Electrical

# Resistor Types and Color Coding

## Definition:

Resistor types and color coding- The components which cause resistance in the flow of current passing through an electrical circuit, or limit it, are called resistors.

or

A resistor is a component, which has a specific resistance value (or a specific quantity). In other words, a component, which is designed for a specific and known value of resistance, is called a resistor.

A resistor is a component designed to have a known value of resistance or those components which are specially designed to have a certain amount of resistance are called resistors. We know that the resistance of any resistor depends on its length (ℓ), its resistivity (, and its area (A), i.e., R = / A. This has been illustrated in figure 1.11.

Figure 1.11 – Factors controlling resistance

## Types of Resistors

There are different types of resistors according to different sizes, shapes, and materials. There are certain resistors, the resistance values of which do not change as a result of a change in applied voltage and temperature. Such resistors are known as linear resistors. (that’s resistors having a value of current directly proportional to an applied voltage, are called linear resistors), whereas the resistance values of some of the resistors change as a result of a change in applied voltage and temperature, they are called non–linear resistors. (In other words, the current value of a linear resistor is not directly proportional to applied voltage).

Generally, different types of resistors having linear characteristics, are categorized into the following two main groups;

(1). Fixed Resistors

(2). Variable Resistors

A further detail of the above–mentioned types f resistors, is as below;

### Fixed Resistors

As is suggested by its name, fixed resistors are a type of resistors, that have fixed values unable to be altered, that’s the resistance value of which turns out to be fixed, are called fixed resistors. During the process of manufacturing, the values of these resistors are fixed permanently, and they are not possible to be changed subsequently.

Fixed resistors can be classified into the following types;

(a). Carbon Composition Resistors

(b). Wire-wound Resistors

(c). Thin Film Resistors

(d). Thick Film resistors

(a). Carbon composition Resistors

A common type of fixed resistor is a carbon composition resistor, which is prepared from a mixture of thin granulated or ground carbon or graphite, insulating filler, or a resin binder. That’s, graphite and carbon are mixed in a certain proportion with a binder (which is a powder of some insulating material) to give it the shape of a resistor. The ratio of carbon and insulating material depends on the specific value of the under-construction resistor. Thus, a resistor of any value can be manufactured through a proper selection of the mutual ratio of carbon powder and insulating powder (or binder).

Normally, these resistors come into existence as a result of the mixing of carbon powder and insulating powder, manufactured in a rod shape. Two metal caps are mounted on both ends of the resistance element, wherefrom two leads are pulled out for a circuit connection. These leads are normally copper made, with tin sheathed on them, so that soldering can be done easily. These leads are called Axial Leads. In order to provide mechanical power to a complete resistor and for its protection, it is encased in an insulating sheath. The resistor’s heat also emits through this sheath, and it also prevents the formation of hotspots on a resistor’s surface. In figure 1.12 (a) and 1.12 (b), different cutaway parts of a carbon composition have been illustrated.

Figure 1.12 (a) – Cutaway view of carbon – composition resistor

Figure 1.12 (b) – Fixed composition resistors

Figure 1.12 (c) – Fixed composition resistors of different wattage ratings

The sizes of these resistors keep changing according to wattage or power ratings. Wattage ratings increase as a result of an increase in size. These resistors are commonly available in the range of one-ohm resistance value to 22 megaohms, whereas their power ratings tend to be between 1/10 to 2 watts. In figure © carbon resistors of a fixed composition having different wattage ratings have been illustrated.

### Characteristics

These resistors are generally cheap, small in size, require less space, tend to be trustworthy and reliable, and are commonly available in different power ratings and resistance values. They can be installed in the circuit pretty easily, and they can withstand excessive voltages. However, they turn out to be less stable (that’s their temperature coefficient is quite high) and they also produce a little sound as compared to other resistors.

(b). Wire – Wound Resistors

The wire-wound resistors are made by means of overlapping resistive wire around an insulating core or rod. The resistance is generally composed of tungsten, manganin, nickel, and chromium alloys, whereas the insulating core is manufactured from porcelain, bakelite, press bond paper, or any other ceramic material. Sometimes, a layer of any ceramic-based enamel is also laid on a wire coil, so that the resistor element could insulate, and metal oxidation also does not take place. When a resistor element is manufactured from this type of wire, then it can function at a very high temperature without breaking down or without causing any kind of variation in its original resistance. Wire-wound resistors are mostly used for high–power ratings.

