RESISTORS

• In the manufacturing of electronic equipment resistors of all types are used in large quantities. Resistor is probably the most common type of electronic component used in electrical and electronic circuits.
• Many different types of resistors are available. The properties of these different resistors vary, and it helps to obtain the right type of resistor for any given design to make sure best performance is obtained. 
• Although many resistors will work in a variety of applications, the type of resistor can be important in some cases. Therefore, it is necessary to know about the different resistor types, and in which applications each type of resistor can be used.

What is a resistor?

A resistor is an electrical component that limits or regulates the flow of electrical current in an electrical circuit.       

• Resistors are used in virtually all electronic and electrical circuits. Resistors, as their name indicates, resist the flow of electricity which is the most important attribute in most circuits.
• There are two main circuit symbols used for resistors. The oldest one is still widely used in North America and consists of a jagged line representing the wire used in a resistor.
• The other resistor circuit symbol is a small rectangle, and this is often termed as the international resistor symbol and it is more widely used in Europe and Asia.

• The unit of resistance is Ohm, Ω and resistor values may be seen quoted in terms of Ohms - Ω, thousands of Ohms or kilo ohms (kΩ) or millions of Ohms Mega ohms (MΩ). The Omega sign is often omitted, and the decimal point replaced by the multiplier: e.g. 1R5 would be 1.5 Ohms, 100R is 100Ω, 4k7 is 4.7 kΩ, 2M2 is 2.2MΩ and so forth.
• There are many different types of resistors. Some are for special applications like being used as variable resistors, and others are used for surge limitation whilst others provide a variable resistance with temperature.

Why are resistors needed?

Resistors are used to provide biasing voltages that control the gain of amplifiers. They are used to limit currents to safe levels and prevent overheating. They provide a way to sense current and voltage for circuit control and more.

How do resistors work?

A conductor has low resistance, while an insulator has much higher resistance. Devices called resistors allow us to introduce precisely controlled amounts of resistance into electrical circuits. A resistor works by converting electrical energy into heat, which is dissipated into the air.

 Basic distinction of resistor types

The first major category into which the different types of resistors can be fitted depends on 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 be changed to "adjust" the circuit. 
• 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.

Variable resistors and potentiometers are widely used for all forms of control:- everything from the volume controls on radios and sliders in audio mixers to a host of areas where a variable resistance is required.

A potentiometer is a three-terminal resistor with a sliding or rotating contact, forming an adjustable voltage divider. When only two terminals, one end and the slider are used, this will serve as a variable resistor or rheostat.

 Resistor Color Coding

Resistor Color Coding uses colored bands to easily identify a resistors resistive value and its percentage tolerance. There are many different types of resistors available which can be used in both electrical and electronic circuits to control the flow of current or to produce a voltage drop in many different ways. But in order to do this, the actual resistor needs to have some form of “resistive” or “resistance” value. Resistors are available in a range of different resistance values from fractions of an Ohm (Ω) to millions of Ohms. Obviously, it would be impractical to have available resistors of every possible value, for example, 1Ω, 2Ω, 3Ω, 4Ω etc. because literally tens of hundreds of thousands, if not then tens of millions of different resistors would need to exist to cover all the possible values. Instead, resistors are manufactured in what are called “preferred values” with their resistance value printed onto their body in colored ink.  

4 Colored Bands

The resistance value, tolerance, and wattage rating are generally printed onto the body of the resistor as numbers or letters when the body of the resistor is big enough to read the print, such as large power resistors. But when the resistor is small such as a 1/4 watt carbon or film type, these specifications must be shown in some other manner as the print would be too small to read.

So to overcome this problem, small resistors use colored painted bands to indicate both their resistive value and their tolerance with the physical size of the resistor indicating its wattage rating. These colored painted bands produce a system of identification generally known as Resistors Color Code.

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Procedure to find value of Resistor with color Coding

 1- The resistance of many resistors can be determined by reading a series of colored bands imprinted on the resistor body. In this scheme each color  represents a different decimal digit, as shown below.

2- The first three bands of the color code are used to specify the nominal value of the resistance, and the fourth, or tolerance band, gives the percent deviation from the nominal value that the actual resistor may have. Due to manufacturing variations, the actual resistance may be anywhere in a range equal to the nominal value plus or minus a certain percentage of that value.

3- The first two color bands specify the first two digits of the nominal value, and the third band represents the power of 10 by which the first two digits are multiplied 4- The example below demonstrates these computations.

Uses and Applications of Resistors

• Resistors are used in the power control circuit.
• It is used in DC power supplies.                          
• Resistors are used in filter circuit networks.
• It is used in amplifiers, oscillators, telecommunication, and digital multimeters.
• It is used in wave generators.
• Resistors are used in transmitters, modulators, and demodulators.
• It is used in medical instruments.
• It is used in instrumentation applications.
• Resistor is used in voltage regulators.
• It is used in feedback amplifiers.

Timing and Frequency

Many circuit designs use a resistor connected to a capacitor to provide a timing source. Light flashers, electronic sirens and many other circuits depend on this feature. The capacitor, which holds an electric charge like a cup holds water, takes a certain amount of time to fill with current, and the resistor determines how fast the capacitor fills up. If you multiply the ohm value of a resistor by the farad value of a capacitor, you get a time value measured in seconds; as resistance increases, the circuit's time also increases. 

Voltage Divider

A voltage divider is a “daisy chain” of resistors connected one after the other, forming a series circuit. If the resistors are all the same value, the voltage drop across each is equal; otherwise it is a proportion determined by the resistance of each resistor and the total resistance of all the resistors in the divider. Voltage dividers are useful for components that need to operate at a lesser voltage than that supplied by the input.                                                                                                                                                                                                                                                                                                                                                                                                                                                                          

Transistors and LEDs

Transistors and LEDs are devices sensitive to electrical current; too much current will destroy them, but too little prevents them from working properly. A resistor of the correct value, placed in the circuit, allows transistors, LEDs and other semiconductor components to function in the current range that suits them best.

Resistors for Heating

Because resistors convert electrical energy into heat, they make good heating elements for toasters, heaters, electric stoves, and similar devices. Traditional light bulbs work because the very high temperature from their resistance turns a metal filament white-hot, producing light. A formula, P = I2 * R, where P is heating power in watts, I is current in amps, and R is resistance in ohms, determines the amount of heat given off by a resistor. 

User Control of Circuit Functions 

Some types of resistors are variable, letting you set their resistance by sliding a slider or turning a knob. Varying resistance changes the amount of current flowing in a circuit. You can, for example, use a variable resistor to control the loudness of an amplifier, the pitch of a musical tone, or the speed of a motor.