the microfarad (f..LF) or picofarad (pF) range. A capacitor is little more than two conductive plates separated by an insulator (called a dielectric), as shown in 2-17. The amount of capacitance is determined by plate area, the distance between each plate, and the specific dielectric material. Large values of capacitance can be achieved by rolling up a plate-dielectric assembly and housing it in a canister. When voltage is applied across a capacitor, current flows in and electrons are stored as a static charge. As the capacitor charges, its current flow decreases. This continues until the capacitor is fully charged. At that point, no additional current flows, and the voltage across the capacitor equals the applied voltage. Keep in mind that a capacitor will remain charged even after charging voltage is removed. Large capacitors can store enough energy to present a shock hazard. Ideally, charge should last indefinitely, but internal resistance through the dielectric will eventually bleed off any accumulated charge. There are two general capacitor types that a teclmician should be familiar with: fixed and electrolytic. Fixed capacitors are nonpolarized devices; they can be inserted into a circuit in any orientation. Many fixed capacitors are assembled as small wafers or disks. Conductive plates are typically aluminum foil. Common dielectrics include paper, mica, and ceramic. The assembly is then coated with a hard plastic, epoxy, or ceramic housing to keep out humidity. Larger devices can be assembled in cans that are sealed hermetically. Electrolytic capacitors are polarized components; they must be inserted in the proper orientation with respect to applied voltage. An aluminum electrolytic capacitor is made using two strips of aluminum alloy (one strip is coated to prevent oxidization) separated by a layer of gauze soaked with an electrolyte paste of boric acid, glycerin, and ammonia. The assembly is rolled up tightly and sealed into a metal canister. Each plate is welded to leads that connect the component. Tantalum electrolytic capacitors are constructed differently. An anode of sintered, porous tantalum powder is housed in a silver-plated container with an electrolyte of sulfuric acid or some other electrolyte. The small devices are then dipped in ceramic or epoxy. Like resistors, capacitors tend to be rugged and reliable devices. Because they only store energy (not dissipate it), it is virtually impossible to burn them out. Remember that capacitors do have some internal resistance. Ideally, this would be infinite, but it is actually somewhat less. Under heavy loads, a capacitor's internal resistance can dissipate power and cause heating over time. Capacitors are subject to dielectric breakdown due to electrical causes (like severe voltage overloads) or environmental causes (such as plate corrosion due to humidity penetration). These failures generally manifest themselves as open or short-circuit conditions within a capacitor. Most breakdowns are invisible from the part's exterior, but are measurable with test instruments. Electrolytic capacitors are not only subject to dielectric breakdown and evaporation, but they can also explode if enough energy is applied in the reverse polarity. A failure elsewhere in the circuit (or your incorrect placement) can reverse the voltage across a capacitor. This causes temperature and pressure to rise inside until its enclosure ruptures. The explosion is rather like a frrecracker, a bang with smoke and shards of foil and electrolyte. Take care that aluminum shards do not settle back onto the circuit and short out any other components. Electrolytic capacitors also fail from "fatigue" due to frequency stress. For example, it is not uncommon to find filter capacitors that have failed in switching power supplies. Reversed tantalum capacitors can also rupture or shatter, but most often its outer casing will crack. Heat and pressure might force out some internal material. Make sure that there is no tantalum "spatter" bridging across other components.
