Resistive Materials & Components PDF

Resistive Materials and Components LAWS OF RESISTANCE: A. ELECTRICAL CONDUCTION OF MATERIALS The resistance of any substance depends on the...

Resistive Materials and Components LAWS OF RESISTANCE: A. ELECTRICAL CONDUCTION OF MATERIALS The resistance of any substance depends on the following factors: What is Electrical Conduction? Length of the substance. Electrical conduction in materials refers to the ability of a Cross-sectional area of the substance. material to allow the flow of electric charge, primarily The nature of material of the substance. carried by electrons, ions, or both. Materials are classified Temperature of the substance. based on their electrical conductivity into three main categories: conductors, insulators, and semiconductors. OHM’S LAW: - The Resistance Formula to calculate the material can be CONDUCTORS: derived from Ohm’s Law. As the electrical resistance of a Conductors are materials that permit electrons to flow material depends on the voltage across the material and freely from particle to particle. An object made of a the current flowing through the material, the formula for conducting material will permit charge to be transferred this can be given as the voltage drop across the material across the entire surface of the object. If charge is per unit ampere current flowing through it. transferred to the object at a given location, that charge is quickly distributed across the entire surface of the object. FIRST LAW: The distribution of charge is the result of electron The First Law states that ” conductive material is directly movement. proportional to the length of the material”. According to Properties: this law, the resistance of the material increases with the High electrical conductivity. increase in the length of the material and decreases with Low resistance to electric current. the decrease in the length of the material Conductivity increases with the availability of free electrons. SECOND LAW: The Second Law states that ” the conducting material is Examples: Copper, Aluminum, and Silver inversely proportional to the cross-sectional area of the material”. According to this law, its material increases with Applications: Wiring, electrical circuits, and the decrease in the cross-sectional area of the conductor components. and decreases with an increase in the cross-sectional area. With this, we can conclude that a thin wire has a INSULATORS : larger resistance value compared to a broad wire of a Insulators are materials that have just the opposite effect larger cross-sectional area. on the flow of electrons that conductors do. They do not let electrons flow very easily from one atom to another. THIRD LAW: Insulators are materials whose atoms have tightly bound The resistance of a substance is directly proportional to electrons. These electrons are not free to roam around the resistivity of the materials by which the substance is and be shared by neighboring atoms. made. The resistivity of all materials is not the same. It Properties: depends on the number of free electrons, and size of the Very high resistance. atoms of the materials, types of bonding in the materials Poor electrical conductivity. and many other factors of the material structures. If the Used to prevent the flow of electricity where not resistivity of a material is high, the resistance offered by desired. the substance made by this material is high and vice Examples: Wood, Plastic, and Rubber versa. This relation is also linear. Applications: Electrical insulation, coatings, and safety devices. FOURTH LAW: The Fourth Law states that “the conducting material SEMICONDUCTORS: depends on its temperature”. According to this law when Semiconductors are materials which have a conductivity the temperature of a metallic conductor is increased, it’s between conductors (generally metals) and non- value also increases. conductors or insulators (such as ceramics). Semiconductors can be compounds, such as gallium C. GENERAL PROPERTIES OF RESISTIVE MATERIALS arsenide, or pure elements, such as germanium or silicon. Their conductivity can be controlled by doping, 1. Resistivity- electrical resistance of a conductor of unit temperature, or electric fields. cross-sectional area and unit length. A characteristic Properties: property of each material, resistivity is useful in comparing Moderate conductivity. various materials on the basis of their ability to conduct Conductivity can increase with temperature or electric currents. impurity addition (doping). High resistivity designates poor conductors.Denoted as ρ, Form the basis of modern electronics. measured in ohm-meters (Ω·m). High resistivity materials Examples: Silicon, Germanium, Gallium Arsenide (e.g., nichrome) are preferred for resistors. Low resistivity materials (e.g., copper) are preferred for Applications: Transistors, diodes, solar cells, and conductors. integrated circuits. 