Resistors

RESISTORS

A resistor is an electrical/electronic passive component used to limit the flow of current. The figure represents the various types of resistors.

Specification of Resistors:

The specification of resistors are :

1. Resistance value

2. Tolerance

3. Power rating

4. Thermal stability

1. Resistance value (Ohmic value)

The magnitude of the resistor has been expressed in terms of its resistance value. The resistance value decides the quantity of current flow to be opposed. Thus the resistance value is expressed in ohms. (Ω) or kilo‒ohms (KΩ) or mega‒ohms (MΩ). The resistance value is either printed on the surface of a resistor or by color bands.

2. Tolerance

Tolerance represents the maximum and minimum values of a resistance value. It is specified in terms of percentage. It is been expressed as either +% on the surface of the resistor or with the help of a fourth color band.

3. Power Rating (Wattage Rating)

Power rating specifies the maximum power in watts that the resistor can handle without being destroyed or damaged. It also represents the power dissipation in the resistor in terms of I²R loss (or heat).

4. Thermal Stability

Thermal stability indicates the stability in the resistance value to a maximum specified temperature. or It is the ability of a resistor to maintain the same resistance value with variation in temperature.

Types of Resistor and their Symbols:

Based on their operation following are the types of resistor:

1. Fixed resistor

(a) Wire Wound Resistors

(i) Power type wire wound resistor

(ii) Precision type wire‒wound resistor

(b) Carbon Composition Resistors

(c) Cracked Carbon Resistors or Carbon Film Resistors

(d) Metal Oxide Resistors

(e) Metal Film Resistors

2. Variable resistor

(a) Continuously variable resistors

(i) Potentiometers

(ii) Rheostats

(b) Adjustable or pre‒set resistors

(i) Decade resistance boxes

(ii) Thermistors

(iii) Varistors.

3. Tapped resistors

Symbol and Units

Symbol for a resistor is as shown below.

The units of resistance is Ohms, which is indicated by Ω.

The formula for resistance is

 R = V/I

Where V is Voltage and I is Current. It would really be difficult to manufacture the resistors with each and every value. Hence, few values are chosen and the resistors of such values are only manufactured. These are called as "Preferred Values". In practice, the resistors with near values are chosen to match the required applications.

Color Coding

A process called color coding is used to determine the value of resistance for a resistor, just as shown in the above figure. A resistor is coated with four color bands where each color determines a particular value. The below table shows a list of values which each color

The first two colored bands indicate the first and second digit of the value and the third color band represents the multiplier number of zeroes added number of zeroes added. The fourth color band indicates the tolerance value.

Tolerance is the range of value up to which a resistor can withstand without getting destroyed. This is an important factor. The following figure shows how the value of a resistor is determined by color code.

The five color band resistors are manufactured with tolerance of 2% and 1% and also for other high accuracy resistors. In these five band resistors, the first three bands represent digits, fourth one indicates multiplier and the fifth represents tolerance.

Let us look at an example to understand the color coding process.

Example 1: Determine the value of a resistor with a color code yellow, blue, orange and silver.

Solution

The value of yellow is 4, blue is 6, orange is 3 which represents multiplier. Silver is 10 which is the tolerance value.

Hence the value of the resistor is 46 × 10³ = 46 kΩ

The maximum resistance value for this resistor is

46 ΚΩ or 46000 Ω + 10% = 46000 + 4600 = 50600 Ω = 50.6 ΚΩ

The minimum resistance value for this resistor is

46 ΚΩ or 46000 Ω ‒ 10% = 46000 ‒ 4600 = 41400 Ω= 414 ΚΩ

After having gone through different details regarding resistors, we have some terms to learn. Also we have to deal with different behaviors of a resistor for few types of connections.

1. Fixed resistor

Fixed resistors are one type of linear resistors. A resistor is said to be a fixed resistor, if its value is fixed. The value of fixed resistor can't be varied like a variable resistor as its value is determined at the time of manufacturing itself. The following Fig. 1.1 represent the symbol of a fixed resistor.

The fixed resistors are classified into different types, depending upon their manufacturing processes and the materials used in their manufacturing. The classification is as follows,

1.1 Carbon Composition

The Carbon composition resistors are a blend of carbon particles, graphite and ceramic dust mixed with a binder substance like clay. This mixture is treated with high pressure and temperature. After the whole thing is molded in a case, the leads are fixed.

• Thermal, mass of the carbon composition resistor is higher so as to withstand high energy pulses.

• These resistors have low stability and high noise which is a disadvantage.

The following figure 1.2 shows an image of carbon composition resistor.

Carbon composition resistors are used in Surge protection, Current limiting, and High voltage power supplies.

1.2 Wire wound

A Wire wound resistor is formed by wounding a wire made up of a resistive material around a core. The metallic core acts as a non‒conductive material while the resistive, wire conducts, but with some resistance. The image of a wire wound resistor is as shown below.

