Buchholz Relay

A Buchholz relay is a safety feature of some electrical transformers, choke coils, or high-voltage electrical capacitors and reactors. It is designed to prevent spreading damage in the case of a short circuit, arcing, or other dangerous electrical faults, such as an explosion or deteriorating condition of overheating. The concept for the relay was invented by Max Buchholz, a 20^th century engineer and inventor whose ancestors emigrated to the US from Germany in the 1800s. He first developed the Buchholz relay in 1921, but it wasn't put into widespread use in the US until the 1940s.

Each Buchholz relay acts as a sort of circuit breaker, most often attached to the top of oil-filled electrical transformers where an oil reservoir tank known as a conservator sits. The chief role of the device is to maintain a dielectric constant or insulating property for the transformer, and it can do this by controlling the supply of circulating oil from the conservator, as well as detecting air leaks into the system. Safety switches like the Buchholz relay are an essential component of modern-day power distribution grids. They are designed to minimize damage to broader areas of the system in case of a localized fault, which could otherwise propagate and overload other transformers farther down the line.

The construction of such devices is heavy duty, so that they can withstand high electrical currents and varying climate conditions. The housing is dome-shaped and made from a weatherproof aluminum enclosure with built in mechanical test and trip circuit controls, as well as an inspection window of tempered glass to visually monitor insulating oil levels. The switches in a Buchholz relay are capable of handling voltages from 24 up to 250 volts of eitheralternating current (AC) or direct current (DC), and the insulation of the relay can handle 2,000 volt charges. The insulating oil itself is a form of mineral oil stable at high temperatures or silicon-based fluorinated hydrocarbon compounds which usually have a functional temperature range of between 77° to 239° Fahrenheit (25° to 115° Celsius).

A series of oil floats in a Buchholz relay are used to gauge fault levels in the transformer. Minor electrical faults will generate a small amount of gas in the oil, which will move an upper float and cause the relay to activate an external alarm. Large-scale faults will release enough gas that a tripping switch in the Buchholz relay is activated when a flap on the larger, lower float is rotated by the rising gas, and the relay cuts power to the transformer. An external button on the device is provided for a reset of the system when the reason for the fault has been determined and corrected. If the transformer sustains a minor oil leak or a small amount of air enters the unit, the minor float assembly activates the alarm. When leaks become significant, the tripping switch is thrown by the larger float and the system is shut down.

Variations on the design can include a mercury switch attached to the rotating flap for the lower assembly instead of a float device. Some units have test cocks as well to check whether the floats and mercury switches are working correctly by channeling air through the system and monitoring their response. The relay assembly is often mounted on a heavy-duty cast iron plate and terminals are insulated with ceramics to give the Buchholz relay added strength and durability.

For more details : Buchholz Relay

Polarized Relay

This type of relay has been given more importance on its sensitivity. These relays have been used since the invention of telephones. They played very important roles in early telephone exchanges and also in detecting telegraphic distortion. The sensitivity of these relays are very easy to adjust as the armature of the relay is placed between the poles of a permanent magnet.

For more details : Polarized Relay

Mercury Wetted Reed Relays

Similar in construction and operation to dry reed relays except that a small amount of mercury is added inside the glass tube to provide more consistent contact resistances.
Main Advantages
• More power handling than a dry reed
• Consistent and low contact resistances
Main Disadvantages
• Position sensitive
• Mercury is a sensitive material
• Expensive relays

For more details : Mercury Wetted Reed Relays

Dry Reed Relay

Dry Reed Relays

Contacts are made from ferromagnetic material (reeds). These contacts are encapsulated in glass. An energizing coil is wrapped around the glass and an EMF brings the two reeds together, closing the contacts.
Main Advantages
• Hermetically sealed, reduces oxidation build-up
• High Isolation (about 1014 O)
Main Disadvantages
• EMFs effect adjacent relays – requires shielding of relays in high-density applications.
• Inconsistent contact resistances


For more details : Dry Reed Relay

Reed Relay

Reed relays consist of a coil surrounding a reed switch. Reed switches are normally operated with a magnet, but in a reed relay current flows through the coil to create a magnetic field and close the reed switch.

