Types of Resistors

Resistance is the property of a substance because of which it can oppose the flow of electric current. Some examples of resistive materials are glass, mica, wood, rubber, etc. 

It is quite likely that the first electronic component you would have come access was a resistor, a bead-like object in different colors. Resistors are usually the most common and the most simple instrument used in manufacturing electronic equipment. The main purpose of using a resistor is to maintain a particular voltage and current value in an electronic circuit. Resistors are based on Ohm’s law, which states that the current that flows through a material is directly proportional to the voltage applied across the material. The constant of proportionality is the material’s resistance at a constant temperature. It is expressed as V = I * R., where V is the potential difference in volts between the two ends of the conductor, I is the current flowing through it, and R is the resistance. Resistance is measured in Ohms (Ω). 

This article will explain the fundamentals of resistors and look into the different types of resistors used in electronic devices.

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How do Resistors Work?

We have learned about conductors in science and know that they allow electricity to pass through them. On the contrary, insulators are exact opposites of a conductor, and they do not allow electricity to pass easily through them. These properties result from the resistance of the material. Conductors have a low resistance to electricity flow, whereas insulators resist electricity to a high degree.

The resistance of a resistor depends on the length of the material (l), their resistivity (ρ), and the cross-sectional area (a) of the resistor. As per the law of resistance R = ρ * (l/a).

Let us look into the insides of a wire by zooming in.

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When electrons pass through this wire, some of them swiftly pass right through the gaps in the atoms while others collide with an atom and bounce back. And other times, the electrons hit each other too. Due to this, the flow of electrons is impeded or non-uniform, which offers resistance to it. So we can see here that resistance is the property of the material itself and the interaction of the electron with the material depends on its length, area, and the packing of atoms within it.

Temperature is another factor that affects the resistance of a material. When temperature increases, the resistance of the material increases too. When heat energy is supplied to the wire, its atoms absorb this energy and start vibrating at a higher speed. This makes it more difficult for the electrons to pass through the wire.

Types of Resistors

The classification of resistors is based on their application. There are two broad categories of resistors into which all the other types of resistors can be fitted.

• Fixed Resistors - These are the most widely used resistors. The values of these resistors are determined and fixed during the design phase. These types of resistors are used to set the right conditions within an electronic circuit, and the values of these resistors never need to be changed. The resistance of such resistors does not change if there is a change in voltage or temperature. We will look into different types of fixed resistors in the coming sections.

• Variable Resistors - The resistance of a variable resistor can be changed based on the voltage applied. These resistors are also called VDR (voltage-dependent resistors). A variable resistor has a fixed resistor element along with a slider that taps into the main resistor. There are three connections to this resistor: two are connected to the fixed element, while the third one is the slider. So by using all three connections, the resistor can be made into a variable potential divider. Variable resistors are similar to potentiometers. We will see different types of variable resistors in the following sections.

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Fixed Resistor Types

Many different types of fixed resistors are used; some of the common ones are:

• Carbon Composition - This type of resistor was very common earlier. Still, it is hardly used now as newer forms of resistors came into the market, which is better performance-wise, smaller in size, and cheaper. Carbon composition resistors are composed of carbon granules made into a rod with the use of binders. It is a large resistor with a huge negative temperature coefficient. They generate a high level of noise when current flows through them due to the binder and the granular nature of carbon. These resistors also have erratic and irreversible changes in their resistance, either due to heat or as they get older.

• Carbon Film - These resistors came into existence during the early days of transistor technology when power levels used to be quite low. When a hydrocarbon is cracked onto a ceramic former, we get a carbon film resistor. The resistance of the film which is deposited is set by cutting a helix into it. The resistor has a ceramic tube or a conformal epoxy to protect the carbon tube. These are highly inductive resistors with a temperature coefficient between -100 and -900 parts per million per degree Celsius. 

• Metal Oxide Film Resistor - It is one of the most commonly used resistors, and it has a metal oxide film on a ceramic rod. The resistance of this type of resistor can be adjusted by cutting a helical grove onto the film. The temperature coefficient of metal oxide resistors is between -15 and +15 parts per million per degree Celsius. 

• Wire Wound Resistor - These types of resistors are used in high power applications and are made by winding a high resistance wire on a former. A more expensive variety of this resistor is made by winding wire on a ceramic former and covered by silicone or vitreous enamel. They have a relatively low-temperature coefficient and a high level of reliability.

Variable Resistors Types

Variable resistors can be classified in various ways; the most common classification is based on whether they are adjustable or preset. Few types of variable resistors based on this classification are:

Wire Wound - These are most expensive to produce and are manufactured by winding a resistance wire coil on a former semi-circular shape. The surface of the wire should not be insulated for the slider to come in contact with the fixed resistance element. 

Cermet - These are made of composite materials like metal and ceramic. These resistors are used where high temperatures are expected.

Carbon Composition - These use a combination of carbon and filler materials.