Types of Resistor

On the basis of of nature of value of resistance, the resistor are two types:

(a). Fixed Resistor

(b). Variable Resistor

Fixed Resistor : The fixed resistors are those whose resistance is fixed and can't be changed. 

Fixed Resistor

Types of Fixed Resistor :

(a). Carbon composition resistor
(b). Metal film resistor
(c). Wire wound resistor
(d). Alloy resistor

Variable Resistor : It permits adjustment of resistance value between 0 to maximum. It is essentially consists of a track of some kind of resistive material to which moveable wiper makes contact.

Variable Resistor

Types of Variable resistor : 

(a). Carbon Composition variable Resistor
(b). Wire wound variable Resistor
(c). Cermet type variable Resistor

A another type of resistor called non-linear Resistor.

(a). Thermistor
(b). Voltage dependent Resistor (VDR)
(c). Light dependent Resistor (LDR)

Carbon Composition Resistor : 

Carbon composition resistors (CCR) consist of a solid cylindrical resistive element with embedded wire leads or metal end caps to which the lead wires are attached. The body of the resistor is protected with paint or plastic.

Carbon Composition Resistor

Constructional details of carbon composition resistor are shown in Fig. The carbon composition consisting of finely ground carbon, resin binder and insulating filler is made. The resulting mixture is compressed into shape of resistance and fired in kiln or oven. The ratio of carbon to the insulating filler determines the final value of resistance. The end connection is obtained by metal spraying the ends and soldering tinned copper wire around it, or molding the connecting wires directly and pressing a metal cap over metal sprayed end. Some manufacturers mould the carbon around the leads so that the connection is embedded. This type of construction is mechanically more robust. Finally, the whole resistor is either molded in plastic or given several coats of insulating lacquer to provide electrical insulation and protection from moisture.

Metal film Resistor :

Metal film resistors have a thin metal layer as resistive element on a non-organized body. They are amongst the most common types of axial resistors. Other film type resistors are carbon film and thick and thin film resistors. In most literature referrals to metal film, usually it is a cylindrical axial resistor.

Metal film Resistor

Metal film resistors are manufactured by depositing a film of metal (Nickel chromium) on high grade ceramic rod. The metal film resistor is shown in Fig. These resistors have a non - porous high quality ceramic rods / cores which are coated with a special metallic film (nickel chromium) whose composition and thickness can be controlled by vacuum evaporation and manufacturing process. End caps are force fitted to the coated ceramic substrate along with specially coated axial leads which are welded to ensure good mechanical and clectrical contact. These cores are then helically grooved with high degree of precision so as to achieve the desired value of resistance. To withstand severe environmental conditions, they are treated with a coat of special grade varnish and compounded resin of high thermal stability. The resistors are molded with epoxy powder to assure perfect electrical insulation and moisture protection.

Applications. Since metal film resistors are the most stable resistors among film type resistors, they are generally used in all professional measuring and calibrating equipment such as oscilloscopes, decade resistance boxes, measuring bridges, oscillators, precision voltage dividers and high frequency communication systems.

Carbon Film Resistor : 

The carbon film resistor is a type of fixed resistor that uses carbon film to restrict the electric current to certain level. These types of resistors are widely used in the electronic circuits.

Carbon film Resistor

Carbon film resistors are manufactured by depositing an even film of resistive material (pure carbon) on grade insulator cu as shown in Fig. High grade non - porous electrical insulator cores with fine surface structure are heated in specially constructed furnaces with accurate temperature and vacuum controls. The hydrocarbon is passed through the fornisce. By pyrolytic action, the hydrocarbon cracks at high temperature inside furnace and forms diamond hard crystalline carbon coating on the insulator core. The coated rods are pressfitted with brass zos bich ze prewelded with electrotinned electrolytic copper wire. These are then helically grooved with a high degree of precision so as to achieve the desired value and tolerance. The film is protected by an undercoat of silicon varnish zad a special epoxy paint, which ensure good protection against tropical and climatic exposure ad badity. 

Applications. In view of remarkable long term stability of the value of resistors under severe al conditions, close tolerance, low noise, low temperature and voltage coefficient and long service to base resistors are most suitable type for all types of precision equipments, defense communications and industrial controls and computers.

Wire Wound Resistor :

A wire wound resistor is an electrical passive component that limits current. The resistive element exists out of an insulated metallic wire that is winded around a core of non-conductive material.

Wire Wound Resistor

The wire wound resistor is manufactured by winding resistance wire on to an insulating former. The common materials used are Nichrome (Nickel Chromium), Copper Nickel alloys (Eureka) and alloys of Nickel and Silver. The wire is produced by drawing the material through a suitable die and then annealing it. The wire must have good uniformity, be ductile, resist corrosion and have fairly bigb resistivity. On the larger resistors, terminations are provided by a band of organized metal (ferrule) with lug. Smaller wire wound resistors may have end caps or a lead brazed on to the ends of the resistance wire. The connected lead is then folded and inserted inside the ceramic former. This firmly retains the wire leads and prevents mechanical strain bving appiied to the actual resistance wire winding the whole resistor is then covered in some insulating material (cement, vitrous enamel and lacquer), which provides the resistor excellent protection against moisture and environmental conditions.