Reed relays generally have higher coil resistances than standard relays (1000 for example) and a wide range of supply voltages (9-20V for example). They are capable of switching much more rapidly than standard relays, up to several hundred times per second; but they can only switch low currents (500mA maximum for example).

These types of relays have been given more importance in the contacts. In order to protect them from atmospheric protection they are safely kept inside a vacuum or inert gas.  Though these types of relays have a very low switching current and voltage ratings, they are famous for their switching speeds.

 

For more information: Reed relay

Latching Relay

It is a relay that is set (ON) or reset (OFF) by the input of a pulse voltage. Even after the input voltage is interrupted, this relay maintains its set or reset condition until it receives the next inverting input. It is also called a keep relay.

There are two types of mechanisms for maintaining the set and reset conditions: a magnetic holding type and a mechanical holding type.

There are also two types of coils for applying the set and reset pulse voltages:a single-winding type and a double-winding type.

Basic Operation:

Outline
In these Relays, the input pulse of the set coil causes the operating condition to be maintained magnetically or mechanically, whereas the input pulse to the reset coil side puts the Relay into the reset condition.
 

Outline
In these Relays, the set input pulse causes the operating condition to be maintained magnetically, whereas the reset input pulse (input with inverse polarity of set input) puts the Relay into the reset condition.

 

For more details : Latching Relay

Relay

What is a Relay?

A relay is an electrical actuator that functions as a switch.
It takes in as input an electric current and uses this current to close a switch. Thus, a relay is really a switch that is switched on and off by changes in the current input into it

How Relays Work

Relays work taking an electrical current through them. In response to this current, the coil of the relay produces a magnetic field. This magnetic field then attracts the mechanical switch, closing it.

This is why a relay is an electrical actuator. It uses an electric current to actuate the closing of a switch, and this is through the use of a magnetic field

For more details : Relay, The Relay

Extrinsic Semiconductor

To convert this stable crystal into a semiconductor some impurity is added into it by doping the silicon material. For adding the impurity, both silicon and impurity are melted at high temperature and mixed perfectly. Then the mixture is cooled to form a new type of material known as the semiconductor. It is then cut into the shape of thin semiconductor wafers. It is also called as extrinsic semiconductor, because some external material is added into pure Silicon and a pure silicon crystal is called as the intrinsic semiconductor.

The impurity added into pure silicon is mainly of two types: the trivalent and the pentavalent. In trivalent impurity likeAluminium, Gallium, Indium and Boron, the outermost shell can accept one electron, to become stable. It means that such atom is accepter type atom. Similarly, in pentavalent impurity like Bismuth, Arsenic, Antimony and Phosphorus the outermost shell can give out one electron to become stable. It means that such atom is donor typeatom.

To manufacture N–type semiconductor, pentavalent impurity is added into a pure Si crystal. Due to this, four electrons form covalent bond with four neighboring silicon atoms and one electron becomes a free electron within four Silicon atoms. This free electron is far away from the nucleus of silicon atom and it is free from the covalent bond. Thus, it can carry current through N-type semiconductor, when voltage is applied to it. Hence, in N–type semiconductor, majority charges are electrons to carry current and minority charges are the holes.

In the same way, P–type semiconductor is manufactured. For this, trivalent impurity (like Aluminium) is added into a pure Si crystal. Thus, three valance electrons of Aluminium, form covalent bond with three neighboring Silicon atoms but now one electron becomes LESS for Aluminium, within four Silicon atoms. This vacancy produced due to absence of one electron is called as hole. This hole is considered as +ve charge, because it can accept one electron from outside. Thus, it can carry current through P-type semiconductor, when voltage is applied to it. Hence, in P–type semiconductor, majority charges are holes to carry current and minority charges are the electrons.

