Resistors and Capacitors | Lecture notes Basic Electronics

Resistors are classified into two broad categories i.e. Fixed and Variable. The Resistors whose values cannot

be changed and are fixed are called as Fixed Resistors and the resistors of which we can change the values

as per our requirement are called as Variable resistors.

Fixed Resistors

1. Carbon Film/Cracked Carbon Resistors: As the name suggests, the carbon film resistors have a film

of carbon deposited onto a suitable insulator such as ceramic in this case. These resistors have a

tolerance between ±5% to ±20%.

Construction:

 The substrate used is a ceramic stick of length between 10mm to 45mm and diameter between 3mm

and 8mm. The ceramic sticks are degreased by putting them in 10% HCl soln. in a revolving container.

This process goes on for 45 minutes and after that a 70% ammonia soln. is added in the container

and rotation goes on for 30 to 40 minutes. The sticks are washed in a continuous stream of water to

wash out every chemical. A centrifugal drier dries up the ceramic sticks and removes all moisture.

 The degreased ceramic sticks are put into an electrically heated oven that is rotated. The oven is first

cleaned with Benzene or petrol and the sticks are placed along the entire length. The end covers are

closed and air is removed with the help of vacuum pump. The temperature of oven is risen upto 3000C.

 When the pressure inside the oven reaches a minimum level, the vacuum pump is disconnected and

the stopper of the heated benzene flask is opened. Benzene is sucked onto the chamber. The gas

carbonises on entering the chamber and a uniform film of carbon is deposited on the hot ceramic

sticks. The time and amount of benzene suction in the chamber decides the resistance.

 After cooling, the carbonised sticks are gently transferred to a revolving container and Bakelite or

silicon is poured over them which forms a protective cover over the carbon film

 The caps are made of nickel-coated steel or brass and leads are attached to it. The caps are pressed

onto the sticks to fix them onto the sticks. The resistors are sorted as per their resistance values and

stored in different containers. The leads are soldered to the caps to make electrical contact across

carbon film.

Rating

10ohm to 10Mohm

wattage

0.25watts to 2 watts

Tolerance

5% to 20%

Operating temperature

-40C to 120C

Max. operating Voltage

500V DC

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Resistors are classified into two broad categories i.e. Fixed and Variable. The Resistors whose values cannot be changed and are fixed are called as Fixed Resistors and the resistors of which we can change the values as per our requirement are called as Variable resistors. Fixed Resistors

Carbon Film/Cracked Carbon Resistors : As the name suggests, the carbon film resistors have a film of carbon deposited onto a suitable insulator such as ceramic in this case. These resistors have a tolerance between ±5% to ±20%. Construction:  The substrate used is a ceramic stick of length between 10mm to 45mm and diameter between 3mm and 8mm. The ceramic sticks are degreased by putting them in 10% HCl soln. in a revolving container. This process goes on for 45 minutes and after that a 70% ammonia soln. is added in the container and rotation goes on for 30 to 40 minutes. The sticks are washed in a continuous stream of water to wash out every chemical. A centrifugal drier dries up the ceramic sticks and removes all moisture.  The degreased ceramic sticks are put into an electrically heated oven that is rotated. The oven is first cleaned with Benzene or petrol and the sticks are placed along the entire length. The end covers are closed and air is removed with the help of vacuum pump. The temperature of oven is risen upto 3000C.  When the pressure inside the oven reaches a minimum level, the vacuum pump is disconnected and the stopper of the heated benzene flask is opened. Benzene is sucked onto the chamber. The gas carbonises on entering the chamber and a uniform film of carbon is deposited on the hot ceramic sticks. The time and amount of benzene suction in the chamber decides the resistance.  After cooling, the carbonised sticks are gently transferred to a revolving container and Bakelite or silicon is poured over them which forms a protective cover over the carbon film  The caps are made of nickel-coated steel or brass and leads are attached to it. The caps are pressed onto the sticks to fix them onto the sticks. The resistors are sorted as per their resistance values and stored in different containers. The leads are soldered to the caps to make electrical contact across carbon film. Rating 10ohm to 10Mohm wattage 0.25watts to 2 watts Tolerance 5% to 20% Operating temperature - 40C to 120C Max. operating Voltage 500V DC