Resistors wound through a manganin wire are extraordinarily precious, important in nature, and are normally applied along with sensitive test equipment. For example, weight stone bridge, etc. As the temperature co0efficient of manganin wire tends to be very low, therefore resistors manufactured from such a wire can sustain their resistance values up to a vast range of temperatures. Multiple wire–wound resistors have been displayed in the figure below.

Remember that the resistance of the wire overlapped on the insulating core depends on its length and its specific resistivity. As the wire tends to be bare, therefore a casing of some type of insulating material is essentially placed on this wire as a precaution. Leads are also placed alongside these resistors for connection with the circuit. Wire–wound resistors are generally preferred over carbon composition resistors owing to low resistance and low noise generation. These resistors can generally comply with high currents; therefore, their resistance turns out to be quite low. Normally, their power ratings tend to be within a range of 2 watts to 100 watts, or even higher. Whereas their resistance may range from 1 ohm to 200-kilo ohms. These resistors can operate up to a temperature range of 350℃ and their temperature coefficient ranges from ± 20 ppm/℃ to 200 ppm/℃ (parts per million per degree Celsius = ppm/℃). The power rating of a high-power wound resistor may maximum be up to 500 watts, whereas its resistance will be within a range of 0.1 ohms to 100-kilo ohm.

Remember that this type of resistor is generally used in those places, where a completely accurate, sensitive, and balanced type of current control is required. For example, as a shunt along with an ampere meter, etc. Moreover, their application with high-powered equipment, industries, and test and control equipment is quite common.

(c). Thin Film Resistors

Basically, all thin-film resistors consist of a thin film of a high-grade ceramic rod and a resistive material. That’s, these resistors are manufactured by means of encasing a very thin layer of any conducting material on an insulating rod, plate, or tube. Insulating rods, tubes, or plates are usually fabricated from some high-grade ceramic material or glass. These types of resistors can be divided into the following two types;

(i). Carbon Film Resistor

These resistors consist of an insulating rod or core (remember that this core or rod is also known as a substrate), which is made from high-grade ceramic material. A thin resistive carbon film (layer) is laid above this rod. These resistors produce very low sound, and their operating range tends to be very expansive. These resistors are mostly used in the electronics industry. In figure 1.14, the construction of a carbon film resistor has been illustrated. These resistors are sturdier and more stable as compared to solid carbon resistors.

(ii). Metal Film Resistors

These resistors are also similar to carbon film resistors from the manufacturing point of view. The only difference is that instead of carbon, some metal or metal mixture, metal oxide, nickel-chromium, or a mixture of metals and glass (which are known as metal glaze) are used in them. as a resistive film. That’s metal film resistors are fabricated by coating a thin metal film over a ceramic made insulating rod. These resistors are tiny in size, less costly, and reliable. A high close tolerance can be achieved through this technique of manufacturing resistors. There are some film resistors as well, the temperature coefficient of which tends to be very low i.e., ± 2 ppm / ℃. Their noise level tends to be very low, and they are used mostly in places where achievement of high stability is desired. In figure 1.15, a metal film resistor has been illustrated.

Figure 1.15 (a). Metal film resistor

(d). Thick Film Resistors

The method of manufacturing these resistors is almost exactly the same as being used for the manufacturing of thin-film resistors. The only difference is that instead of using a thin film, a thick film of some resistive material is coated for the manufacturing of these resistors. That’s why these are known as thick film resistors. Their forms are as follows;

(i). Metal Oxide Resistors

These resistors are fabricated through oxidizing of tin chloride on a heated glass rod (substrate). As such, a thick tin chloride film is coated on a glass rod. The thickness of the film depends on the oxidizing time of tin chloride. A vast range of the resistance values of these resistors is available up to very high voltage ratings. These resistors produce very little noise and are capable of strong heat stability.

(ii). Cermet Film Resistors

The interior part of this type of resistor, which consists of insulation, is made from ceramics, above which a film of carbon or any metal alloy is coated. After this, it is fixed in a ceramic metal (which is known as cermet). The basic objective of these types of resistors is to provide accurate resistance values and superior heat stabilization. (That’s these resistors work in an immensely balanced manner in a heat situation, their resistance values also do not change, and these resistors are generally formed in a square or rectangular shape, having underneath leads fixed, with the help of which they can be fitted on any printed circuit board. Therefore, plugging and unplugging these resistors on a circuit board tends to be easy.

(iii). Fusible Resistors

This type of resistor is analogous to wire–wound resistors, and they are designed in such a fashion that whenever the power rating of any circuit exceeds a predetermined specific value, they tend to fuse. That’s why these resistors are called fusible resistors. As such, these resistors not only limit current, they also perform the function of a fuse (that’s these resistors are capable of a double function). The application of these resistors in TV sets, amplifiers, and other precious electronics equipment, is quite common. The value of these resistors normally tends to be less than 10 ohms.