For more details: Extrinsic Semiconductor

Intrinsic Semiconductor

When a semiconductor is in pure form, without mixing of any other impurity in it (like Si or Ge in pure form), such substance is called INTRINSIC SEMICONDUCTOR. Silicon and germanium are tetravalent elements. They have four valance electrons in their outermost orbit as shown below. The nucleus exerts a weak nuclear force on valence electrons. In silicon atomic structure, each valance electron is bonded with its neighboring silicon atom. This type of bonding is known as covalent bonding.

However, when some external energy (like voltage or strong light) is applied on a piece of pure silicon, the covalent bond breaks and valance electrons jump into conduction band to carry electric current. The energy required to break the bond is 1.12eV for silicon and 0.75eV for germanium. When an electron jumps into conduction band, it leaves a vacancy in its own atom. This vacancy is called as hole. Thus, the electron–hole pair is produced simultaneously in silicon atom and their number is ALWAYS EQUAL. A small section of silicon crystal is shown in the following figure –

For more details : Intrinsic Semiconductor

Semiconductor

The magic word semiconductor is composed of two words-Semi and Conductor. Semi means not completely while conductor mean something, which can conduct electricity. Everybody is familiar with "Electricity". It is present everywhere; it runs many appliances in your home and outside the home like TV, Bulb, Freeze, and Microwave Oven etc. In simple terms, the current must past through wires so that the electricity can reach all these appliances. So a conductor is nothing but a material having ability to conduct this electricity. Semiconductors conduct electricity to some extent, less than the conductors, how much do you think? Well, it depends on the type of material or it's mixture and size. A semiconductor is a material that has intermediate conductivity between a conductor and an insulator. It means that it has unique physical properties somewhere in between a conductor like aluminum and an insulator like glass. In a process called doping, small amounts of impurities are added to pure semiconductors causing large changes in the conductivity of the material. Examples include silicon, the basic material used in the integrated circuit, and germanium, the semiconductor used for the first transistors.


Importance : 


To understand the importance of semiconductors let's first understand the difference between electricity and electronics. Both are concerned with generating, transferring, and utilizing electrical energy. The chief difference is that electricity is concerned with using that electrical energy in power applications for heat, light, and motors while electronics is concerned with power control and communications applications such as electronic thermostats, electric motor speed control and radio. Engineering importance of semiconductors results from the fact that they can be conductors as well as insulators. Semiconductors are especially important because varying conditions like temperature and impurity content can easily alter their conductivity. The combination of different semiconductor types together generates devices with special electrical properties, which allow control of electrical signals. Semiconductors are employed in the manufacture of electronic devices and integrated circuits. Imagine life without electronic devices. There would be no radios, no TV's, no computers, no video games, and poor medical diagnostic equipment.

For more details : Semiconductor

Wire wound Resistors

Wirewound resistors are commonly made by winding a metal wire, usually nichrome, around a ceramic, plastic, or fiberglass core. The ends of the wire are soldered or welded to two caps or rings, attached to the ends of the core. The assembly is protected with a layer of paint, molded plastic, or an enamel coating baked at high temperature. Wire leads in low power wirewound resistors are usually between 0.6 and 0.8 mm in diameter and tinned for ease of soldering. For higher power wirewound resistors, either a ceramic outer case or an aluminum outer case on top of an insulating layer is used. The aluminum-cased types are designed to be attached to a heat sink to dissipate the heat; the rated power is dependent on being used with a suitable heat sink, e.g., a 50 W power rated resistor will overheat at a fraction of the power dissipation if not used with a heat sink. Large wirewound resistors may be rated for 1,000 watts or more.

Because wirewound resistors are coils they have more undesirable inductance than other types of resistor, although winding the wire in sections with alternately reversed direction can minimize inductance.

Nichrome is a brand name for a nickel-chromium resistance wire, a non-magnetic alloy of nickel and chromium. A common alloy is 80% nickel and 20% chromium, by weight, but there are many others to accommodate various applications. It is silvery-grey in colour, is corrosion resistant, and has a high melting point of about 1400 °C (2552 °F). Due to its relatively high resistivity and resistance to oxidation at high temperatures, it is widely used in heating elements, such as in hair dryers, electric ovens and toasters. Typically, Nichrome is wound in wire coils to a certain electrical resistance, and current passed through to produce heat.