Carbon Composition Resistors : The sticks are entirely made up of carbon and no substrate is used. Graphite granules are crushed in rolling mills. Silica and synthetic resin is added as binder and a dough is prepared. The amount of carbon is decided by the ohmic range and wattage.  The dough of required composition is produced in various thickness as per requirements. The output of punch extruder is cut to make required length sticks. Grooves are cut onto both ends of stick to connect the leads. The sticks are dried and baked at 1000C in ovens. The ovens are filled with nitrogen and the sticks are cooled and given a synthetic coating. A dab of silver solution is put in the end grooves and on the leads and the leads are pushed into the grooves. The product is again heated at 150C and this dries the silver. They are labelled and painted as per their resistance values.
Ceramic resistors : This is another variety of carbon composition resistors where the silica is replaced by ceramic granules. The powdered carbon and ceramic constituents are mixed with binding resin as to make sticks. Baking is done at temp. between 1000C and 1200C. The ceramic makes the resistors ideal for operation at high temperatures.
Metal-film Resistors : These come in various types, each with its own process. Nickel-Film resistors: The substrate used is ceramic but deposition process is entirely diff than that of carbon. Electroplating is used for metal deposition. Ceramic is covered with a layer of industrial graphite. Using a Nickel rod as anode and and electrolyte of nickel phosphate, a film of nickel is deposited on the ceramic substrate which is used as cathode. The thickness of nickel decides the ohmic range. The leads are mounted using solder dipping process or by direct soldering. Rating 10ohm to 10kohm wattage Less than 1 watt Tolerance Less than 1% Operating temperature Upto 200C Max. operating Voltage Upto 250V

Variable resistors :

Carbon track potentiometers : A layer of carbon is deposited on Bakelite substrate in the shape of an annular ring. A sliding contact (wiper) of beryllium copper alloy brings only a portion of the track into the circuit providing desired resistance. Construction: The carbon track is composed of a proportionate mixture of ground graphite, varnish and filler suitable for spraying. The mixture is put in a spray gun and sprayed uniformly over the Bakelite sheet. After hot air drying, the carbon painted sheets are baked at 150C in an oven. The resistance depends on the thickness of the sprayed layer, amount of carbon and the mixture. Suitable rings of carbon are cut out as required after heating the Bakelite sheet, using a pressure punching machine. The two ends of the track are drilled and contact lugs are connected to them. The lugs are given a touch of silver to ensure firm electrical contact. A slip ring and a wiper and one more contact lug are attached in the middle of the annular ring. The shaft is mounted along with the slip ring mechanism. When the shaft is turned, the wiper end slides over the carbon track, while the sliding contact provides a link to centre lug and a variable resistance is connected in circuit. The entire arrangement is encased in a metallic cover with insulation on top. a. Non-Linear Tracks : The process above gives a linear change in resistance with angular rotation of shaft. A non- linear variation of resistance is achieved as follows: The annular ring is sprayed with a single layer. Then one-tenth of the track Is covered and ten coatings are given. Again, two-tenth of the covered and ten coatings are given. In this way, a track on non-uniform thickness can be made to achieve nonlinear variations. Rectilinear tracks : Carbon tracks can also be fabricated by a moulding process when substrate and track are moulded simultaneously. It is possible to attach a moving and fixed part in the mould itself. The carbon track is laid on a straight substrate. The moving contact is strip with carbon. The wiper slides on the track.

Presets: Resistive trimmers or presets are a kind of potentiometer having a metallic wiper that can be moved with screw driver. Their tracks are either carbonized or metallic cermet. The substrate used is Bakelite or phenolic sheet.
Wire Wound Potentiometer :

A capacitor (originally known as a condenser) is a passive two-terminal electrical component used to temporarily store electrical energy in an electric field. Capacitors are widely used as parts of electrical circuits in many common electrical devices. Unlike a resistor, an ideal capacitor does not dissipate energy. Instead, a capacitor stores energy in the form of an electrostatic field between its plates. When there is a potential difference across the conductors (e.g., when a capacitor is attached across a battery), an electric field develops across the dielectric, causing positive charge +Q to collect on one plate and negative charge −Q to collect on the other plate. If a battery has been attached to a capacitor for a sufficient amount of time, no current can flow through the capacitor. However, if a time-varying voltage is applied across the leads of the capacitor, a displacement current can flow. An ideal capacitor is characterized by a single constant value, its capacitance. Capacitance is defined as the ratio of the electric charge Q on each conductor to the potential difference V between them. The SI unit of capacitance is the farad (F), which is equal to one coulomb per volt (1 C/V) Capacitors are widely used in electronic circuits for blocking direct current while allowing alternating current to pass. In analog filter networks, they smooth the output of power supplies. In resonant circuits they tune radios to particular frequencies. In electric power transmission systems, they stabilize voltage and power flow A capacitor consists of two conductors separated by a non-conductive region. The non-conductive region can either be a vacuum or an electrical insulator material known as a dielectric. Examples of dielectric media are glass, air, paper, and even a semiconductor depletion region chemically identical to the conductors. A capacitor is assumed to be self-contained and isolated, with no net electric charge and no influence from any external electric field. The conductors thus hold equal and opposite charges on their facing surfaces, and the dielectric develops an electric field. In SI units, a capacitance of one farad means that one coulomb of charge on each conductor causes a voltage of one volt across the device. An ideal capacitor is wholly characterized by a constant capacitance C, defined as the ratio of charge ±Q on each conductor to the voltage V between them C = Q V Because the conductors (or plates) are close together, the opposite charges on the conductors attract one another due to their electric fields, allowing the capacitor to store more charge for a given voltage than if the conductors were separated, giving the capacitor a large capacitance. Sometimes charge build-up affects the capacitor mechanically, causing its capacitance to vary. In this case, capacitance is defined in terms of incremental changes: C = dQ/dV

