rocking ece: Apr 11, 2008

Friday, April 11, 2008

Different resistors

Resistor types

- an overview of the different types of resistor that are available and how these different resistor types can be used.

The unit of resistance is the ohm, and the chief parameter for any resistor is its resistance. However there are a number of other parameters that are also important. In view of these other resistor parameters there are several different resistor types that are available. In fact choosing the right type of resistor for a given application can be important. Although many resistors will work in a variety of applications the type of resistor can be important in some cases. Accordingly it is necessary to know about the different resistor types, and in which applications each type of resistor can be used.

Basic distinction of resistor types

The first major categories into which the different types of resistor can be fitted is into whether they are fixed or variable. These different resistor types are used for different applications:

Fixed resistors: Fixed resistors are by far the most widely used type of resistor. They are used in electronics circuits to set the right conditions in a circuit. Their values are determined during the design phase of the circuit, and they should never need to be changed to "adjust" the circuit. There are many different types of resistor which can be used in different circumstances and these different types of resistor are described in further detail below.

Variable resistors: These resistors consist of a fixed resistor element and a slider which taps onto the main resistor element. This gives three connections to the component: two connected to the fixed element, and the third is the slider. In this way the component acts as a variable potential divider if all three connections are used. It is possible to connect to the slider and one end to provide a resistor with variable resistance. Further details of variable resistor can be found on the variable resistors page accessible through the "Related Articles" list which can be found on the left hand side of this page below the main menu.

Fixed resistor types

There are a number of different types of fixed resistor:

Carbon composition: These types were once very common, but are now seldom used. They are formed by mixing carbon granules with a binder which was then made into a small rod. This type of resistor was large by today's standards and suffered from a large negative temperature coefficient. The resistors also suffered from a large and erratic irreversible changes in resistance as a result of heat or age. In addition to this the granular nature of the carbon and binder lead to high levels of noise being generated when current flowed.

Carbon film: This resistor type is formed by "cracking" a hydrocarbon onto a ceramic former. The resulting deposited film had its resistance set by cutting a helix into the film. This made these resistors highly inductive and of little use for many RF applications. They exhibited a temperature coefficient of between -100 and -900 parts per million per degree Celcius. The carbon film is protected either by a conformal epoxy coating or a ceramic tube.

Metal oxide: This type of resistor is now the most widely used form of resistor. Rather than using a carbon film, this resistor type uses a metal oxide film deposited on a ceramic rod. As with the carbon film, the the resistance can be adjusted by cutting a helical grove in the film. Again the film is protected using a conformal epoxy coating. This type of resistor has a temperature coefficient of around + or - 15 parts per million per degree Celcius, giving it a far superior performance tot hat of any carbon based resistor. Additionally this type of resistor can be supplied to a much closer tolerance, 5% or even 2% being standard, with 1% versions available. They also exhibit a much lower noise level than carbon types of resistor.

Wire wound: This resistor type is generally reserved for high power applications. These resistors are made by winding wire with a higher than normal resistance (resistance wire) on a former. The more expensive varieties are wound on a ceramic former and they may be covered by a vitreous or silicone enamel. This resistor type is suited to high powers and exhibits a high level of reliability at high powers along with a comparatively low level of temperature coefficient, although this will depend on a number of factors including the former, wire used, etc.

Resistor power dissipation

Although the resistance is the key parameter for any resistor, another parameter which is very important is the amount of power it can dissipate. When current passes through a resistor power is dissipated and this manifests itself in the form of heat. In turn this cases the temperature of the resistor to rise, and if too much current passes through the resistor, the temperature rise can be too great and it can cause the resistance to change, or in extreme cases it can cause damage to the resistor.

The power dissipated in a resistor is easy to calculate. The basic equation for power is:

W = V x I

where:
W = power in watts
V = voltage in volts
I = current in amps

It is often easier to combine this equation with Ohm's Law to create a more useful equation which calculates the power dissipated from a knowledge of the resistance and the voltage across it:

W = V² / R

Where
R = resistance in ohms.

All resistors have a power dissipation rating specification. This is the maximum power that they are designed to dissipate. The resistor type should be chosen so that this power level is never exceeded in operation. In fact good design practice dictates that the maximum power dissipation should be well inside this. Many electronics design companies operate a practice where they state that the maximum actual dissipation should never exceed around 60% of the rating of the particular type of resistor. By doing this, the reliability of the circuit is improved.

Summary

Although resistors may be thought of as simple electronics components to use, there are a number of parameters that need to be considered when choosing the correct resistor type. Parameters apart from just the resistor are important. Voltage withstand, power dissipation and the actual type of resistor itself all have an impact on the performance. Which a variety of resistor types available, it is necessary to choose the correct one for any particular application. In this way the best performance can be assured.

points on cell site

The genius of the cellular system is the division of a city into small cells. This allows extensive frequency reuse across a city, so that millions of people can use cell phones simultaneously.

A typical cell phone can communicate on 1,664 channels or more!

A walkie-talkie can transmit about 1 mile (1.6 km) using a 0.25-watt transmitter. A CB radio, because it has much higher power, can transmit about 5 miles (8 km) using a 5-watt transmitter

Each cell has a base station that consists of a tower and a small building containing the radio equipment.

