Resistance

Resistance

The property of a substance which opposes the flow of electric current (or electricity) through it is called Resistance OR Resistance is the ability of a circuit which opposes current.

Mica, Glass, Rubber, Wood etc. are the examples of resistive materials. The unit of resistance is OHM (Ω) where 1Ω = 1V/1A which is derived from the basic electrical Ohm’s law = V = IR.

Other definitions of Ohm“Ω “are as follows;

If there is a potential difference of 1 volt between two ends of the conductor and the flowing current through it is 1Ampere, then the resistance of that conductor would be 1 Ohm (Ω). OR

If 1 ampere of current is flowing through a resistance, and 1 joule per second (1Watt) energy (in the form of heat) is generated, then the measurement of that resistance is 1 Ω.

Ohm is the measurement quantity of resistance, which produces one joule of energy (in the form of heat) in one second, when one ampere of current is flowing through it. The reciprocal of the resistance is called conductance.

Resistor

Those components and devices which are specially designed to have a certain amount of resistance and used to oppose or limit the electric current are called resistors.

Resistance of a resistor depends on their length (l), resistivity (ρ) and its cross sectional area (a) which is also known as laws of resistance.

R = ρ (l/a)

Types of Resistors

Resistors are available in different size, Shapes and materials.

There are two basic types of resistors.

1. Fixed Resistors

As the name tells everything, fixed resistor is a resistor which has a specific value and we can’t change the value of fixed resistors.

Types of Fixed resistors

     1.     Carbon Composition Resistors

     2.     Wire Wound Resistors

     3.     Thin Film Resistors

      4.     Thick Film Resistors

1. 1) Carbon Composition Resistors

A typical fixed resistor is made from the mixture of granulated or powdered carbon or graphite, insulation filler, or a resin binder. The ratio of the insulation material determines the actual resistance of the resistor. The insulating powder (binder) made in the shape of rods and there are two metal caps on the both ends of the rod.

There are two conductor wires on the both ends of the resistor for easy connectivity in the circuit via soldering. A plastic coat covers the rods with different color codes (printed) which denote the resistance value. They are available in 1 ohm to 25 mega ohms and in power rating from ¼ watt to up to 5 Watts.

Generally, they are very cheap and small in size, hence, occupy less space. They are reliable and available in different ohmic and power ratings. Also, fixed resistor can be easily connected to the circuit and withstand for more voltage.

In other hand, they are less stable means their temperature coefficient is very high. Also, they make a slight noise as compared to other types of resistors.

     1.2) Wire wound Resistors

Wire wound resistor is made from the insulating core or rod by wrapping around a resistive wire. The resistance wire is generally Tungsten, manganin, Nichrome or nickel or nickel chromium alloy and the insulating core is made of porcelain, Bakelite, press bond paper or ceramic clay material.

The manganin wire wound resistors are very costly and used with the sensitive test equipments e.g. Wheatstone bridge, etc. They are available in the range of 2 watts up to 100 watt power rating or more. The ohmic value of these types of resistors is 1 ohm up to 200k ohms or more and can be operated safely up to 350°C.

in addition, the power rating of a high power wire wound resistor is 500 Watts and the available resistance value of these resistors are is 0.1 ohm – 100k Ohms.

Advantages and Disadvantage of Wire wound Resistors

Wire wound resistors make lower noise than carbon composition resistors. Their performance is well in overload conditions. They are reliable and flexible and can be used with DC and Audio frequency range. Disadvantage of wire wound resistor is that they are costly and can’t be used in high frequency equipments.

Application/Uses of Wire Wound Resistors

Wire wound resistors used where high sensitivity, accurate measurement and balanced current control is required, e.g. as a shunt with ampere meter. Moreover, Wire wound resistors are generally used in high power rating devices and equipments, Testing and measuring devices, industries, and control equipments.

1. 3) Thin Film Resistors

Basically, all thin film resistors are made of from high grid ceramic rod and a resistive material. A very thin conducting material layer overlaid on insulating rod, plate or tube which is made from high quality ceramic material or glass. There are two further types of thin film resistors.

1. Carbon Film Resistors

2. Metal Film Resistors

1. 3. 1) Carbon Film Resistors

Carbon Film resistors contains on an insulating material rod or core made of high grade ceramic material which is called the substrate. A very thin resistive carbon layer or film overlaid around the rod. These kinds of resistors are widely used in electronic circuits because of negligible noise and wide operating range and the stability as compared to solid carbon resistors.

