There are many different types of resistors and they each vary in their composition and other characteristics to maximize their specific functionality. Some resistor types and their optimal use are briefly discussed below.
1. Linear Resistors
A linear resistor maintains its resistance value regardless of the variation of the current flowing through or the voltage applied across. Thus the current is always proportional to the voltage and V/I remains constant, making the characteristic curve a line. Linear resistors can be further classified as fixed or variable.
1.1 Fixed Resistors
Fixed resistors are used to set the desired conditions in a circuit, and are what are most generally thought of when hearing the term “resistors”. Their values are chosen or calculated during the design phase of the circuit, and they should never need to be changed to modify the circuit. Some fixed resistors below are classified according to characteristics and/or composition.
1.1a Wire Wound Resistors
Wirewound resistors are constructed using a conductive wire wound around a non-conductive core. The length, thickness and alloys of the conductive wire can be varied to control the resistance value. Wire wound resistors can range from small precision surface mount components to large tubular power resistors. These resistors are typically used in electronic instruments where high accuracy and more power dissipation are required. They are characterized by high stability at higher temperatures, wide resistance range and high accuracy.
1.1b Carbon Composition Resistors
Carbon composition resistors are made from fine carbon particles (graphite, ceramic dust and resin) mixed with a binder and baked into a solid form. Varying the length of the carbon composition body determines the resistance value. The thermal mass of the entire carbon composition body which is responsible for conducting energy gives these resistors the ability to withstand high energy pulses resulting in a higher energy capability. Some applications of carbon composition resistors include circuit protection, current limiting, high voltage power supplies, high power or strobe lighting, and welding.
1.1c Film Resistors
Film resistors are generally manufactured by depositing resistive pure metals or oxide film onto an insulating ceramic base. Thin film uses vacuum deposition while thick film fires a special paste (a mixture of glass and metal oxides) to deposit the metallic film onto the insulating substrate. Thin film is high precision, has better temperature coefficient and is more stable. Thin film resistors are therefore used in measuring or monitoring equipment and medical or audio applications. Thick film resistors are used in almost any electrical device with battery or AC connection where high requirements are not critical and prices are lower.
1.2 Variable Resistors
Variable resistors consist of two connections of a fixed resistor element and a slider which taps onto the main resistor element, giving three connections to the component. 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 are widely used for all forms of control, from volume controls on radios and sliders in audio mixers to a host of areas where a variable resistance is required.
When a variable resistor is used as a potential divider by using 3 terminals, it is called a potentiometer. Strictly a potentiometer is a component where there is a fixed resistor which has a slider to provide a potential division from the voltage at the top. Potentiometers are most commonly used as voltage dividers to obtain a manually adjustable output voltage at the slider from a fixed input voltage across the two ends.
A rheostat is used to control the current flowing in a circuit. It uses only two connections: one connection to the resistive element and the other connection to the wiper. Rheostats were used (as dimmers) to control light intensity, control the speed of motors, control heaters and ovens; generally as power control devices. However, because of their relatively low efficiency, in power control applications they are now replaced by switching electronics. Nowadays, preset variable resistors (trimmers/trimpots), wired as rheostats are used in circuit tuning, fabrication or calibration.
2. Nonlinear Resistors
While the majority of resistors are standard fixed resistors or variable resistors, there are other resistor types that are used in some more specialized applications. For non-linear resistors the voltage and current characteristics vary non-linearly. The voltage and current values vary depending upon other factors like temperature and light.
The resistance of thermistors vary with temperature and they are used for measurement, control and device protection. NTC (Negative Temperature Coefficient) thermistors protect devices from overvoltage conditions by decreasing the resistance when the temperature increases while PTC (Positive Temperature Coefficient) thermistors protect devices from overcurrent conditions by increasing the resistance when the temperature increases.
Varistors are voltage dependent resistors with high electrical resistance (at low voltages) which decreases as the voltage increases. They eliminate high voltage transients and are often used as surge suppressors to protect sensitive circuits from destructive voltage spikes. They can also be used as elements for compensation or control in circuits to provide optimal operating conditions.
2.3 Photoresistors or LDR (Light Dependent Resistors)
Photoresistors react to light and exhibit light conductivity by decreasing their resistance with increasing incident light intensity. They are used for light-sensing and/or light intensity measurement and in some cases in audio compressors to create changes in signal gain. Photoresistor light sensitivity has certain wavelength ranges and varies with different light wavelengths.
2.4 Magneto Resistors
Magneto resistors have a variable resistance which is directly proportional to the external magnetic field. They make use of magnetoresistance, enabling them to measure magnetic field presence, strength, position (angular, rotary or linear) and direction. They are often used in magnetic field sensing devices such as electromagnetic compasses and magnetometers.
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