Fixed Capacitors

Ceramic Capacitors: The Dielectric in this case is a ceramic compound in the form of a tube or a disc. Tubular Ceramic: The tubes are usually 1.5mm to 3mm in diameter and 15 to 35mm long. Steatite is powdered and used as base material. The powder is mixed with flux, compressed and heated up to below its melting point. After cooling it is ground again and mixed with water and gum. The paste formed is extruded into a hollow tube shape, which is cut as per required length. The tubes are now baked at 1200, cooled and lacquered with solver solution both inside and outside. After the silver coat is dry, the pieces are again heated to 700C. Radial leads are soldered to silver coat and cheaper pieces are coated with lacquer while plastic is used for better quality. Ceramic Disc : Here, the ceramic paste used is thinner than tubular shape so it can be coated onto a glass substrate in the form of a sheet. After oven treatment, thin metallic layers of silver or platinum solution are deposited on ceramic. After drying, the piece is baked again and circular sheets are cut from each disc. Plastic moulding is used it cover the product. Labelling is done in numbers or colour codes. Multi - layer ceramic : The layers of ceramic used are only a fraction of a mm thick. Ceramic paste is made into a slurry and layered on a moving belt in an oven. A doctoring blade is used to maintain uniformity. Binders are used to give the slurry strength. After drying, the film is separated from the steel belt and punched into ceramic sheets. Metallic sheets are printed onto the ceramic sheets and are layered onto one other in a stack. These blocks are heated at 1000C and contact leads are soldered.

 The coils formed are rolled up to form a capacitor. For a polarized cap, two foils one oxidized and other un-oxidized are made to sandwich two layers of paper and rolled up. The paper must be dipped in the same electrolyte used before. For a non-polarized cap, both foils must be oxidized.  The encapsulation is usually made of aluminium. Small pieces get radial or axial leads while larger pieces get lugs. They have strong covers made of steel and are insulated with seals above and below. Small pieces are covered with PVC and rating is labelled. b. Tantalum : Aluminium is replaced by tantalum foils coated with tantalum pentaoxide. Electrolyte used for anodizing is lithium chloride up to 30V or H2S04. Boric or succinic acid is used for etched foil type at 75V up to 150V.  Foil type tantalum can be plain or etched. The oxide is formed on both sides of foil. Some porous paper is dipped in the same electrolyte which is used before and spaced between foils. The formation can be polarized or non-polarized. The lugs are connected after the foil is rolled up and aluminium caps are replaced by brass or copper caps with silver soln inside. Teflon is used to seal the package.  Solid capacitors use a solid electrolyte as manganese dioxide. Tantalum powder is pressed in metallic dies and heated to form solid pellet and are later soaked in manganous nitrate. The nitrate pellets are covered with manganese dioxide during electrolysis. The pellets are sprayed with graphite and a metallic layer copper. A contact penetrated the pellet and forms the anode and cathode is given to copper covering and whole package is encapsulated in plastic.  In wet pellet capacitor, tantalum is not formed in electrolysis but pellet is directly used as anode and continually dipped in a lithium chloride soln in a silver sup. The cup serves as cathode and the unit Is sealed at anode.

Ganged Capacitors Air Gang : An Air gang capacitor has variable capacitance. It consists of two sets of interleaved metallic vanes, one of which the stator is fixed and the other rotor is movable. The vanes or plates are made from cadmium plated aluminium. The rotor plates are stacked on a rotating shaft and grounded to the frame through a flat phosphor-bronze spring which presses against the shaft. The stator plates are made up of two or three sections mounted on insulated strips each having an independent lug. The rotor vanes have slots that are cut to match the multisection stator when rotor is turned. The highest capacitance occurs when the sets are fully meshed. Air is the dielectric medium between the plates. When the rotor is turned the vanes unmesh and capacitance goes down to 15Pf from 500Pf. Using diff shapes of rotor or stator we can change the variation I capacitance.

Resistors and Capacitors, Lecture notes of Basic Electronics

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