1. 3. 2) Metal Film Resistors

Metal film resistors are same in construction like Carbon film resistors, but the main difference is that there is metal (or a mixture of the metal oxides, Nickel Chromium or mixture of metals and glass which is called metal glaze which is used as resistive film) instead of carbon. Metal film resistors are very tiny, cheap and reliable in operation. Their temperature coefficient is very low (±2 ppm/°C) and used where stability and low noise level is important.

1.4) Thick Film Resistors

The production method of Thick film resistors is same like thin film resistors, but the difference is that there is a thick film instead of a thin film or layer of resistive material around. That’s why it is called Thick film resistors. There are two additional types of thick film resistors.

1. Metal Oxide Resistors

2. Cermet Film Resistors

3. Fusible Resistors

1.4.1) Metal Oxide Resistors

By oxidizing a thick film of Tin Chloride on a heated glass rod (substrate) is the simple method to make a Metal oxide Resistor. These resistors are available in a wide range of resistance with high temperature stability. In addition, the level of operating noise is very low and can be used at high voltages.

1.4.2) Cermet Oxide Resistors

In the cermet oxide resistors, the internal area contains on ceramic insulation materials. And then a carbon or metal alloy film or layer wrapped around the resistor and then fix it in a ceramic metal (which is known as Cermet). They are made in the square or rectangular shape and leads and pins are under the resistors for easy installation in printed circuit boards. They provide a stable operation in high temperature because their values do not change with change in temperature.

1.4.3) Fusible Resistors

These kinds of resistors are same like a wire wound resistor. When a circuit power rating increased than the specified value, then this resistor is fused, i.e. it breaks or open the circuit. That’s why it is called Fusible resistors. Fusible restores perform double jobs means they limit the current as well as it can be used as a fuse.

They used widely in TV Sets, Amplifiers, and other expensive electronic circuits. Generally, the ohmic value of fusible resistors is less than 10 Ohms.

1. 2) Variable Resistors

As the name indicates, those resistors which values can be changed through a dial, knob, and screw or manually by a proper method. In these types of resistors, there is a sliding arm, which is connected to the shaft and the value of resistance can be changed by rotating the arm. They are used in the radio receiver for volume control and tone control resistance.

Following are the further types of Variable Resistors

1. Potentiometer

2. Rheostats

3. Trimmers

1.2.1) Potentiometers

Potentiometer is a three terminal device which is used for controlling the level of voltage in the circuit. The resistance between two external terminals is constant while the third terminal is connected with moving contact (Wiper) which is variable. The value of resistance can be changed by rotating the wiper which is connected to the control shaft.

1.2.2) Rheostats

Rheostats are a two or three terminal device which is used for the current limiting purpose by hand or manual operation. Rheostats are also known as tapped resistors or variable wire wound resistors.

What is the main Difference between Potentiometer and Rheostats?

Basically, there is no difference between Potentiometer and Rheostat. Both are variable resistors. The main difference is the use and circuit operation, i.e. for which purpose we use that variable resistor?

For example, if we connect a circuit between resistor element terminals (where one terminal is a general end of the resistor element while the other one is sliding contact or wiper) as a variable resistor for controlling the circuit current, then it is Rheostats.

In the other hand, if we do the same as mentioned above for controlling the level of voltage, then this variable resistor would be called a potentiometer.

1.2.3) Trimmers

There is an additional screw with Potentiometer or variable resistors for better efficiency and operation and they are known as Trimmers. The value of resistance can be changed by changing the position of screw to rotate by a small screwdriver.

They are made from carbon composition, carbon film, cermet and wire materials and available in the range of 50 Ohms up to 5 mega ohms. The power rating of Trimmers potentiometers are from 1/3 to ¾ Watts.

Uses / Application of Resistors

Practically, both types of resistors (Fixed and Variable) are generally used for the following purposes.
Resistors are used:

I. For Current control and limiting

II. To change electrical energy in the form of heat energy

III. As a shunt in Ampere meters

IV. As a multiplier in a Voltmeter

V. To control temperature

VI. To control voltage or Drop

VII. For protection purposes, e.g. Fusible Resistors

VIII. In laboratories

IX. In home electrical appliances like heater, iron, immersion rod etc.

X. Widely used in the electronics industries

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