Gadget Download

The industry of electronic circuits uses resistors of various different types available in the market. The properties of these resistors vary and are different for each type being ruled by their manufacturing and construction process.


By: S. Prakash


Over a time period, the resistors of various types which were and are being used in the production of electronics have undergone continuous change.

The resistors which were used previously consisted of lead as their component along with being very large in size when compared to the resistors of the present day which resulted in low performance level of the former.

The current day resistors are comparatively smaller in size along with performing at a high level.

Resistors of variable and fixed types

The most major and basic category in which a resistor can be differentiated is on their nature of being either of variable or fixed type. The applications for which these resistors of different types are used differ respectively.

Fixed Resistors: The resistor which is used most widely in the industry is the fixed resistors. The electronic circuits use the fixed resistors for correcting and setting the right and appropriate conditions within their circuits.

The determination of the values of the resistors is carried out in the circuit’s design phase. These values are not required to be adjusted or changed in any manner with respect to the circuit.

The decision as to which resistor type needs to be used is dependent on the various circumstances in which they are to be used. These resistor types have been described in further detail in the subsequent sections.

Variable Resistors: The variable resistors consist of two elements, namely a fixed resistor element. The main element of the resistor is tapped onto by the slider present in the resistor.



Thus, this provides the resistor’s components with three connections. Out of these three connections, the fixed element is fixed to the two connections while the slider is the third connection.

Thus, this enables the components to act as an agent of the variable potential divider.

This also required that they use the three connections altogether. The variable resistance can be provided to the resistor by connecting the one end of the resistor with the slider.

Potentiometers, presets and rheostats are some of the common examples of variables resistors

Resistors of Fixed Types

The various different fixed resistor types are as follows:

Carbon composition: The carbon composition resistors were very common previously but the currently their usage has decreased considerably.





The carbon resistors are manufactured by mixing the granules of the carbon with an element which acts as a binder and this mixture in turn is made in the shape of small rods.

The carbon resistors had a disadvantage in terms of suffering from a very high negative temperature coefficient.

This is due to their comparatively large size when viewed from the current day standards.

The carbon composition resistors also suffered another downfall wherein due to aging of the resistor with time or an exposure to excessive heat, the carbon composition resistor goes through irreversible changes which are erratic and large.

Additionally, a large amount of noise is generated in the carbon composition resistor when the current flows through it because of the carbon’s granular nature and its association with the binder.

Carbon Film (CFR 5%): The carbon film resistor is manufactured by the induction of the process of a hydrocarbon’s cracking into a former made up of ceramic.



The resistance of the film which is deposited as a result of the above process is set by making a cut into the film in the shape of helix. This has resulted in very high inductance in the carbon film resistors and thus most of the RF applications cannot use it much.

A -900 ppm/ºC to -100 ppm/ºC of temperature coefficient is exhibited by the carbon film resistors. A ceramic tube or a conformal epoxy coating is used to protect the carbon film.

Metal Oxide Film (MFR 1%): The metal oxide film resistor has come to become the resistor which is used in the current day industry at a wide scale along with another resistor type of the metal film type.





The metal oxide film resistor type uses a film of metal oxide instead of a carbon film to be deposited on the ceramic rod.

The deposition of the metal oxide which can be found on the ceramic rod can include tin oxide. There are two ways in which the component’s resistance is adjusted.

Firstly, at the initial stages of the manufacturing process, the deposited layer’s thickness is controlled. Thereafter the adjustment is done in more accurate way by cutting a grove in the shape of helical form in the film.

Again, as in the previous case, the conformal epoxy coating is heavily coated on the film to protect it.

The ±15 ppm/ºK of temperature coefficient has been observed in the metal oxide film resistor which results in a very high and superior function of this resistor when compared to any other resistor which is carbon based.

 Additionally, the tolerance levels to which these resistors are supplied are very close including the standard tolerance levels of ±2%, ±1%, and ±5% being available.

Also, when compared with the resistors which are carbon based, there is very low exhibition of noise in these resistors.

Metal Film: There is a great similarity which can be observed between the metal oxide film resistor and the metal film resistors in terms of their performance and appearance.

A metal film is used by this resistor in place of the metal oxide film which is used in the metal oxide film resistor. The metal film which is used in the resistor can include nickel alloy.

Wire Wound: The applications which require very high power in general use this type of resistor. A wire is wounded around a former in order to manufacture this type of resistors.





The resistance of these wires is higher than that of the normal resistance. The varieties of these resistors which are expensive consist of the wire which is wound on a former made up of ceramic along with a cover of the silicone or citreous enamel over it.

The temperature coefficient of these resistors is very low along with a reliability of very high level being exhibited by these resistors when exposed to high power which enables it to operate at high performance level.

But these properties are also dominated by various other factors such as type of the wire being used, type of former being used, and more.

Thin Film: The majority of the resistors which are of surface mount types use the technology of the thin film. The resistors based on this technology are used widely in the current day industry where the number goes up to billions here.