Each cell is typically sized at about 10 square miles (26 square kilometers). Cells are normally thought of as hexagons on a big hexagonal grid,

A cellular network is a radio network made up of a number of radio cells (or just cells) each served by a fixed transmitter, known as a cell site or base station.These cells are used to cover different areas in order to provide radio coverage over a wider area than the area of one cell. Cellular networks are inherently asymmetric with a set of fixed main transceivers each serving a cell and a set of distributed (generally, but not always, mobile) transceivers which provide services to the network's users.

Cellular networks offer a number of advantages over alternative solutions:

increased capacity
reduced power usage
better coverage

Different types of resistors

Axial

Carbon

composition

Carbin film

Ceramic

Cermet

Chip

Conductive plastic

Deposited film

Fixed

Fixed tapped

Hybrid

Metal film

Metal oxide

Miniature

Noninductive

Pc board

Potentiometer

Power

Precision

Radial

Rheostat

Sliding

tapped

Different capacitors and uses

Capacitors are one of the most widely used electronics components. Passing only alternating or changing signals, capacitors are used in a variety of applications. There are a wide variety of types of capacitor including electrolytic, ceramic, tantalum, plastic, sliver mica, and many more. Each type has its own advantages and disadvantages can be used in different applications.

In essence the construction of a capacitor is very simple, although in practice a lot of research and development has been put into capacitor technology. The basic component consists of two plates that are insulated from one another. In between them there is an insulating medium known as the dielectric. The value of the capacitor is dependent upon the area of the plates, the distance between them and the dielectric constant of the material or dielectric between them. The greater the area of the plates, the closer they are together and the greater the value of the dielectric constant the greater the value of capacitance.

Today capacitors are able to provide relatively high levels of capacitance within components that occupy a small volume. This is achieved in a number of ways. One is to have several sets of plates, and another is to place the plates very close to one another, having a thin layer of dielectric placed between them. In addition to this special insulating dielectric materials have been developed to enable high levels of capacitance to be achieved.

The method of construction is also important. In some capacitors the plates may be flat, and normally these capacitors will have rectangular, or more exactly cuboid shapes. Some will be tubular and in these capacitors the plates will be wound round on each other. The reasons for these types of construction are normally dependent upon the way in which the capacitors must be manufactured. The final stage in the construction of a capacitor is to place it in a protective casing. In some instances it may be dipped in an insulating coating, in others it may be contained within a metal can.

Some capacitors are what are termed polar or polarized. When this is the case the capacitor has a positive and a negative connection and it must be placed in circuit so that the voltage across it is in a particular sense. If the voltage is incorrectly placed across the component then it may be damaged. Fortunately many capacitors, and in particular low value ones are non-polar and can be placed in circuit either way round.

Although there is a large variety that are available the most commonly used are ceramic, plastic film types, electrolytic and tantalum. These names refer to the type of dielectric that is used within the capacitor.

Ceramic
Ceramic capacitors are normally used for radio frequency and some audio applications. These capacitors range in value from figures as low as a few picofarads to around 0.1 microfarads. In view of their wide range and suitability for RF applications they are used for coupling and decoupling applications in particular. Here they are by far the most commonly used type being cheap and reliable and their loss factor is particularly low although this is dependent on the exact dielectric in use. Their stability and tolerance is not nearly as good as silver mica types, but their cost is much less.

There are a number of dielectrics that can be used. For low values a dielectric designated "COG" is normally used. This has the lowest dielectric constant but gives the highest stability and lowest loss. Where higher values are required in a given size, a dielectric with a higher dielectric constant must be used. Types with designations X7R and for higher values, Z5U are used, however their stability and loss are not as good as the capacitors made with COG dielectric.

Silver Mica
Silver mica capacitors are not as widely used these days as they used to be. However they can still be obtained and are used where stability of value is of the utmost importance and where low loss is required. In view of this one of their major uses is within the tuned elements of circuits like oscillators, or within filters.

Values are normally in the range between a few picofarads up to two or possibly three thousand picofarads.

For this type of capacitor the silver electrodes are plated directly on to the mica dielectric. Again several layers are used to achieve the required capacitance. Wires for the connections are added and then the whole assembly is encapsulated.

Plastic film capacitors
There is a number of different types of plastic film capacitors. Polycarbonate, polyester and polystyrene are some of the most common. Each has its own properties, allowing them to be used in specific applications. Their values may range anywhere from several picofarads to a few microfarads dependent upon the actual type. Normally they are non-polar. In general they are good general-purpose capacitors that may be used for a variety of purposes, although their high frequency performance is not usually as good as that of the ceramic types.

Electrolytic
This type of capacitor is the most popular type for values greater than about 1 microfarad. It is contructed using a thin film of oxide on an aluminium foil. An electrolyte is used to make contact with the other plate. The two plates are wound around on one another and then placed into a can that is often aluminium.