For more details : Wire wound Resistors

Variable Resistor

Variable resistors consist of a resistance track with connections at both ends and a wiper which moves along the track as you turn the spindle. The track may be made from carbon, cermet (ceramic and metal mixture) or a coil of wire (for low resistances). The track is usually rotary but straight track versions, usually called sliders, are also available.

Variable resistors may be used as a rheostat with two connections (the wiper and just one end of the track) or as a potentiometer with all three connections in use. Miniature versions called presets are made for setting up circuits which will not require further adjustment.

Variable resistors are often called potentiometers in books and catalogues. They are specified by their maximum resistance, linear or logarithmic track, and their physical size. The standard spindle diameter is 6mm.

The resistance and type of track are marked on the body:
    4K7 LIN means 4.7 k linear track.
    1M LOG means 1 M logarithmic track.

Some variable resistors are designed to be mounted directly on the circuit board, but most are for mounting through a hole drilled in the case containing the circuit with stranded wire connecting their terminals to the circuit board. 

For more details : Variable Resistor

Trimmer Resistor

A trimmer resistor usually called a trim pot is a small variable resistor it is used in circuits to do preset it is trimmed with a trimmer screwdriver it is made up with a round fiber disc with a layer of carbon with a wiper that run on the carbon layer to change the resistance it always have 3 connecting pins where the center one is connected to the wiper and the other two to opposite ends of the carbon strip.

For more details : Trimmer Resistor

Precision Resistor

 Precision resistors are critical components in applications such as analog and mixed signal integrated circuits. Reducing the variation of the resistance values of precision resistors over the operational temperature range is critical to maintaining the stability of an analog or mixed signal circuit. In RF circuit applications, precision resistors are needed for I/O circuitry implementing both radio frequency CMOS an RF SiGe technology. High tolerance resistors are important for accurate prediction of models and statistical control. In RF devices and circuits, high tolerance resistors are needed that have good linearity; a low temperature coefficient of resistance (TCR) which is the normalized first derivative of resistance and temperature, and provides an adequate means to measure the performance of a resistor; a high quality factor (Q); and are suitable for high current applications.

For more details : Precision Resistor

Potentiometer

Potentiometers are used in circuits, when it is necessary to alter the resistance. Dark/light and temperature sensors usually have these components, as the potentiometer / variable resistor allows the circuit to be made more or less sensitive (they can be turned up or down - reducing or increasing resistance).

The long handle on the potentiometer / variable resistor can be turned clockwise or anticlockwise, altering the value of the resistance. The range of resistance varies, for example: 0 to 100 ohms 0 to 1M ohms

This can be seen when using circuit simulation software such as Circuit Wizard. The resistance value of a potentiometer / variable resistor can be altered. The animations starts with a 4.67% resistance level and reaches a resistance level of 28.33%. It can go all the way up to 100% resistance, preventing current flow, in a circuit. The resistance is altered by using the computer’s mouse, to turn the potentiometer’s dial.

The circuit below is a temperature sensor. When the temperature drops below 3 degrees centigrade, an LED illuminates. Increasing the resistance value of the potentiometer / variable resistor, by turning its handle, makes the circuit less sensitive. For instance, the temperature sensor would require a higher temperature (e.g. 8 degrees centigrade) before the LED illuminates. Circuit explanation in detail: When the thermistor is warmed up by an hair drier its resistance drops, this will take a few seconds. As its resistance drops current begins to flow from positive 9volts to negative 0 volts. Current flows into the base of the transistors allowing the LED to light. The handle of the potentiometer / variable resistor can be turned up or down, to increase or decrease resistance, in this way it can make the circuit more or less sensitive.

