Resistors
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September 1, 2022
A resistor is a passive electrical component with two terminals that implements electrical resistance as a circuit element. Resistors have a variety of purposes in electronic circuits, including lowering the flow of current, adjusting the level of a signal, dividing voltages, and terminating transmission lines.
High-power resistors that are able to convert many watts of electrical power into heat can be utilized in a variety of control systems, including those for motors and power distribution, as well as in amplifiers for the purpose of load testing.
Types of Resistors
Fixed Value Resistor
The ohmic resistance of a fixed value resistor is predetermined, and the resistor's value cannot be adjusted. The fixed resistor is the type of resistor that is used the most frequently and is also one of the electrical components that is used the most frequently overall. Depending on their intended use, fixed resistors can be purchased in axial leaded or surface mount packages, in addition to other, more specialized packaging options. In the past, the most common type of resistor was an axial leaded resistor; however, due to the many benefits of surface mount devices, SMD resistors are now the most widely used type.
Variable Resistors
A resistor known as a variable resistor is a resistor in which the value of the electric resistance may be changed. In its most basic form, a variable resistor is an electromechanical transducer. Its operation typically involves sliding a contact, also known as a wiper, over a resistive element. A variable resistor can be converted into a potentiometer by connecting it to three separate terminals in order to perform the function of a potential divider. It is known as a rheostat and operates as a variable resistance when only two terminals are utilized in its construction. There are variable resistors that can be controlled electronically as opposed to being controlled by mechanical action, and these resistors do exist. Digital potentiometers are just another name for these resistors.
Applications of Resistors
Potential Dividers - When connected in series, two or more resistors will produce a voltage at the point where they meet that is proportional to the ratio of their individual values. This functionality is utilized in circuits on a regular basis for the generation of intermediate voltages.
Biasing Resistors - In order for transistors and a wide variety of other devices to function properly, their AC and DC operating characteristics as well as their gain levels must be calibrated. This is accomplished by using a number of resistors and is referred to as biasing.
Op-amp Gain and Feedback - The gain and feedback functionality of the majority of op-amp circuits must be set by resistors that are located external to the amplifier chip. Resistors are the primary way by which this can be accomplished.
Current Limiting - The amount of current that passes through an element in a circuit can be restricted through the use of resistors. This is an important safety function that is present in many different circuits, for example, restricting the amount of current that can flow into an LED in order to control its level of brightness.
Impedance Matching - It is necessary for the impedance of a circuit's receive end and its transmit end to be identical in order to achieve maximum power transmission at high frequencies. At least a portion of this requirement can be satisfied using resistors.
Current Measuring - Many circuits need to know how much current is flowing, however, it is much easier to measure voltage, so inserting a resistor into the circuit to ‘develop’ a voltage – remember Ohm's law- is a common technique for measuring current
Data and Address Bus Pullups - This functionality assists in alleviating noise problems that occur on high-speed computer busses. When a data bus tri-states or is driven high, it is often essential to pull it into a known state or to make sure its output high is much above the switching point of other logic elements on the bus. A pull up helps to do this, and it is one of the ways in which a pull up can be used.
Materials Used for Resistors
Carbon Film Resistors - Because of their lower tolerance and higher noise characteristics, carbon film resistors have mainly been supplanted by metal varieties for use in applications that are intended for general use. Despite this, they are still put to use in a variety of specialized contexts.
A carbon layer is formed on top of a ceramic substrate during the manufacturing process of the resistor. This has a variety of positive implications. In the first place, the film is not inductive for the most part; in the second place, the ceramic is a great insulator for both heat and electricity; and in the third place, the pattern has a large cross-section. Therefore, because of their low inductance, they are ideal for high-frequency applications. Additionally, because of their bigger cross-section, they are able to withstand greater working voltages and better survive transients than many other varieties. As a consequence of this, even though they are utilized less frequently, they are nonetheless easily accessible.
Metal Film Resistors - On top of an insulating substrate, a layer of metals like ruthenium is deposited to create a metal film resistor. In comparison to carbon film resistors, their structure is easier to mass produce and results in reduced physical dimensions. Additionally, they are offered in a wide variety of types and packaging options.
Because of their lower noise, tighter tolerances, and typically better temperature coefficients, metal film resistors have largely supplanted carbon film resistors in most standard applications. This is owing to the fact that metal film resistors are used in most standard applications. Although they are frequently thought of as a "standard" resistor, they have progressed from a lower performance base to the point where they are now offered with quite high precision (0.1%) and low TCRs in the single-digit PPM/C range. This is a result of their evolution from a lower performance base. It is important to keep in mind that through hole and surface mount (chip resistor) varieties of metal film resistors are both available for purchase.
Wirewound Resistors - In specialized applications, such as high precision instrumentation, where tolerances considerably better than 0.01% and very low-temperature coefficients (TCR) are required, wirewound resistors offer some qualities that make them appealing.
They are also frequently the resistor type of choice for applications that require high power (examples rated up to 100s of watts are available). It is important to note, however, that due to the fact that these resistors are made up of wire that is wound around an insulated core, which, by definition, makes them an inductor, it is not recommended to use these resistors for high-frequency applications, and they are typically not available in surface mount form.
Metal Oxide Resistors - These things are very much like metal film resistors, with the exception that the element that provides resistance is an oxide (often tin). Their performance is subtly distinct from that of metal film in the sense that, in general, they are superior to metal film components for use in applications requiring higher voltages and greater powers.
To compensate for this, however, they are often provided in narrower ranges and may have lower tolerances and TCRs. To reiterate, you can choose from surface mount or through-hole varieties of these components.
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