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Title: Things you may like to know about Resistors
Summary:
Resistor is a component that is used in electronic circuits and electrical networks to primarily resist flow of current. It is a passive component with two (or more) terminals and of variety of types as per functions. The latest SMD resistors have no leads but contact surfaces to connect electrically.
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What is resistor? 

Resistor is a component that is used in electronic circuits and electrical networks to primarily resist flow of current.   It is a passive component with two (or more) terminals and of variety of types as per functions.  The latest SMD resistors have no leads but contact surfaces to connect electrically.  According to Ohm’s law, in a DC circuit, (and also in AC circuits, in case of pure resistance) the current through a resistor is inversely proportional to its resistance, and directly proportional to the voltage across it.  Typical applications and uses of resistors are:  to reduce current flow, adjust signal levels, to divide voltagesbias active elements,  dissipate electrical power (as heat).   


Resistor Value calculation, naming, colour codes and symbol 

Resistors are defined in terms of its resistance value in Ohms (or in Kilo-Ohms – K).  Also, rating of resistors is defined in terms of watts.  Resistor size changes with variation in watts value for same value of resistance Wattage of resistors varies from 1/8th W to few kW depending upon types).  There are various ways of mentioning resistor value and one way is by colour codes.  There are 3 or more colour bands with one band for tolerance and accordingly value in terms of Ohms for any resistor can be defined.  Details are in attached image. 

Another important factor associated with resistor value is its tolerance, i.e. permissible variation in actual value.  Resistors with tolerance of 1% or less are used in precision electronic circuits and these are depending upon their structure, like MFR or CERMET resistors. 

SMD resistors have different methods of calculation of their values.  Because of vary small size of SMD resistors, they have a different system of coding.  The most commonly seen codes are the three- and four-digit system and an Electronic Industries Alliance (EIA) system called EIA-96.

There are resistors with zero (0) value also to be used in specific applications. 

Types of resistors 

Resistors are produced with a wide variety of materials and manufacturing processes. Each resistor material has its typical properties and specific areas of use. The main types that are used in electrical engineering are summed below. 

ü  Wire wound (WW)

The wire wound resistor is made by winding the metal wire around a metal core. In wire wound resistors, metal wire is used as the resistance element and metal core is used as the non-conductive material. Normally the resistive wire is insulated, so that adjacent wires do not short together.

A Nichrome or Manganin is commonly used as the metal wires, because they provide high resistance to the electric current and operates at high temperature. Most commonly used core materials include plastic, fiberglass, or ceramic.

Wire wound resistors were one of the first types of resistor to be manufactured in the earliest days of electrical science and then wireless. They were superseded in many applications by carbon resistors and then metal oxide and metal film resistors. However, today, wire would resistors are still used as the resistor of choice in many applications.

ü  Carbon Composition (CCR)

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.  These resistors are the most commonly used resistors in the 1960’s and earlier. However, now-a-days, carbon composition resistors are rarely used because of its high cost and low stability. The carbon composition resistors are also called carbon composition or carbon resistors. 

ü  Carbon Film (CFR)

Carbon film resistors are a significant improvement on carbon composition. However, in comparison to metal film and metal oxide film, the commercially available range steadily decreases. Metal and oxide film are not more expensive to produce, and have overall better properties.  Carbon film resistors are a fixed form type resistor. They are constructed out of a ceramic carrier with a thin pure carbon film around it, ( The carbon film is deposited on a ceramic former) that functions as resistive material.

 

ü  Metal Film (MFR)

Metal film resistors have a thin metal layer as resistive element on a non-conducting 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. However, thin film chip resistors use the same manufacturing principle for the metal layer. The appearance of metal film resistors is similar to carbon film resistors, but their properties for stability, accuracy and reliability are considerably better.


 

ü  Metal Oxide Film

Metal-oxide film resistors are made of ceramic rod that is coated with a thin film of metal oxides, such as tin oxide. Metal oxide film resistors must not be confused with metal oxide varistors, made of zinc oxide or silicon carbide.  Metal Oxide Resistors belong to the family of fixed form axial resistors similar to metal film resistors and carbon film resistors. Instead of a metal film, these resistors use a metal oxide resistance material that is coated on top of a ceramic substrate. 

ü  Foil

The metal foil resistor has the best precision and stability properties of all resistor types. The foil is made of an alloy of usually Nichrome with additives. It is mounted on a ceramic carrier with high heat conductivity. The foil has a thickness of only several micrometres. The desired resistance value is achieved by a photoetched resistive pattern in the foil. The metal foil resistor has a low Temperature Coefficient of Resistance (TCR), good long-term stability, low noise, low capacitance, fast thermal stabilization and no inductance. 

Depending upon material, construction and applications, resistors are available in different types, such as: 

Ø  Fixed resistors

Ø  Variable resistors, like:

§  Potentiometer

§  Rheostat

§  Trim pot

§  Ballast




    Ø  Resistance dependent on a special characteristic:

    §  Thermistors (NTC  and PTC) as function of temperature change

    §  Photo resistor – Light Dependent Resistors (LDR) value changing as per changing light level

    §  Varistor – Voltage Dependent Resistors (VDR)  value changing as per changing voltage

    §  Magneto resistor (MDR) changing as per magnetic field

    §  Strain Gauges value changing with mechanical load 




How resistors are manufactured? 

Resistors can be constructed out of a variety of materials.  Manufacturing process and technology differs with type of material and construction.  Most common modern resistors are made out of either a carbon, metal, or metal-oxide film. In these resistors, a thin film of conductive (though still resistive) material is wrapped in a helix around and covered by an insulating material.  The process involves depositing a film of resistive material such as tin oxide on a tiny ceramic chip. The edges of the resistor are then accurately ground, or cut with a laser to give a precise resistance (which depends on the width of the resistor film), across the ends of the device.  Exact elaborate processes of manufacturing, however will differ from resistor type to type. 

New about resistors 

SMD (Surface Mount Devices) resistors are new in the technology upgradation of resistors. SMT (Surface Mount Technology that uses SMD components) was invented to both reduce the size of components and significantly reduce the time it takes to manufacture a circuit. SMDs, are essentially very small size resistors but without leads, are mounted on PCB surface and are connected in circuit in wave soldering process.  SMT manufacturing is a new technology with many advantages of manufacturing including accuracy, speed and quality.

 

 

 

 

 

CREDITS: Resistor Guide, Wikipedia, Manufacturers websites

This is a brief overview of this topic.  The subject itself is vary vast and it’s not expected to cover every aspect here.  Technical details in this article are compiled from best known sources.  These are of course open for any corrections and additions.



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21/01/2023  BY GRLpGpAG      
 
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