Non-Leaded and Leaded types of Resistors

The manner in which the components or the resistors are connected acts as an important determinant of the differentiation of the components and the resistors.

The manner in which the components were connected previously has changed over the time majorly due to the use of the techniques of mass production and the circuit boards being used at a widespread level.

This is especially true for the components which the mass production process incorporates.

On the basis of the method of connection, the two major categories of the resistors are as follows:

Leaded Resistors: Since the times when the electronic components had first come into use, the leaded resistors had also come into use since those times.

The lead which came from the element of the resistor were required wherein the components were required to be connected in various different forms to the terminal posts.

Their usage has not stopped till date and only technique has changed wherein in the current practices where there is more use of the printed circuit boards, the holes present in the boards are used to insert the lead and then the reverse side is used to solder it where one can find the tracks.

Surface Mount Resistors: The since the time when the technology of the surface mount have been introduced, there has been a significant increase in the surface mount resistors.



The technology which is used to manufacture the surface mount resistor is the thin film technology. Through this technology, the resistor can obtain the values in full range.

Resistors which are based on SMT technology are called SMT resistors which are one of the members of the SMD family or surface mount device family.


By: S. Prakash


The electronic equipment of various types such as televisions, commercial communication equipment, cell phones, research equipment of high technology and MP3 player uses the SMD resistors.

Basic Construction of SMD Resistor

 

 

The shape of the SMD resistor is rectangular.

There is a presence of a metallized area in the chip resistors at any one side of their body which in turn enables the contact of the printed circuit board with the chip resistor by using the solder.

A ceramic substrate is one of the components of the resistor and a metal oxide film is deposited over it. The resistance of the resistor is determined by the actual film’s length and thickness.

Since metal oxide is used to manufacture the SMD resistors which enable the resistor to be highly stable along with the tolerance level also being high. The element with which a ceramic substrate is made up of is high alumina ceramic.

The use of the high alumina ceramic in the SMD resistors provide stable insulation on the basis of the resistive metal oxide element on which the resistor is set down.


The terminations of the SMD resistors are also important.

The contact which the SMD resistor needs to make with the chip resistor’s resistive element needs to be reliable while at the same time it should provide solderability of very high levels.

Such high levels are achieved through the usage of the nickel based layer to make an internal connection. At the same time and outer layer which is tin based is used to make the outer connection thereby achieving solderability of very high levels.

Packages of SMD Resistors

There are variety of different packages in which the SMD (Surface mount resistors) come. The size in which the packages of the chip resistors come has decreased significantly because of the advancement of the technology over a period of time.

Specifications of SMD Resistors

There are a large number of companies which are involved in the manufacturing of the SMD resistors. The specifications of the SMD resistors thus vary based on the specifications provided by the manufacturer.

Thus, while deciding on the requirement of the SMD Resistors, one needs to check the rating of the manufacturer for the given SMD resistor. At the same time one can anticipate the rating at a general level.


Power Rating: Due consideration needs to be provided to the power rating in a given design.

The power levels of the designs which use the SMD resistors are of very smaller levels when compared to the circuit designs which use wire ended components.


Tolerance: The tolerance values of the Surface Mount Resistors which use the metal oxide film for their manufacturing is very close.

The tolerance levels which are available at a large scale include 1%, 5%, and 2%. Additionally, the values of the 0.1% and 0.5% can be achieved for the applications which are specialist applications.


Temperature Coefficient: The temperature coefficient values of the Surface Mount Resistors which use the metal oxide film for their manufacturing is very high.

The temperature coefficients levels which are available at a large scale include 100 ppm/C and 25,50 ppm/C.

Applications of SMD Resistors

There are a variety of designs for which the use of the surface mount resistors is done.

The size of the SMD resistors enables them to have various advantages such as: they can be used for the techniques of the automatic assembly; provide high performance at radio frequencies; and high suitability for the circuit boards which are compact in nature.

The capacitance and the inductance of the SMD resistor are spurious in nature because of their size. Thus, one needs to be careful while the power dissipation of the SMD resistors are calculated since the power levels which are dissipated is very low.

There are many different names by which the LDR (Light Dependent Resistors) are known as and includes names such as photoresistor, photocell, photoconductive cell, and photoconductor.


By: S. Prakash


In the domestic equipment industry, the term which is most prevalent and used at a large scale in the instructions and the data sheets is “photocell” term.

There are a variety of uses to which the LDR or photoresistor can be put to due to their being an element which is both photosensitive and also available at low cost as well.

Thus, LDR has been continuously used for a long period of time in a range of applications such as photographic light meters, burglar and smoke detectors, in street lamps to control the lighting, flame detectors, and card readers.

The generic term of “photocell” is used for the Light Dependent Resistors within the general literature.

 

Discovery of LDR

As discussed above, the LDR has been continuously used for a long period of time. The early forms of the photoresistors were manufactured and came in the market in the early nineteenth century.