These capacitors are polarised, and care should be taken to ensure they are placed in circuit the correct way round. If they are connected incorrectly they can be damaged, and in some extreme instances they can explode. Care should also be taken not to exceed the rated working voltage. Normally they should be operated well below this value. Also in power supply applications significant amounts of current may be drawn from them. Accordingly capacitors intended for these applications have a ripple current rating which should also not be exceeded. If it is, then the component may become excessively hot and fail. It is also worth noting that these components have a limited life. It is often as little as 1000 hours at the maximum rating. This may be considerably extended if the component is run well below its maximum rating.

These capacitors have a wide tolerance. Typically the value of the component may be stated with a tolerance of -50% +100%. Despite this they are widely used in audio applications as coupling capacitors, and in smoothing applications for power supplies.

These are normally contained in a tubular aluminium can, each end being marked to show its polarity.

Tantalum
Ordinary aluminium electrolytic capacitors are rather large for many uses. In applications where size is of importance tantalum capacitors may be used. These are much smaller than the aluminium electrolytics and instead of using a film of oxide on aluminium they us a film of oxide on tantalum. They do not normally have high working voltages, 35V is normally the maximum, and some even have values of only a volt or so.

Like electrolytic capacitors, tantalums are also polarised and they are very intolerant of being reverse biased, often exploding when placed under stress. However their small size makes them very attractive for many applications.

Summary
The most suitable way to summarise the various types of capacitor and the applications for which they are suited is in a table.

Application	Suitable types	Reasons
Power supply smoothing	Aluminium electrolytic	High capacity, high ripple current
Audio frequency coupling	Aluminium electrolytic Tantalum Polyester / polycarbonate	High capacitance High capacitance, small size Cheap, but values not as high as electrolytics
RF coupling	Ceramic COG Ceramic X7R Polystyrene	Small, cheap, low loss Small cheap, but higher loss than COG Very low loss, but larger than ceramic
RF decoupling	Ceramic COG Ceramic X7R	Small, low loss. Values limited to around 1000 pF Small, low loss, higher values available than for COG types
Tuned circuits	Silver mica Ceramic COG	Close tolerance, low loss Close tolerance, low loss, although not as good as silver mica

5 AC 0NLINE 2-2

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1 6. N o i s e l i m i t e r i s a n al o go u s to ( a) R e c t i fi e r ( b ) C l i p p e r ( c ) C l a m p e r ( d ) AG C

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4 AC ONLINE 2-2

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2 . T h e S i gn a l t o N o i s e r a t i o of S S B - S C s ch e m e i s ( a) z e r o ( b ) 3 ( c ) 1 ( d ) 2

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1 3. S e l e c ti v i ty of a r e c e i ve r i s d e p e n d e nt on ( a) m i x e r a n d I F ( b ) I F am p l i fi e r ( c ) R F & M i x e r ( d ) m i x e r

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1 5. Fo r b e s t s e l e c t i vi ty s t a b i l i ty a n d fi d e l i ty, th e I F mu s t b e ( a) l ow ( b ) a rb i t r a ry ( c ) h i g h ( d ) m e d i u m

1 6. T h e c i r c u i t t h at r e d u c e s t h e g ai n of R F am p l i fi e rs w h e n th e i n p u t s i g n al i n c r e a s e s b e yo n d a p r e d e t e r m i n e d va l u e i s ( a) S i m p l e AVC ( b ) d i ff e r e nt i at i o n ( c ) d e l aye d AVC ( d ) C r y s t al fi l t e r

1 7. I n d i c a t e t h e f al s e s ta t e m e nt : I f t h e I nt e r m e d i a t e f r e q u e n c y i s h i g h ( a) t h e l o c a l o s c i l l a t or n e e d n o t b e h i gh l y s ta b l e ( b ) i m a ge f r e q u e n c y r e j e c t i on i s ve r y go o d ( c ) t ack i n g w i l l b e i m p r ove d ( d ) t h e s e l e c t i v i ty w i l l b e p o or

1 8. A ty p i c a l s q u e l ch c i rc u i t ( a) c u t s off t h e a u d i o am p l i fi e r w h e n t h e c ar r i e r i s b l o ck ( b ) c u t s off t h e I F am p l i fi e r w h e n t h e AG C i s m a xi mu m ( c ) c u t s off t h e I F am p l i fi e r w h e n t h e AG C i s m i n i mu m ( d ) e l i m i n a t e s t h e RF i nt e r f e re n c e w h e n t h e s i gn a l i s we ak

1 9. T h r e e p o i nt t r a cki n g i s a ch i e ve d w i t h ( a) d o u b l e s p ot t i n g ( b ) b l o ck i n g ( c ) Pa d d e r C a p ac i t o r ( d ) d o u b l e c o nve r s i o n

2 0. C o m p an d i n g i s u s e d ( a) I n P C M r e c e i ve r s t o re d u c e i m p u l s e n o i s e ( b ) To ove r c om e q u ant i z i n g n oi s e i n P C M ( c ) To i n c r e as e t h e p owe r c ont e nt of t h e m o d u l a te d s i g n al ( d ) To p ro t e c t s m a l l s i gn a l s i n P C M f r om Qu a nti s i n g n oi s e

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3 AC ONL;INE 2-2