## Variable Resistors

As its name suggests, variable resistors are a type of resistors, the resistance of which can be changed or controlled through a dial, knob, screw, or any other suitable source according to requirements. Or resistors, which are designed in such a fashion that their values can either be changed manually or through some automatic adjustment, are known as variable resistors. In other words, resistors the resistance of which can be changed from zero to up to a specific value, are called variable resistors. These types of resistors are used for controlling volume and tone in a radio receiver. There is a sliding arm in these types of resistors, which is associated with the shaft, by means of turning in a complete circle, the resistance value can be changed.

A variable resistor is of the following types;

(i). Potentiometers

(ii). Rheostats

(iii). Trimmers

(i). Potentiometers

These are variable resistors having three terminals, which are used to control voltage levels in any circuit. Its outer two terminals are fixed (that’s a constant resistance value that exists between these terminals), whereas the middle or third terminal is associated with moving contact or wiper (that’s middle terminal is variable). The value of resistance can be controlled according to requirement, by rotating a wiper connected with the control shaft. When the control shaft turns, the wiper moves above the resistive element (which is either carbon or graphite made). As such, the resistance of the mean terminal and mean resistance of any outer terminal, change. By adjusting the control shaft on any side of the mean terminal, the output of any part or value of input voltage can be measured. As voltage level continues to change as a result of such application of a potentiometer, therefore, it functions as a divider. This type of potentiometer is also known as a variable composition resistor. Their value can be up to 10 megaohms.

(ii). Rheostats

Rheostats are also known as variable wire – wounded resistors. These are manufactured through winding nichrome resistance wire on any ceramic core. The accomplished assembly is enclosed in a safety casing.

A rheostat is in fact a variable resistor having two or three terminals. The circuit current can be controlled by changing its value by hand. Rheostats are also known as tapped resistors. A metal band is solidly coated around a resistor’s element. By making a connection with one end of a resistor’s element and sliding contact or wiper, if it is used as a variable resistor for controlling the current, it is known as a rheostat (and if voltage is controlled by means of establishing a connection between both ends of an element and wiper, then such a variable resistor is known as a potentiometer). Basically, there is no difference between a rheostat and a potentiometer. Both are variable resistors, only their application is different.

(iii). Trimmers

These are type of potentiometers or variable resistors, alongside which a small screw is being fitted for the achievement of a better performance and working efficiency. A small screw is inserted within it, which is turned a number of times to alter its resistance value. They are manufactured from carbon composition, carbon film, cermet, and wire materials. They can either be formed with single turns, or may be multi turns as well. Trimmers are available within a range of 150 ohms to 5 megaohms, whereas their power ratings normally tend to be ¼ to ¾ watts.

## Non – Linear Resistors

We know that non – linear resistors are those resistors, wherefrom the current passing through, does not change according to the ohm’ law, rather it changes as a result of a change in the applied voltage or body temperature. For example, if the resistance value changes as a result of body temperature, then such resistors are known as thermistors. And if resistance changes owing to a change in applied voltage, then such resistors are known as varistors or VDR (Voltage Dependent Resistors). The non – linear resistors are of the following types;

(1). Thermistors

(2). Photo Resistors or Photo Conductive Cell

(3). Varistors

### Thermistors

A thermistor is a two-terminal device, which turns out to be too sensitive for temperature. That’s a thermistor is a type of variable resistor, which can immediately sense even a slightest of a change occurring in temperature. The resistance of a thermistor is indirectly proportional to the temperature, that’s the ratio of a change between resistance and temperature tends to be indirect or reversed. That’s resistance declines as a result of a rise in the temperature, whereas it increases as a consequence of a decrease in temperature. In other words, the thermistor has a negative temperature coefficient (NTC).

The temperature of a thermistor can be changed internally as well as externally. If an addition is made in the value of current flowing through it, its temperature increases, due to which, its terminal resistance declines. If heat is provided through any external source, then an increase occurs in the body temperature of a thermistor, as a result of which resistance decreases. These thermistors are manufactured from the metal oxide of cobalt, nickel, strontium, and manganese. In figure 1.25, symbols and characteristics of the thermistor have been demonstrated.

Remember that some of the thermistors have a positive temperature coefficient (PTC). These thermistors are manufactured from a doped barium titanate semiconductor material, and their resistance increases as a result of an increase in temperature.