2. Temperature Coefficient of Resistance: Indicates how resistance changes with temperature. Materials like B. FUNDAMENTAL LAWS OF RESISTIVITY nichrome have stable coefficients, making them ideal for Resistivity precision applications. - Resistivity or Coefficient of Resistance is a property 3. Mechanical Strength: Resistive materials must of substance, due to which the substance offers withstand mechanical stress without degrading. opposition to the flow of current through it. Resistivity or 4. Stability: The ability to maintain resistance under varying Coefficient of Resistance of any substance can easily be conditions like temperature and load. calculated from the formula derived from Laws of Resistance. D. CLASSIFICATION OF RESISTORS: below about 10KΩ), meanwhile on the right side, is a Film 1. Classification of Resistors by Construction type resistor. Having a spiral construction do tend to FIXED RESISTORS: exhibit the properties of inductors (which are basically Fixed Resistors: Provide a constant resistance value. spirally wound coils of wire) but this is not usually a Examples: Carbon film, metal film, wire-wound resistors. problem until used at frequencies in the MHz range. Film VARIABLE RESISTORS: type resistors that do not have a spiral track, such as Variable Resistors: Allow adjustment of resistance. surface mount resistors remain purely resistive up to Examples: Potentiometers, rheostats. hundreds of MHz. 2. Classification of Resistors by material Carbon Composition Resistors: Power Dissipation Made of carbon and binder. This refers to the maximum power a resistor can dissipate Low cost and widely used in general applications. without overheating. Resistors are typically produced in Metal Film Resistors: standard power ratings, often in fractions of 1 watt, with High precision and stability. larger carbon and metal resistors available in ratings Used in sensitive electronic circuits. ranging from 1 watts to approximately 5 watts. Wirewound Wire-Wound Resistors: resistors generally offer power ratings up to around 25 Wires wound around a core. watts, though specialized wirewound resistors with Ideal for high-power applications. significantly higher power ratings are also manufactured, Ceramic resistors: often tailored to meet the specific requirements of High resistance and excellent thermal stability. equipment manufacturers. 3. Special Purposes Resistors: Thermistors: Resistance changes with temperature. Power De-rating Application: Temperature sensors in HVAC systems. Typical maximum temperatures for carbon composition Photoresistors: Resistance varies with light intensity. resistors would be around 100 to 120°C and for metal and Application: Automatic street lights. oxide film types, about 150°C. Wirewound resistors can Varistors: Resistance changes with voltage. operate at higher temperatures up to around 300°C. Application: Surge protectors. Maximum Temperature F. Important Parameters pertaining Resistors: Resistors are designed to function within a specific · Temperature Coefficient temperature range, ensuring that parameters such as · Frequency Response tolerance and temperature coefficient remain as · Power Dissipation specified. Outside this range, these characteristics are no · Power De-rating longer guaranteed. Prolonged exposure to high · Maximum Temperature temperatures causes a gradual increase in a resistor's · Maximum Voltage resistance, particularly noticeable in resistors with initially · Safety Symbols high resistance values. C Temperature Coefficient Maximum Voltage The resistance of a resistor depends on its length, cross- The voltage developed across a resistor as current flows sectional area, and the resistivity of its material. When through it places an electrical stress on the materials from temperature changes, the resistance typically varies which the resistor is made. If this voltage exceeds the slightly because manufacturers use materials with permitted maximum there is a likelihood of a sudden resistivity minimally affected by temperature, resulting in breakdown of the resistor and a voltage flash over. The a low temperature coefficient. This coefficient, expressed maximum voltage varies greatly between different types in parts per million (ppm), indicates the fractional change of resistor from just a few volts for some surface mount in resistance per degree Celsius (°C). types to several thousand volts for some specialist high voltage resistors. Frequency Response Ideally, resistors should act as pure resistors, without any Safety Symbols of the characteristics of other types of components and when they are used in DC circuits they do. In AC circuits, however, some resistors may have characteristics that make them unsuitable for a particular purpose. At high frequencies, some resistors also have characteristics of capacitance and/or inductance. Because of this, they will When servicing equipment it is advisable to use have a property called reactance, like resistance but replacement components supplied by the original dependent on the frequency of AC signals passing manufacturer as far as is possible. In addition, certain through the component. The frequency response of a critical