Usually a nichrome wire or a manganin wire is used to wind the core because they offer high resistance. Whereas plastic, ceramic or glass is used for core.

• Wire wound resistors are very accurate.

• They work excellently for low resistance values and high power ratings.

These are the oldest type of fixed resistors, but are being used even now.

1.3 Thick Film

The film resistors have a resistive layer on a ceramic base, whose thickness defines the type they belong to. The thickness of resistive layer on thick film resistors is much higher than thin film resistors. Thick film resistors are produced by firing a special paste, which is a mixture of glass and metal oxides, onto the substrate.

There are three main types in thick film resistors like Fusible resistors, Cermet film resistors, and Metal oxide film resistors.

1.4 Fusible Resistors

The Fusible resistors are similar to wire wound resistors. But these resistors along with providing resistance, act as a fuse. The image of a fusible resistor is as shown below.

In this resistor, the current flows through a spring loaded connection, which is placed closely to the body of the resistor. The blob that is attached to the spring resistor takes the heat generated by the resistor due to the current flow. If this heat is increased, the attachment to the blob gets melted up and opens the the connection.

Hence we can say that, these resistors limit the current, but if the circuit power rating exceeds a specified value, these resistors act as a fuse to open or break the circuit. The value of these resistors is usually of less than 10 Ohms.

These resistors are generally used in TV sets, amplifiers and other expensive electronic circuits.

1.5 Cermet Film Resistors

The Cermet film resistors are the film resistors made up of a special material called Cermet. Cermet is a composite alloy made by combining Ceramic and Metal. This re resistance combination provides the advantages in both of these materials like high temperature resistance and wear resistance of ceramic along with flexibility and electrical conductivity of a metal.

A metal film layer is wrapped around a resistive material and is fixed in a ceramic metal or cermet substrate. Leads are taken to make the connections easy while fixing on a PCB. They offer high stability as temperature cannot affect their performance.

1.6 Metal Oxide film resistors

A Metal oxide film resistor is formed by oxidizing a thick film of Tin chloride on a heated glass rod, which is a substrate. They have high temperature have high temperature stability and can be used at high voltages. These resistors have low operating noise.

Metal oxide film resistors differ with metal film ones only regarding the type of film coated. Metal oxide is a metallic compound like tin with oxygen to form tin oxide, which is coated as a film on the resistor. The resistivity of this resistor depends upon the amount of antimony oxide added to the tin oxide.

1.7 Thin Film

Thin film resistors have a resistive layer of width 0.1 micrometer or smaller on the ceramic base. Thin film resistors have a metallic film that is vacuum deposited on an insulating substrate.

Thin film resistors are more accurate and have better temperature coefficient and is more stable. The thin film resistors are further divided into two types such as ‒

• Carbon film resistors

• Metal film resistors

Carbon film resistors

A Carbon film resistor is made by depositing a carbon film layer on a ceramic substrate. The carbon film acts as the resistive material to the current and the ceramic substance acts as an insulating substance. Metallic caps are fixed at both the ends and copper leads are drawn out.

The following figure 1.3 shows the construction of a carbon film resistor.

The main advantages of these resistors are their high stability, wide operating range, low noise, and low cost. The carbon film resistors are the most preferred ones over carbon composition resistors due to their low noise.

Metal Film Resistors

The film coating makes the difference between metal oxide film resistors and metal film resistors. A thin film of metallic substance such as nickel chromium is used to coat the resistor in a metal film resistor whereas a film of metal oxide like tin oxide is used to coat the resistor in a metal oxide resistor.

Metal film resistors have low temperature coefficient of resistance, which means the resistance is less affected by the temperature.

Wattage

While using a resistor, if the flow of current increases, the resistor dissipates some heat. If this value crosses a certain critical value, the resistor may get damaged. The wattage rating of a resistor is printed on some higher value resistors in order to avoid such situation.

Wattage is the amount of electric power expressed in watts. Electric power is the rate of transfer of electrical energy.

Power P=VI=I²R

2. Variable Resistors

The variable resistor is a passive, three terminal device that can adjust its resistance via third terminal located between two terminals so that the obstruction to the flow of current goes up and down. Therefore, variable resistor circuit symbol has an arrow which represents resistance variation. The electrical symbol of variable resistor shown in Figure 1.4.

The resistance of a variable resistor can be changed between zero to a certain maximum value with its third terminal. When the circuit diagram of the variable resistor in Figure 1.4 is carefully examined, you can see constant resistance exist between the terminals 1 and 3. Terminal 2 (in the middle) is the only terminal which has the ability to move. Therefore, in order to change resistance you must use anyone of the side terminals with the moving terminal.

