In this tutorial, we will discuss resistors and their types. These are passive electronic and electrical components that present electrical resistance in a circuit as an element. It consists of two terminals that reduce current flow, divide voltages, and adjust signal levels. We also use them in high-power circuits to reduce power in the form of heat dissipation and as a load to test power generators. The working principle of resistors is Ohm’s law. Ohm’s law states that voltage is directly proportional to circuit resistance and the circuit current flowing in a circuit. The main working principle of every type is the same, but the only difference is in their current, voltage, power, and building material. Its SI unit is ohm. It is available on the market in the microohms to megaohms range.
Series and Parallel Combination of Resistors
In a series combination, the resistance increases or adds up when two or more resistors with the same values connect in series, but the passing current remains the same across all of them while the voltage divides across each one. Similarly, when we connect two or more identical resistors in parallel, the overall resistance decreases. However, the voltage across each resistor remains the same, and they divide the current among themselves.
The resistor was first invented by Georg Simon Ohm in 1827 at the University of Erlangen. He was a mathematician who published numerous research papers on the conduction of heat in molded circuits, and he also presented Ohm’s law. Since then, various types have been developed based on their manufacturing materials. Currently, we use a wide range of resistances in electronic and electrical systems. We will discuss them in more detail in the following paragraphs. We can see its shape, along with its electrical symbol, in the figure below.
Figure 1 Resistor with its Electrical Symbol
Types of Resistors
Different types of resistors are available on the market for use in electronic and electrical systems. These resistors have different properties based on their construction and manufacturing system, which makes them suitable for different applications. Typically, we can categorize all these resistors into two types: linear and non-linear resistors.
Linear resistors are those whose values change with temperature and applied voltages. In other words, in these types of resistors, current is directly proportional to applied voltages. Normally, they have two types of linear properties: fixed and variable.
Fixed resistors are those whose values are fixed, meaning they cannot change. These further divide into four types, which are as follows:
Wire Wound Resistors
A wire-wound resistor is one that has an insulating rod or core around which the wire is wrapped. Resistor wire is generally made up of magnetism, tungsten, or nickel material, and the core is made up of press-bound paper, ceramic clay material, beaklike material, etc. If we talk about the cost of wire, then magnetism wire wound resistors are very costly, and these are mainly useful in sensitive test equipment such as wheat stone bridges, etc. Their power rating, which is easily available on the market, is from 2 watts to up to 100 watts. Their ohmic resistance is 1 ohm to 200 kOhm, and these can operate safely at 350 °C. For a simple wire-wound type, refer to the figure below.
Figure 2 Simple Wire Wound Resistor
Carbon Composition Resistors
These types have a mixture of graphite, powdered carbon, resin binder, or insulation filler. The ratio of insulating material in these resistors determines their values. These have rod shapes and two metal caps that connect at both ends of this rod, which is why their soldering is easy on the PCB. They are very reliable, cheap, and small in size; therefore, they occupy less space. They are available on the market in different ohms and power ratings. We can see a simple carbon-composition type in the figure below.
Figure 3 Simple Carbon Composition Resistor
Thin Film Resistor
Typically, thin-film resistors have a resistive material of high-grid ceramic rod. On this insulating rod, which is normally available on the market in the shape of a glass or tube, a thin layer of conductive material overlays it to form a thin-film resistor. This further divides into two types: the first is a carbon film, and the second is a metal film.
Carbon Film Resistor
The type consists of a high-grid ceramic material rod that is the substrate, and on this substrate, a very thin resistive layer of carbon overlays to form a carbon film resistor. Because they have negligible resistance, high stability, and a wide operating range in comparison to other types, they are mainly useful in electronic circuits.
Metal Film Resistor
The construction of a metal film resistor is almost the same as that of a carbon film, but the only difference between them is that its core has a metal material instead of carbon. They are very cheap, tiny, and reliable in operation. Its temperature coefficient is very low—almost + 2 pp/mC0. Their uses include places where low noise and stability are not so important. For a metal film type, refer to the figure below.
Figure 5 Metal Film Resistor
Thick Film Resistor
The manufacturing method of a thick film type is the same as that of a metal film or a thin film resistor, but the difference is only that it has a thick layer of resistive material instead of a thin layer. Therefore, we call them thick film resistors.
Variable resistors are those whose values change by moving the knob. These further divide into three types.
It is the type of resistor whose resistance value changes by rotating the wiper that connects to the control shaft. It consists of three terminals; the resistance between the external terminals is fixed. Whereas the resistance between one external and one central terminal is variable. Their use includes controlling the voltages of electronic circuits.
A rheostat is a device that consists of three terminals and is useful for limiting the current manually or by hand. We also refer to them as wire-wound variable resistors or tap-changing resistors. These are available on the market in different structures, but their purposes are the same.
A trimmer is a type of potentiometer that consists of an addition screw whose position changes with the help of a screwdriver. By changing the position of this screw, the value of resistance changes.
In circuits, resistors dissipate power in the form of heat. We can calculate the power dissipation using Ohm’s law and the formula P = I^2 * R, where P represents the power dissipated, I represents the current flowing through them, and R represents the resistance value. It is crucial to understand power dissipation to ensure that these operate within their power ratings and do not overheat.
The resistance of a resistor can change with temperature, and this change is quantified by its temperature coefficient. The percentage change defines the temperature coefficient due to resistance per degree Celsius of temperature change. Different types have different temperature coefficients, which can be either positive or negative. Understanding the temperature coefficient is crucial in applications where we need to maintain a consistent resistance value over a wide range of temperatures.
Networks and Arrays
The networks combine multiple resistors into a single package, providing a convenient way to connect them together. Thus, they include applications where the minimization of space and component count is necessary. Sometimes, we use various configurations, such as series, parallel, or a combination of both, to achieve specific resistance values. Engineers widely use these networks in electronic circuits and systems that require precision and compactness.
In conclusion, this tutorial provides an in-depth overview of resistors and their types. It covers their combinations and categories on the basis of materials used during production. We also discuss subcategories of some of the types for a better understanding of their types. Hopefully, this was helpful in expanding your knowledge.
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