### Photo Resistors or Photo Conductive Cells

These are type of semiconductor devices that two terminals and the terminal resistance of which changes as a result of light intensity. In other words, resistors the terminal resistance of which changes as a consequence of a variation in the light reflected on its surface, are known as photoresistors. The materials, from which these semiconductors are constructed, are called photoconductors. For example, cadmium sulfide, lead sulfide, etc.

As soon as the intensity of light on the surface of the photoresistor increases, then the number of free carriers or electron – holes pairs produced on the surface of semiconductor material as a result of this light energy, increases. As a result, the resistance of semiconductor material decreases. That’s the resistance of a semiconductor is inversely related to the quantity of light energy. In other words, the resistance of a photocell changes and is reduced as a result of an increase in light energy, and as a consequence of a decrease in light energy, its resistance increases. These cells or resistors are also capable of a negative temperature coefficient (NTC).

Uses

These types of resistors are commonly used in burglar alarms, door openers, flame detectors, light meters, light-activated relay controls, for controlling lights of street lamps in industrial and commercial areas, and photographic equipment, etc.

### Varistors

Varistors are voltage-dependent resistors, which are used for eliminating high voltage transients. They are briefly known as VDR. A special type of variable resistor used to protect circuits from destructive voltage spikes is called varistors.

There feature is that whenever an increase occurs in the voltage found parallel to any sensitive equipment or system (as a result of lighting or power lines fault), it reduces them pointedly up to a safe limit (i.e., change voltage levels). This turn out to be possible as a result of a change in their resistance values. Hence, electronic instruments are protected against the risk of any kind of damage. In other words, varistors are such variable resistors, a change in the resistance of which depends on the applied voltage.

Uses of Resistors

Practically, resistors (fixed as well as variables) are generally used for the following objectives. Remember, that resistors maintain the same characteristics both on AC as well as DC.

(I). For controlling or limiting current

(II). For the conversion of electric energy to thermal or kinetic energy

(III). As a shunt in ampere meters

(IV). As a multiplier in voltmeters

(V). For controlling the temperature

(VI). For controlling or dropping voltage

(VII). For protection, e.g., fusible resistors

(VIII). In laboratories and observatories

(IX). In household applications e.g., heater, iron, emersion rod, etc.

(X). Its application in electronics industries is carried out on a very vast scale.

## Power Rating of a Resistor

At the time of installation of resistors in the electronics circuits, apart from resistance, their power ratings are also taken into consideration, which depends on a resistor’s own resistance and on the square of light passing through it (i.e., I2R watt). Resistors of any value can be manufactured in different power ratings. The normal resistance of a carbon resistor tends to be 1, 1/2, 1/4, 1/8, 1/10, and 2 watts, whereas wire – wound resistors consist of a power rating from 1/4 watt to 200 watts. Carbon resistors can normally work efficiently and correctly up to 85°C whereas wire wound resistors up to 300°C. Their characteristics sometimes change if the temperature increases beyond this limit.

In both situations, the resistor’s power rating reflects the maximum power quantity, which a resistor can handle without crossing the limit of a working temperature. Power rating actually tends to be a physical characteristic of any resistor, which depends on the structure as well as the size of a resistor. The resistors having higher power ratings, usually turn out to be quite bulky in size, and as the resistor’s power rating keeps declining, their size also becomes smaller. Moreover, high wattage resistors can always operate at high temperatures. For example, as wire – wound resistors are larger in size than the carbon resistors, therefore, they can sustain higher temperatures. It should be remembered here that apart from power rating, voltages of both types of resistors are also described, which means that these resistors are damaged beyond this voltage rating.

## Dissipation of Power in Resistors

The maximum amount of power, that a resistor dissipates without crossing a safe limit of the extreme temperature, is known as the dissipation of a resistor. When supply is provided to a resistor fitted on any electrical circuit, the flow of current starts through this resistor as a result of potential difference, the quantity of which tends to be inversely related to the resistor’s value. The resistor provides resistance in the flow of this current, due to which electrical energy converts into heat energy. This heat energy emits from the surface of a resistor. As such, dissipates consistently by converting from electrical energy to heat energy. This process is called the dissipation of power, and it is mentioned in watts.