Wire wound resistor can be classified in three categories :

(i) Low power upto 2 watts

(ii) Medium power 2 watts to 6 watts (iii) High power more than 6 watts.

Applications. These resistors find immense use in regulated power supplies, multimeters, wheatstone bridges and for most purposes where a reliable power dissipation component is needed.

Alloy Resistor : 

Nickel chromium alloy is evaporated on to a glass or ceramic substrate. The evaporated film has good adhesion to the base hence a stable resistor. A resistive film of 50 Aº thickness bas been found to be the thinnest film with very good stability. Nickel chromium can be evaporated from boats containing NiCr alloy or it can be sublimed by heating of wire of NiCr directly by passing a current through it. These resistors are used in precision applications.

High value Resistor : 

High value resistors (HVR) are electronic components used for special applications, such as in bio-medical systems, measurement equipment and some condenser microphones. While discrete resistors in the range of few tens of megohm are very common, making giga-ohm range resistors is more complicated.

High value Resistor

High value resistors range upto ten million mega ohm (10 to 1013). These resistors are carbon composition type. Resistive element consists of carbon film on insulating ceramic rod which is sealed in evalua: ed glass tube. End connections are made by wires of copper clad nickel iron alloy The moisture effects are minimized by coating outside of the glass tube with silicon lacquer. The element can be spiralled for control of fine resistance. These resistors are used in measurement work for atomic energy where it is often necessary to detect very small currents such as photon counting, radiation monitoring etc.

Typical data for high value resistors: Voltage coefficient = 0.02 -0.1% / V Temperature coefficient = 0.05 -0.25% / Shelf stability = 2% per year.

Carbon Composition variable Resistor : 

Carbon composition potentiometers or variable resistors are the most commonly used type. The material used is a mixture of carbon and a filler material, the combination determining the resistivity of the carbon composition film of the potentiometer element.

Carbon Composition variable Resistor

Low power carbon variable resistors are manufactured from carbon composition either in solid track type or a coating of carbon film. Both of these consist of an annular ring as shown in Fig. 6.7 of substrate material (ceramic or plastic). A resistive carbon layer is grown over it. The moveable contact slides over it. The carbon track or film can be either film type or molded type. In molded type, resistive material and base plate are molded together with stamping, terminal lugs and bushing all being inserted during molding operation In film typc, carbon resistive paste is sprayed on base annular ring. Different variations can be obtained controlling the width of track or composition of material. These resistors are made to follow linear and logarithmic variations.

Carbon composition type variable resistors are available in the following construction:

 (i) Single turn type
 (ii) Slide type

Wire Wound Variable Resistor :

A wire wound resistor is an electrical passive component that limits current. The resistive element exists out of an insulated metallic wire that is winded around a core of non-conductive material.

Wire wound variable Resistor

Wire wound type resistors consist Nichrome or other resistance wire wound over some insulating former which is in the form of arc as shown in Fig. 6.8. The sliding contact moves on the edge of element.

Wire wound resistors can be made

(i) Single turn

(ii) Multi turn

(iii) Ganged

Cermet type Variable Resistor : 

Cermet compositions are screen printed on ceramic substrate and Aired in tunnel kiln at temperatures between 750 ° C and 850 ° C. Prior to firing conductor pads are screened on to the substrates and fired at about the same temperature. The process of firing clement results in a resistance clement which is permanently fused to its substrate. The variation in resistance is obtained by using threaded screw or wiper assembly depending upon the configuration used. The cermet resistors are made as single turn or multi turn.

Comparison of Variable Resistor :

Thermistor : 

A thermistor is a type of resistor whose resistance is dependent on temperature, more so than in standard resistors. The word is a combination of thermal and resistor.

Themistor

Themistor is a contraction of the term "Thermal resistors" and is used for devices which have high temperature co - efficient. While metallic conductors temperature co - efficient of resistance, certain oxides have values ​​ranging from about - 5% per ° C to 60% per ° C.

There are two types of thermistors according to the sign of temperature co - officient of resistance. These are:

(i) Negative temperature co - efficient (NTC) thermistors whose resistance decreases with temperature.

(ii) Positive temperature co - efficient (PTC) thermistors whose resistance increases with rise in temperature.