Inductor
An inductor is used to store a magnetic charge for much the same reasons that a capacitor is used to store an electrical charge. Advances in solid-state electronics have rendered inductors essentially obsolete for traditional applications such as resonant circuits and filters, but they remain invaluable for such high-energy components as transformers, motors, and solenoids. All inductors are measured in henries (H), although some small inductors are measured in millihenries (mH) or microhenries Q.LH). A transformer is actually a combination of inductors working together. It is composed of several important parts (as shown in 2-18), a primary winding, a secondary winding, and a core structure. An ac voltage is applied across the primary winding. The ac voltage is constantly changing its value and reversing its polarity over time. As a result, the magnetic field generated in a primary coil also fluctuates. When the fluctuating magnetic field intersects a secondary winding, an ac voltage is created (or induced) across it. This principle is known as magnetic coupling. Notice that the primary and secondary coils are wound around the same core. Acommon core concentrates magnetic energy and provides efficient coupling to the secondary coil. Although 2-18 shows only one secondary coil, there might be several secondary coils in a transformer. The actual voltage generated in a secondary coil depends on the ratio of primary windings to secondary windings. This is known as the turns ratio. If your secondary coil contains more turns than the primary coil, then ac induced on the secondary will be greater than primary voltage by a factor of the turns ratio. For example, if a transformer has 250 turns in its primary and 500 turns in its secondary, its turns ratio is 1:2 (0.5 tr). This means that 10 Vac applied to the primary will yield [10/0.5 tr] 20 Vac. Such an arrangement is known as a step-up transformer. If the situation were reversed with 500 turns in the primary and 250 turns in its secondary, your transformer's turns ratio would be 2:1 (2 tr). If 30 Vac were applied to the primary, its secondary voltage would be [30/2 tr] 15 Vac. This is called a step-down transformer. Current is also stepped in a transformer, but opposite to the direction of voltage steps. If voltage is stepped down, current is stepped up by the same ratio, and vice versa. For the step-up transformer above, if your 10-Vac input carries a current of 1 A, the 20-Vac output would only supply a current of [1 x 0.5 tr] 0.5 A. With a stepdown transformer, an input of 30 Vac at 1 A would supply an output of 15 Vac at [1 x 2 tr] 2 A. Ideally, output power should equal input power. For example, an input of 30 Vac at 1 A is [30 V x 1 A] 30 W, and the output of 15 Vac at 2 A is [15 V x 2 A]30 W. In reality, output power will always be slightly less than input power due to losses in the windings and core; no device is 100% efficient at transforming power.
Diodes
Diodes are two-terminal semiconductor devices that allow current to flow in one direction only, but not in the other. This property is known as rectification. As detailed in Chapter 8, rectification is essential to the operation of every power supply. Because diodes only operate in one direction, they are polarized devices as shown in figure 2-19. General-purpose rectifier-type diodes are available in a variety of case styles depending on the amount of current that must be carried. Glass-cased diodes are often used for low-power (or "small-signal") applications. Plastic or ceramic-cased diodes are moderate power devices, generally used for power supplies, circuit isolation, and inductive flyback protection. Metal-cased, stud-mounted devices are for high-power rectification.
Transistor
Transistors are three-terminal semiconductor devices whose output signal is directly controlled by its input signal. This kind of operation makes transistors particularly well suited for signal amplification and switching tasks. Two major families of transistors have been developed, as shown in the schematic diagrams of s 2-22 and 2-23. Bipolar transistors are common, inexpensive, general-purpose amplifiers and switches. You will probably encounter bipolar transistors in most conunercial printer circuits. Field effect transistors (FETs) and metal oxide semiconductor FETs (MOSFETs) are also used, but not as often. Unfortunately, there is just not enough room in this book to discuss the characteristics and operation of each transistor family, but it is important that you realize they are all transistors, and that you can identify them on sight should they ever appear on a schematic.
OptoIsolator
Optical isolators, also known as optoisolators, are used as position and proximity sensors within the printer. They can also be used to electrically isolate signals between circuits. A basic optoisolator is shown in 2-26. The signal to be isolated (usually digital) is fed to the "transmitter" portion of the device. Transmitters can use visible light or infrared (IR) LEDs. Light travels across a physical gap to the "receiver." Instead of current flowing into a transistor's base lead, a photo transistor's base generates current when stimulated by light. Notice the schematic symbol for a standard phototransistor. 'Vhen the LED turns on, its light activates the phototransistor, which reproduces the original signal in its output circuit.
Integrated Circuits
Integrated circuits (ICs) are the most powerful and diverse group of electronic components that you will ever deal with. They are the "building blocks" of modern electronics that can take the form of amplifiers, memories, microprocessors, digital logic, oscillators, regulators, or a myriad of other analog or digital IC functions. It is impossible to determine the specific function of an rc just by looking at it. You must refer to the printer's schematic (or manufacturer's data for the particular part) to determine the function of each IC pin.
ICs are manufactured in a variety of package styles as illustrated in 2-27. Dual in-line packages (DIPs) are the oldest and still most common IC package found in electronic circuits. Single inline packages (SIPs) are often found on densely packed printed circuits where board space (or "real estate") is limited. Densely packaged ICs such as microprocessors or complex ASICs are sometimes packaged in plastic leaded chip carrier (PLCC) packages. ICs intended for surface mounting are manufactured in specialized surface-mount packages. Integrated circuits generally do not show any outward signs of failure, so it is necessary to check suspect ICs carefully using the appropriate test equipment while the IC is actually operating in a circuit. As a rule, whenever an IC must be replaced, insert an IC socket in its place on the printed circuit board, then plug in the I C. This prevents you from having to desolder the printed circuit again should the IC need to be replaced in the future. If there is no test equipment available, your best course is to replace the subassembly containing the suspect part.
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