In other words, power dissipation in any resistor means that automatically generated heat, which is equal to the product of a resistor’s value and square of a current, (i.e., W = I2 =R)

This power dissipated in the form of heat is represented by watts. Remember that a resistor also gains heat from other resistors fitted on a circuit and other nearby equipment through radiation. Therefore, power dissipation in any circuit should be less than the applied resistor’s actual power rating. The flow of charge (Q) is known as current, and the flow of current in any resistor becomes possible only due to an electric potential (V), whereas power (W) is a work ratio resulting from the transferring of this charge, hence power dissipation in a resistor can also be represented by the following relation;

W = V Q

## Color Coding of Resistors

Color coding is a type of method, by means of which the total resistance value of a resistor and its tolerance can be determined through different color bands and stripes designed on a resistor. The resistance value (in ohms) of a number of variable and fixed resistors is printed on their body or cover, whereas some of the resistors are so small in size, that writing resistance values above them is almost impossible. Therefore, instead of printing their value on these resistors, a system of color-coding was devised, through the application of which their resistance can be determined. In this color-coding system, 4 or 5 bands of different colors are designed on the outer cover of the resistor from the left end towards the right end. Every color band has a distinct numerical value, which can be ascertained with the help of the following table. Color bands are always read from left to right. In figure 1.27, a color band system has been illustrated, the value of which can be read as follows with the help of the following table.

(i). The first band reflects the first digit of a resistor’s resistance value

(ii). The second band reflects the second digit of a resistor’s resistance value

(iii). The third band reflects the number of zeros along with the predetermined first two bands. The third band is also known as a multiplier.

(iv). The fourth band reflects the capacity relating to dearth or abundance, or tolerance.

For example, a 5% tolerance will mean that there is a capacity or accommodation of a change of up to ±5% in the actual resistance value within the color-coded value. Let us assume that a resistor is of 100-ohm power, and its tolerance is ±5%. It means that the minimum value of this resistor is 95 ohms, whereas the maximum value of the resistor is 105 ohms. Remember that if the 4th band does not exist on the resistor, then tolerance of the resistor is treated as ±20%.

Figure 1.27 – Color-coded bands on a resistor

(v). if a 5th band also exists on any resistor, it refers to the reliability factor of that resistor, which represents a failure percentage of any resistor after its usage for 1000 hours.

Different colors and their numerical values have been illustrated below;

 Color Numerical figure Decimal Multiplier Value Tolerance Power of 10 Multiplier Value Black Brown Red Orange Yellow Green Blue Violet Gray White Silver Gold None 0 1 2 3 4 5 6 7 8 9 – – – 100 101 102 103 104 105 106 107 10-2 (alternate) 10-1 (alternate) 10-2 (alternate) 10-1 (alternate) – 1 10 100 1000 10000 100000 1 million 10 million 0.01* 0.1* 0.01* 0.1* – ±10% ±5% ±20%

Example 1; Find the resistance value in ohms and the percent tolerance for each of the color-coded resistors shown in figure 1.28.

Figure 1.28

Solution;

First band is red = 2

2nd band is violet = 7

3rd band is orange = 3 zeros

4th band is silver = 10% tolerance

R = 27,000 Ω ±10% Ans.

(b). First band is brown = 1, 2nd band is black = 0, 3rd band is brown = 1 zero, 4th band is silver = 10% tolerance

R = 100 Ω ±10% Ans.

©. First band is green = 5, 2nd band is blue = 6, 3rd band is green = 5 zeros, 4th band is gold = 5% tolerance

R = 5, 600, 000 Ω ±5% Ans

Example 2; Determine the resistance and tolerance ratings for the following color codes;

(i). Gray, Red, Black, Gold

(ii). Blue, Green, Red, Silver

Solution;

In this case:

Gray     Red      Black    Gold

↓          ↓          ↓          ↓

8         2         0         ±5%

Resistance value is 82Ω ± 5% Ans.

(ii). In this case;

Blue      Green   Red      Silver

↓          ↓          ↓          ↓

6         5         10          ±10%

Resistance value is 65 x 102 Ω ±10%

Or 6500 Ω ± 10 % Ans.

## Troubles in Resistors

A fundamental defect frequently witnessed in different types of resistors is that they turn out to be open. When these resistors are checked with any multi-meter (or AVO meter), they denote an infinity value. Remember that before checking a resistor, it should be unplugged from the circuit. Further, a knob of the meter is also necessary to be set on ohm.  The reason behind the resistor being open is that whenever a large quantity of current passes through the resistor, it gets overheated. Due to overheating, its body color changes, which clearly reflects that it has become open. When a resistor gets open, the current stops flowing through it. As such, that concerned part of the circuit, on which this resistor has been fitted, also becomes an open circuit.

Some variable resistors stop working properly after some time. If such resistors have been used for volume control, then noise will also be audible along with sound on the radio receiver’s or amplifier’s output. Its reason is that the moving wiper above the resistor does not touch properly with the resistance element during turning. This happens due to the misplacing of the wiper or erasing of carbon in case of carbon resistors. For achieving a correct output, the replacement of such resistor is better.

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