NTC thermistors are two terminal devices which are manufactured in three basic shapes as shown in Fig. For disc and rod shapes the constituent oxides are mixed in suitable proportion, a binder is added and mixture is pressed or extruded into the required shape. The ceramic body is then heat treated at temperatures around 1200 ° C. During this process the binder material is burnt out, the constituent oxide reacts to form the desired composition and material sinters to form a hard ceramic body. Silver paste is either painted or sprayed into opposite faces of the disc or ends of the rod and then fired into ceramic to form ohmic contact. The leads are attached to this ohmic contact by soldering and final device is coated with insulating point or resin layer. Bead thermistors are manufactured by forming a small blob of semiconducting oxide mixture between two parallel platinum or platinum alloy wires. The wires are heat treated to about 1200°C and thermistor material shrinks on wire forming a sintered. The Platinum wires are then welded / soldered to thicker supporting leads. The bead is finally encapsulated in gas filled or evacuated glass envelope.

PTC thermistors are produced by using either Ge or Si on semiconducting barium titanate. The latter type PTC thermistors are known as switching PTC thermistors. The former type are prepared by forming an ohmic contact to opposite faces of rectangular block of suitably doped silicon. Axial lead wires are attached usually by soldering to the contact areas and unit is encapsulated either in glass or molded in epoxy resin. For producing switching PTC thermistors care is taken to use high purity materials since unwanted impurities can cause large variations in final device properties. Oxides, carbonates of required elements are mixed in proportion. The dried mixed is calcined at about 1100 ° C and reduced in particle size, dried, mixed with binder and pressed into desired shape. They are then sintered at about 1350 ° C. Leads are attached to the opposite faces of disc or ends of rod. The V - I characteristic of the PTC has three regions as shown in Fig.

VI characteristics for a PTC thermistor

The region OA presents the region where ohm's law is obeyed. Point A indicates the temperature at which PTC switch characteristic commences. AB represents the region of increasing temperature. At B. the maximum resistance of the curve is reached and subsequent NTC region causes a tum over in VI characteristic in this region.

Applications of Thermistors. Thermistors are used in the following applications:

(i) Thermistors are used in the field of temperature measurement. Its large temperature co - efficient provides good accuracy and resolution.

(ii) Thermistors are used for temperature compensation over a wide temperature range.

(iii) Measurement of power at high frequency.

(iv) Measurement of thermal conductivity. (v) Thermal relay.

(vi) In control devices actuated by temperature.

Specifications of Thermistors. Specifications of Thermistors are:

(a) Resistance at 25 ° C

(b) Thermal time constant
(c) Switch temperature
(d) Tolerance (Max. Voltage at 55 ° C
(e) Temperature range.
 (f) Temperature co - efficient (Dissipation factor temperature range

Voltage Dependent Resistor (VDR) :

It is a resistor whose electrical resistance varies considerably with applied voltage. It is also called varistor or voltage sensitive resistor.

 These are of two types:

 (a) Symmetrical varistors

 (b) Non - Symmetrical varistors.

Symmetrical varistors. These have its resistance same for both positive and negative voltage i.e. they have seme VI characteristics in both directions. These varistors are manufactured by pressing or extruding a mixture of silicon carbide grains, clay and graphite into desired geometrical shape. These are available in rods or disc form. The flat faces of disc are metallized and wires are soldered to it. The assembly is dipped in PVC coating or epoxy coating. These Yaristcrs can be made from 50 V to 10 KV by controlling thickcess and mixture composition during manufacture

(b) Non - Symmetrical or Asymmetrical Varistors. These varistors do not have the same resistance for the same positive and negative applied voltages. These are like rectifiers i.e. they have very high resistance in one direction and very low in the other direction. They are copper oxide and selenium oxide rectifiers.

Copper oxide varistors are manufactured as disc or plates. These are made of high purity copper metal plate on which a layer of coprous oxide is produced by oxidation. Ohmic contact with oxide layer is achieved by evaporation of gold or silver electrode on to hard coprous oxide layer.

Application. These are used in :

(a) Protective circuits for protection of circuit or component from inductive surges.

 (b) Sensing element in voltage stabilizers.

 (c) For increasing meter sensitivity.

 (d) For limiting anode peak voltage.

 Specifications. The main specifications of varistors are:

 (a) Max. continuous voltage.

 (b) Maximum energy.

 (c) Maximum non-repetitive current. (d) Typical capacitance.

Voltage Dependent Resistor (VDR) :

These resistors change their resistance with variations of incident light energy. These are made in disc shapes with wire lead ends on one side.

LDR construction

They have a ceramic substrate over which a layer of cadmium sulphide (CDS) is deposited in zig zag form to increase the length hence resistance value as shown in figure. Depending on the layer the resistance changes. Electrodes are formed by evaporating metal in vacuum. Leads are connected and put in plastic case as shown in Fig.

Applications. LDRs are mainly used in: 

(a) Automatic Brightness control in TVs.

 (b) Proximity switches.

 (c) Light related control systems.

 (d) Optical encoding

Specifications. Main specifications are:

 (a) Resistance range

 (b) Power rating

 (c) Dark resistance

 (d) Response time

 (e) Switching speed