resistors in any piece of equipment may be resistor tells us at what frequencies the resistor still acts labelled as a safety component with a small symbol. as a pure resistor, without any significant effects The markings shown are not universally adopted associated with these other types of frequency- however, so when servicing any electronics equipment, dependent components. close attention must be paid to manufacturer’s service manuals for the particular equipment being worked on. G. Resistor Codes Resistor values are often indicated with color codes. Practically all leaded resistors with a power rating up to one watt are marked with color bands. The color code is given by several bands. Together they specify the resistance value, the tolerance, and sometimes the On the left side is an example of a Carbon composite reliability or failure rate. The number of bands varies from resistor, it acts as pure resistors at frequencies in the three to six. At a minimum, two bands indicate the Megahertz (MHz) range (at least those with a resistance resistance value, and one band serves as a multiplier.  The resistance values are standardized; these values are 6 Band Resistor called preferred values. 4 Band Resistor Resistors with 6 bands are usually for high precision resistors that have an additional band to specify the temperature coefficient (ppm/˚C = ppm/K). The most common color for the sixth band is brown (100 ppm/˚C). This means that for a temperature change of 10 ˚C, the resistance value can change 1000 ppm = 0.1%. For the 6 band resistor example shown above: orange (3), red (2), brown (1), brown (x10), green (1%), red (50 ppm/°C) represents a 3.21 kΩ resistor with a 1% tolerance and a 50 ppm/°C temperature coefficient. Color Code exceptions: Reliability Band Single Black Band or Zero-ohm resistor 5 band resistor with a 4th band of gold or silver The four-band color code is the most common variation. These resistors have two bands for the resistance value, Deviating Colors one multiplier and one tolerance band. In the example shown here, the 4 bands are green, blue, red and gold. E. Selection Considerations of Resistors By using the color code chart, one finds that green stands for 5 and blue for 6. The third band is the multiplier, with Selecting the appropriate resistor for a specific application red representing a multiplier value of 2 (102). Therefore, requires considering various factors to ensure optimal the value of this resistor is 56 · 102 = 56 · 100 = 5600 Ω. performance and reliability. Here are the key factors to The gold band means that the resistor has a tolerance of consider when choosing a resistor: 5%. The resistance value lies therefore between 5320 and 5880 Ω (5560 ± 5%). 1. Resistance Value The resistance value, measured in ohms (Ω), determines 5 Band Resistor how much current will flow for a given voltage. Calculate Resistors with high precision have an extra band to indicate a third significant digit. Therefore, the first three the required resistance using Ohm's Law (V=IR) with the circuit's voltage and current requirements. If the exact bands indicate the significant digits, the fourth band is the resistance value is unavailable, consider using series or multiplication factor, and the fifth band represents the parallel resistor combinations to achieve the desired tolerance. For the example shown here: brown (1), yellow value. (4), violet (7), black (x 100 = x1), green (0.5%) represents a 147 Ω resistor with a 0.5% tolerance. 2. Power Rating The power rating indicates how much power a resistor can dissipate without failing. It is critical to select a resistor with a power rating that is at least twice the expected power dissipation in normal operation. For high-power applications, consider resistors rated for three to four times the calculated dissipation 3. Tolerance Tolerance specifies how much the actual resistance can vary from its nominal value, expressed as a percentage. Common tolerances include ±5%, ±1%, and ±0.1%. For critical applications requiring high precision, choose resistors with lower tolerance values 4. Temperature Coefficient The temperature coefficient of resistance (TCR) indicates how much the resistance changes with temperature variations. For applications in fluctuating temperatures,  select resistors with a low TCR to maintain stability and https://www.esa.int/ accuracy 5. Voltage Rating BULLET PROOF VEST Ensure that the resistor's voltage rating exceeds the Another characteristic where this might come into play maximum voltage it will experience in the circuit. This would be ballistic resistance, meaning whether a fabric prevents breakdown and failure under high-voltage can withstand projectiles. This is a preferred feature in conditions protective apparel, such as bulletproof vests. Bulletproof vests are made of three layers—the layer on the outside 6. Mounting Type is for everyday use, and the layer on the inside is Decide between surface-mount technology (SMT) or protective, known as the ballistic panel, with a comfort through-hole mounting based on