For more details : Potentiometer

Fixed Resistor

A fixed resistor is part of an electric circuit and is used to reduce the flow of electricity. Resistance is measured in Ohms and is typically shown as the number and then the units. For example, a 750 Ohm resistor would be written as 750-Ohm. The size of the resistor is based on the number of Ohms and can range from pin to book size.

Resistors come in two classes: fixed and variable. A fixed resistor is set at a specific value and cannot be changed. A variable resistor is able to manage flows at a specific level and below. This is an important distinction and determines when and where a resistor should be used.

When selecting a resistor, it is important to consider three things: the value of the resistor, tolerance, and power rating. The value of the resistor is measured in Ohms. The tolerance indicates the upper and lower bounds of actual performance. This is measured in plus and minus percentage. For example, a tolerance of 10 percent means that the resistor performs within a 10 percent range of the resistance value listed in the specifications.

Power rating shows the upper limit of power that can be managed by the fixed resistor. This is measured in watts. To calculate the power, multiply the resistance value of the resistor by the square root of the current. If the power rating is exceeded, the resistor will fail. A rule of thumb is to use a resistor with a power rating two times higher than the actual power needed.

There are two kinds of fixed resistors: carbon and metal film. Carbon film resistors are designed for general use and are fairly cheap to produce and purchase. These units have a tolerance of 5 percent, with power ratings of 1/8 Watts (W), 1/4W, and 1/2W. The primary issue with this type of resistor is the fact they generate electrical noise.

A metal film fixed resistor is best utilized when a higher tolerance is required. These units have a greater level of accuracy than carbon film resistors, due to the nature of the materials used. There is a corresponding increase in price, but it may be well worth the incremental cost to protect the other components of the circuitry.

When reviewing the different types of resistors, think about the intended use of the circuit. Select a manufacturer with a good reputation for quality and consistency. Take the time to test the resistor and the circuit before installation to ensure all the specifications are correct.

For more details : Fixed Resistor

Chip Resistors

Chip resistors are passive resistors with a form factor of an integrated circuit (IC) chip. Typically, they are manufactured using thin-film technology. There are two basic configurations for chip resistors: single resistor and resistor chip array. Single chip resistors are standard, passive resistors with a single resistance value. Resistor chip arrays contain several resistors in a single package. For both configurations, performance specifications include resistance range, tolerance, temperature coefficient of resistance (TCR), power rating, operating direct current (DC) voltage, current rating and operating resistors. For resistor chip arrays, the number of resistors in the package is also an important parameter to consider. 

Chip resistors are made from many different materials. Carbon-composition resistors consist of powdered carbon, an insulating material, and a resin binder. Cermet resistors are made of ceramic and metallic materials. Carbon film, ceramic composition, metal alloy, metal foil, tantalum, and wirewound chip resistors are also commonly available. Metal-film resistors are produced by depositing a resistive element onto a high-grade, ceramic rod. They are similar, but not identical to metal-oxide resistors. Thick-film chip resistors are made by stenciling a resistive metallic paste or ink onto a base in a process similar to silk screening. By contrast, think-film chip resistors are formed by vapor-deposition and then trimmed to a specific value. 

Chip resistors differ in terms of categories, applications, features and packing methods. Categories include fusible, current sensing, high current, high frequency, high voltage, power, precision, surge and telecom chip resistors. Chip resistors for automotive, aerospace, medical or military applications are also available. In terms of features, some chip resistors feature a heat sink or electrically-isolated case. Others are air-cooled, water-cooled, or non-inductive. Chip resistors with a fireproof case or non-flame coating are also available. Devices with an electrically-isolated case or NEMA enclosure are also available. Packing methods for chip resistors include tape reel, rail, bulk pack, tray, and shipping or stick magazine. 

Organizations such as Underwriters Laboratories (UL), the International Electrotechnical Commission, and the Canadian Standards Organization (CSA) maintain standards for a variety of passive resistors. Chip resistors that are destined for sale in European Union (EU) nations must meet the Restriction of Hazardous Substances (RoHS), End-of-Life Vehicles (ELV), and Waste Electrical and Electronics Equipment (WEEE) directives. RoHS compliant chip resistors contain only minimal levels of lead, cadmium, hexavalent chromium, polybrominated biphenyl and polybrominated diphenyl ether. ELV compliant devices contain only minimal amounts of lead, cadmium and mercury.