This was made available through the discovery of the “selenium’s photoconductivity” in 1873 by the scientist Smith.

A range of different varieties of the photoconductive devices have been manufactured since then. An important progress in this field was made in the early twentieth century, especially in 1920 by the renowned scientist T.W. Case who worked on the phenomenon of photoconductivity and his paper, “Thalofide Cell- a new photoelectric cell” was published in 1920.


During the next two decades in the 1940s and 1930s, a range of other substances were studied in detail which included PbTe, PbS, and PbSe. Further in 1952, the photoconductors of these substances were developed by Simmons and Rollin using germanium and silicon.

Symbol of the Light Dependent Resistors

The circuit symbol which is used for the photoresistor or the light dependent resistor is a combination of the resistor action which indicates that the photoresistor is light sensitive in nature.




The basic symbol of the light dependent resistor consists of a rectangle which symbolizes the resistor action of the LDR. The symbol additionally consists of two arrows in the incoming direction.

The same symbol is used to symbolize the sensitivity towards light in the phototransistors and photodiodes.


The symbol of the “resistor and arrows” as described above is used by the light dependent resistors in majority of their applications.

But there are few cases where the symbol used by the light dependent resistors depicts the resistor encased within a circle. This is evident in the case when circuit diagrams are drawn.

But the symbol where there is absence of circle around the resistor is a more common symbol used by the photoresistors.

Mechanism of Photoresistors

The photoconductor which is placed between the two contacts is used as a component by the photocell or the photoresistor. The resistance of the photoresistors undergoes a change when there is an exposure of the photoresistor to the light.


Photoconductivity: The mobile carriers are generated when the photoconductor’s semiconductor materials used absorbs the photons results in the mechanism which works behind the light dependent resistors.

Although the materials which are used by the photoresistors are different, they are all semiconductors.

But when they are used in the form of photoresistors, then these materials act as resistive elements only where there is absence of PN junctions. This results in the device to become passive in nature purely.


The photoresistors or the photoconductors are basically of two types:

Intrinsic Photoresistor: The photoconductive material which is used by a specific photoresistor type enables the charge carriers to get excited and change to the conduction bands from their initial valence bonds respectively.

Extrinsic Photoresistor: The photoconductive material which is used by a specific photoresistor type enables the charge carriers to get excited and change to the conduction bands from their initial valence bonds or impurity respectively.

This process requires non-ionised impurity dopants which are also shallow and requires this to take place when light is present.

The design of the photocells or extrinsic photoresistors is done specifically considering the long wavelength radiations such as the infra-red radiations in most of the cases. But the designing also considers the fact that any type of thermal generation needs to be avoided since they are required to operate at temperatures which are very low.

Basic Structure of Photoresistors

The number of natural methods which are commonly observed for the manufacturing of the photoresistors or the light dependent resistors is very few in number.

A light sensitive resistive material is contained by the light dependent resistors which are constantly faces exposure to light. As discussed above, there is a specific section which is possessed by the light sensitive resistive material which is required to be in contact with both or one of the ends.


A semiconductor layer which is active in nature is used in a general structure of a photoresistor or a light dependent resistor and an insulating substrate is further used for depositing the semiconductor layer.

In order to provide the semiconductor layer with the conductivity of the required level, the former is observed to be normally doped lightly. Thereafter, on any one side of the area which is exposed, there is a placement of the contacts done.


One of the key issues in the basic structure of the light dependent resistor or photocell is its material’s resistance.

There is a minimization of the contacts which is done in order to ensure that there are changes occurring in the resistance when exposed to the light. In order to achieve this state, the surrounding area of the contacts is observed to be doped heavily which results in the reduction of the resistance in the given area.


The shape of the surrounding area of the contact has been observed to be mostly in the interdigital pattern or the zig zag form.

This enables the maximization of the exposed area along with the reduction in the levels of the spurious resistance which in turn results in the enhancement of the gains by contracting the distance between the two contacts of the photoresistors and making it small.

There is also a possibility of the usage of the semiconductor material such as polycrystalline semiconductor depositing it on a substrate. One of the substrates which can be used for this is ceramic. This enables the light dependent resistor to be of low cost.

Applications of the Photoresistors

The most attractive point of the light dependent resistor or a photoresistor is that it is of low cost and thus is widely used in a variety of electronic circuit designs.

Apart from this their rugged features and simple structure also provides them with an advantage.

Although the photoresistor lacks various features which are found in a phototransistor and a photodiode, it is still an ideal choice for a variety of applications.

Thus, LDR has been continuously used for a long period of time in a range of applications such as photographic light meters, burglar and smoke detectors, in street lamps to control the lighting, flame detectors, and card readers.


The factor which determines the photoresistor properties is the material type which is used and thus the properties can vary accordingly. Some of the materials used by the photoresistors possess constants of very long time.

Thus, it is quintessential that the photoresistor type si selected carefully for specific applications or circuits.