Operation Principles of Variable Resistors

Variable resistors are widely used in electric circuits to adjust the value of current or voltage, since the resistance of variable resistors can be set to a certain value. Variable resistors allow you to adjust the value of voltage by changing the resistance and keeping current constant. To adjust the input voltage, a voltage source is connected to the terminals 1 and 3 as shown in Figure 1.5. The output voltage between terminals 1 and 2 can be calculated by the voltage division formula

Voltage Division Formula

Construction of a Variable Resistór

Although there are different types of variable resistors, their working principle is the same. When the inside of a variable resistor is examined such as Fig. 7,6, there is a fixed resistance called the resistive track which is between terminals 1 and 3. Terminal 2 is connected to the knob and the slider (wiper) has a direct contact with the knob. The resistance between terminals 1 and 2 or 2 and 3 can be changed don by adjusting the knob in the middle as represented by red circle in Fig. 1.6.

Types of Variable Resistors

There are different types of variables resistors which all have almost the same working principle that was illustrated in the previous sections. However, terminal configuration and resistance value of a variable resistor can be adjusted with respect to various environmental parameters. These different types of variable resistors include:

1. Potentiometer

2. Rheostat

3. Photoresistor

4. Force sensitive resistor

5. Thermistor

6. Humistor

1. Potentiometers

As mentioned in previous sections, variable resistors are often used to control voltage or current. Potentiometers is one of the most popular types of variable resistors. A Potentiometer is simply called as a Pot. This is a three‒terminal resistor having a shaft which slides or rotates. This shaft when operated forms an adjustable voltage divider. A potentiometer also measures the potential difference voltage in a circuit. A path of resistive material with resistance of low to high value is laid internally and a wiper is placed so that it connects the resistive material to the circuit. This is mostly used as a volume controller in TV sets and Music systems.

The following fig. 1.7 shows an image of a Potentiometer.

Out of the three terminals, two are used. One is connected to the resistive element, and the other is connected to the knob. As seen in the above figure, by rotating the knob above, the position of the resistance differs. We know that resistance depends on the area of cross‒section, length, and specific resistance. So by varying the position of the knob, the length varies and hence resistance varies.

This change in the resistance cause change in voltage, and hence the potential difference. A simple example of this potentiometer is changed in the speed of the ac motor by applying a varied armature voltage. They are also frequently used as power control devices. They can control voltage, current. Light intensity, sound, etc. Also used in heaters, oven, electric motors, and many other electrical appliances.

They are preferred in applications where voltage control is required. There are mainly two groups of potentiometers known as mechanical and digital. Mechanical potentiometers such as linear and rotary potentiometers have accuracy problems in vibrational circumstances. Digital potentiometers are commonly used due to sensitivity problem of mechanical potentiometers. One of the most fundamental uses of digital potentiometers is to come up with resistance drift problem that occurs in challenging environmental conditions. Since digital potentiometers can be adjusted by communication protocols such as I²C, they are also quite useful in cases where mechanical resistance adjustment isn't possible.

2. Rheostats

The construction of a rheostat is similar to that of a potentiometer. However, the rheostat's moving terminal is short circuited with one of the side terminals as shown in Fig. 1.8. Rheostats are preferred in applications where resistance adjustment or current limitation required.

This is also called a linear rheostat. In this type, the resistance is adjusted by varying the position of the wiper shown on the horizontal bar. The wiper is in connection with the resistance material below. This element has two ends for obtaining variable resistance. As the wiper is moved from one end to another, a variable resistance is obtained. This type is often used in laboratories. The wiper is placed a top an insulated ceramic core. And a long wire is wounded around the core. They are also called as wire wound resistors. Due to the nature of the movement of the wiper, they are also called slide rheostats.

As compared to potentiometers, rheostats carry more current. In other words, the current rating of rheostats is very high. It may range up to 10 A. The high rating is because of the wire. Since the current‒carrying capability depends on the cross‒sectional area of the conductor, to carry high currents, the thick wire is used to wound around.

3. Trimmer

Trimmer is both a variable resistor and a potentiometer measures potential difference measures potential difference. This Trimmer Potentiometer is, in short called as Trim Pot. If these are used as variable resistors, then they are called as Preset Resistors.

4. Photoresistors

Photoresistors also known as light dependent resistors (LDRs) are a common type of variable resistor. Their resistance changes with respect to the intensity of incoming light due to the photoelectric effect. Photoresistors can be preferred in environments where intensity of the light varies.

5. Force Sensitive Resistor

As the name suggests, the resistance of a force sensitive resistor changes with respect to the applied force level. They are generally used in robotic applications such as inside of the grippers of a robot.

Applications of Variable Resistors

Variable resistors are found in many of the devices/electronics we have in our homes.

Some of these include radios, speakers, microphones, TVs, oscillators, smart home control devices etc. Potentiometers are generally used in home electronic appliances where speed or volume level control needed.

• Rheostats are used where current or resistance levels should be adjusted. A common example is the dimming of lights. In summary, variable resistors are popular in applications where voltage control or current adjustment required.