For more details : Chip Resistors

Ceramic resistor

Resistors are electronic components that provide specific amounts of resistance to an electrical current in electronic circuits. Ceramic resistors can fall into many different classes of resistors. Which classes these are often depend on who is describing the resistor. To a layman or electrician, a ceramic resistor is often any resistor encased in ceramic. Engineers and technicians, on the other hand, define ceramic resistors as those that use ceramics to control a resistor’s resistive value.

Ceramics are a very common internal component of many different types of resistors. In acarbon film or resistive wire resistor, the resistive material adheres to the outside of a ceramic core, usually in the shape of a cylinder. These cores provide a non-conductive base to hold the conductive components of the resistor in place, and give the resistor its general shape and size.

While an excellent electrical insulator, ceramic is also an excellent heat conductor. This property of ceramic allows the cores of these resistors to endure low to moderate power electrical current throughput without overheating and becoming damaged. Ceramic’s use, however, is not limited to the internal components of resistors.

Because of its insulation and thermal properties, ceramic is used to externally insulate and provide even greater thermal endurance to some types of resistors. The most common of these types are resistors made of resistive wire spun around a ceramic core and then encased in a block or cylinder of ceramic material. Combining external ceramics with metals and internal ceramics allows these types of resistors to endure very high temperatures without damage.

The construction of a true ceramic resistor, often called a carbon composite resistor, is different from most other types, even though it also uses ceramics. Ceramic resistors are made of a combination of finely powdered carbon and ceramic material. These two powders combine in specific ratios to determine the final value of the resistor.

The higher ratio of carbon in the mix, the lower resistive valve the ceramic resistor will have. A higher ratio of ceramic material, on the other hand, will mean a higher resistive value of the resistor. Once the proper ratios are established, the mixture is compressed to create its shape and then kiln fired to set the ceramic. It is common for these types of resistors to have an external shell of pure ceramic material to serve as an external insulator.

True ceramic resistors are widely used in many different types of electronic circuits and devices. While these types of resistors can endure very high operation temperatures, they also create significant amounts of electrical noise. Due to this fact, a ceramic resistor seldom finds use in sensitive radio receivers or other devices particularly susceptible to interference

For more details : Ceramic resistor

Carbon Film Resistor

It's exactly what is says. A resistor made of a film of carbon deposited on a substrate. Cuts are made in the film to allow generation of different resistance values. 
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A spiral is used to increase the length and decrease the width of the film, which increases the resistance. Varying shapes, coupled with the resistivity of carbon, (ranging from 9 to 40 µΩm) can make for a variety of resistances. Carbon film resistors feature a power rating range of 1/6 W to 5 W at 70 degree celcius. The resistance ranges from 1 ohm to 10M ohm. The carbon film resistor features an operating temperature of -55 to degree celcius to 155 degree celcius. It has 200 to 600 volts maximum working voltage range. 

Thick film resistors became popular during the 1970s, and most SMD resistors today are of this type. The principal difference between "thin film" and "thick film resistors" isn't necessarily the "thickness" of the film, but rather, how the film is applied to the cylinder (axial resistors) or the surface (SMD resistors). In thick film resistors the "film" is applied using traditional screen-printing technology. 

Thin film resistors are made by sputtering the resistive material onto the surface of the resistor. Sputtering is a method used in vacuum deposition. The thin film is then etched in a similar manner to the old (subtractive) process for making printed circuit boards: ie the surface is coated with a photo-sensitive material, then covered by a film, irradiated with ultraviolet light, and then the exposed photo-sensitive coating, and underlying thin film, are etched away. 