the design requirements layer against the body. The layers that constitute the and space constraints of your application ballistic panel are usually made of aramid fibers (Kevlar) or ultra-high-molecular-weight polyethylene. When a 7. Special Conditions projectile hits a bulletproof vest, it does not allow for Consider any special conditions that may affect surface penetration; instead, it takes on the energy impact performance, such as: and dissipates it—often through deformation—so that High-frequency applications: Choose resistors with low less energy of impact penetrates the body beneath. parasitic capacitance and inductance. Pulse power applications: Select resistors designed to handle transient conditions without damage. Environmental factors: Ensure robustness against humidity, vibration, or extreme temperatures. 8. Material and Construction Type Different resistor materials offer various performance characteristics: Carbon Composition: Suitable for high-energy pulse applications but less stable. Metal Film: Offers better tolerance and stability than carbon film. Wire Wound: Good for high precision and power Reference:https://ojp.gov/pdffiles1/nij/099859.pdf applications but may have issues at high frequencies. Metal Foil: Provides superior precision and low noise ULTRA-VIOLET RAY RESISTIVE MATERIAL levels, ideal for high-performance applications One recognized visual material destroyer that causes fading, strength loss, and brittleness is ultraviolet light. H. Presentation of Resistive Material Products Because of their strong molecular connections, inorganic Manufacturing materials like glass and ceramics naturally resist UV light. For UV stabilization, organic polymers such as PTFE SPACE SUITS (Teflon), PVC, ABS, and polycarbonate can incorporate Space suits stop astronauts from suffering catastrophic stabilizers, such as UV absorbers and HALS. By events while conducting experiments in space. They efficiently absorbing UV radiation and releasing them as shield from catastrophic concerns such as radiation, heat, these stabilizers prevent damage to the polymer micrometeoroids, and orbital debris, and extreme chains. These materials can be used for packing, car temperatures. The components of a space suit that parts, and outdoor furniture. It can last for many years if it control temperature include the outer and inner layers of is designed to withstand exposure to sunlight and uses the suit, plus the Liquid Cooling and Ventilation Garment the right UV-resistant materials. (LCVG) and Thermal Control Layers (TCLs). The TCLs— Reference: https://www.americanchemistry.com/ standard insulation materials like Mylar—reflect away from the body in hot situations; in cold situations, they FIRE-PROOF CLOTHING hold heat close to the body. The specific layer of a space Fire-resistant clothing, or turnout gear, protects suit which protects against micrometeoroids is the firefighters from extreme heat and flames and an array of Micrometeoroid Garment Layer (MGL). The materials other hazards. They are constructed of fabrics such as from which space suits are made are high tensile fabrics Nomex, Kevlar, Modacrylic and PBI. They possess such as Kevlar or Nomex. outstanding thermal stability, flame resistance, and heat protection properties. These composite fibres put together are quite heat resistant preventing the path of direct harm to firefighters and allowing them to do their jobs without putting themselves in danger. FIREWALL Firewalls are critical components in building construction designed to prevent the spread of fire within structures. They serve as passive fire protection systems, which means they do not actively extinguish fires but instead contain them, allowing for safer evacuation and minimizing property damage. The materials used to construct firewalls are critical for their effectiveness and compliance with safety regulations. Firewalls are built using various fire-resistant materials to prevent the spread of fire. Some commonly used materials are: Reference:https://www.nasa.gov/humans-in- space/astronauts/spacesuits/  Concrete, including precast panels and cast-in-place options, offers robust protection with fire-resistance ratings from one to four hours; Gypsum board, particularly Fyrchek, is commonly used in commercial settings for higher fire resistance, while standard gypsum board is suitable for less critical applications; Brick walls provide significant fire resistance due to their density and are prevalent in both residential and commercial buildings; Steel requires additional fireproofing, such as intumescent paints or concrete encasement, to enhance its effectiveness; Modular wall systems like Speedpanel combine lightweight cores with reinforced shells for fire and acoustic protection, ideal for multi-unit buildings. Other materials include; fire-resistant insulation (like mineral wool) to improve thermal resistance and advanced ceramics (such as FIBERFRAX) for specialized applications. The choice of material depends on the required fire-resistance rating and local building codes.