Thin film resistors, like their thick film counterparts, are then usually trimmed to an accurate value by abrasive or laser trimming. 
Because the time during which the sputtering is performed can be controlled, the thickness of the film of a thin-film resistor can be accurately controlled. The type of the material is also usually different consisting of one or more ceramic (cermet) conductors such as tantalum nitride (TaN), ruthenium dioxide (RuO2), lead oxide (PbO), bismuth ruthenate (Bi2Ru2O7), nickel chromium (NiCr), and/or bismuth iridate (Bi2Ir2O7). 
By contrast, thick film resistors may use the same conductive ceramics, but they are mixed with sintered (powdered) glass and some kind of liquid so that the composite can be screen-printed. This composite of glass and conductive ceramic (cermet) material is then fused (baked) in an oven at about 850 °C. 

Traditionally thick film resistors had tolerances of 5%, but in the last few decades, standard tolerances have improved to 2% and 1%. But beware, temperature coefficients of thick film resistors are typically ±200 or ±250 ppm/K, depending on the resistance. Thus a 40 kelvin (70° F) temperature change can add another 1% variation to a 1% resistor. 

Thin film resistors are usually specified with tolerances of 0.1, 0.2, 0.5, and 1%, and with temperature coefficients of 5 to 25 ppm/K. They are usually far more expensive than their thick film cousins. Note, though, that SMD thin film resistors, with 0.5% tolerances, and with 25 ppm/K temperature coefficients, when bought in full size reel quantities, are about twice the cost of a 1%, 250 ppm/K thick film resistors.

For more details : Carbon Film Resistor

Carbon Composition Resistors

The most common type of Composition Resistor is the Carbon Resistor, also known as Carbon Composition. It consists of a solid cylindrical resistive element with embedded wire leads or metal end caps to which the lead wires are attached (Figure 1). The carbon composition resistors in the early 20th century have uninsulated bodies where the lead wires were wrapped around the ends of the resistance element rod and soldered.

Figure 1  Structure of the Carbon Composition Resistor

It can be considered as the oldest design and, more often, the cheapest of the general purpose resistors that are used in electrical and electronic circuits. These resistors are made from a molded carbon powder that has been mixed with a phenolic binder to create a uniform resistive body. The body may also be protected with paint or plastic. As seen in the image below, a mixture of finely ground carbon dust or graphite (similar to pencil lead) is used to manufacture their resistive element. A non-conducting ceramic (clay) powder is used to bind the mixture all together along with resin. The carbon granules are mixed with a filler material and inserted into a tubular casing or molded into a cylindrical shape with metal wires or leads attached to each end to provide the electrical connection.

Figure 2  Carbon Composition Resistors

The overall resistive value of the mixture is determined by the ratio of carbon dust to powdered ceramic or the amount of carbon added to the filler mixture. The resistive value is denoted by the color-coded markings to the outer insulating material. Low resistance is a result of higher concentrations of carbon, making it a weak conductor.

For more details : Carbon Composition Resistors

Resistor

A resistor is a component of an electrical circuit that resists the flow of electrical current. A resistor has two terminals across which electricity must pass, and is designed to drop the voltage of the current as it flows from one terminal to the next. A resistor is primarily used to create and maintain a known safe current within an electrical component.

Resistance is measured in ohms, after Ohm's law. This rule states that electrical resistance is equal to the drop in voltage across the terminals of the resistor divided by the current being applied to the resistor. A high ohm rating indicates a high resistance to current. This rating can be written in a number of different ways depending on the ohm rating. For example, 81R represents 81 ohms, while 81K represents 81,000 ohms.

The amount of resistance offered by a resistor is determined by its physical construction. Acarbon composition resistor has resistive carbon packed into a ceramic cylinder, while a carbon film resistor consists of a similar ceramic tube, but has conductive carbon film wrapped around the outside. Metal film or metal oxide resistors are made much the same way, but with metal instead of carbon. A wirewound resistor, made with metal wire wrapped around clay, plastic, or fiberglass tubing, offers resistance at higher power levels. For applications that must withstand high temperatures, materials such as cermet, a ceramic-metal composite, ortantalum, a rare metal, are used to build a resistor that can endure heat.

For more details : Resistors