Carbon film resistor is a fixed resistor, in which, a carbon film is deposited over an insulating substrate. Helix is cut on to this film to create long narrow resistive path. Value of resistance can be varied by changing pitch of the helix. Smaller the width means larger the resistance, since the decrease in pitch width will increase the length of the film.
Carbon composition resistor consists of, mixture of carbon granules and powdered ceramic. It is relatively smaller in size and it can withstand high energy pulses. It contain mixture of different components each may have different temperature properties. So temperature coefficient is poor in carbon composition resistors. In modern application, where high precision resistors are required, carbon composition resistors are not used.
Construction of metal film resistor is similar to that of carbon film resistor. In this, carbon film is replaced by metal film. In circuits that require good tolerance, better stability, low noise property and low temperature coefficient, metal film resistors are most preferred.
Wire-wound resistor is generally constructed by winding a metal wire usually nichrome around a ceramic, plastic, or fiberglass core. The structure is protected with a layer of paint, molded plastic, or an enamel coating. Resistance value depends on the type of material, thickness and length of the wire. Usually wire-wound resistors have very high power ratings, 50 watts or more, hence usually used along with heat sink. Coil shape of this type of resistor causes an inductor effect, so wire-wound resistors are not preferred for high frequency applications. Inductance effect can be minimized by winding the wire in sections alternately in reversed direction.
Wire Wound Resistor
Surface mount resistors uses surface mount technology, in which, devices are directly placed on PCB. SMD resistors are very small in size, so have very high component density. They are constructed by depositing resistive film on a ceramic chip and cutting edges using laser for precise value. In applications, where power dissipation and size of the device is important, SMD resistors are most preferred.
Network resistors are the combination of resistance with one common terminal and identical values at all other pins. Network resistors are used in applications such as pull up/pull down, DAC etc. Resistors are available in single in-line package, dual in-line package, surface mount and through-hole types.
Potentiometer is a three terminal variable resistor, with one sliding contact and two fixed contacts. If all three terminals in a potentiometer are used, it will work like a voltage divider. Or if two fixed contacts are only used in the circuit, then, the device behaves like a fixed resistor. For variable resistor, one sliding contact and any other fixed contact is used. Variable resistors are used for applications like volume control.
Rheostat is similar to trimmer but it is not as a potential divider. Only 2 terminals are used in rheostat, instead, of the 3 terminals like in potentiometers. Connection is made at one end of the resistive element, the other at the wiper of the variable resistor. By moving the wiper we can change the resistance value of the device.
Decade resistance box consist of group of resistors of different range of values. Switches are used to select a particular resistor from each range to get the required value. Resistors from each group are connected in series so final value is added sum of selected resistor. They are usually used in precise testing applications.
In rotary variable resistor, moving arm will have a rotary motion to slide around a circular track.
Thermistors are temperature sensitive resistors. They exhibit a large change in resistance proportional to a small change in temperature. Therefore, Thermistors are used as temperature sensors, current limiters etc. Thermistors can be classified into two types PTC (positive temperature coefficient) and NTC (negative temperature coefficient). In PTC, resistance of the device increases with the temperature and in NTC, resistance decreases with increase in temperature.
Mostly thermistors are constructed of semiconductor material which has a negative temperature coefficient. In normal cold conditions, resistance of the device is high due to limited number of free electrons. Electrons are tightly bonded with positive ions in their respective atoms, therefore they cannot move freely. As temperature increases, these bonds will break and more free electrons are created. This increases conductivity, therefore current flow through the device is proportional to the temperature. Thermistors have very high speed response to any changes in temperature. So they are preferred over other temperature sensors.
Figure shows the simple circuit for temperature sensor using thermistor. Resistor R1 pulls thermistor voltage to reference voltage Vref. Current from the voltage source flows through resistor R1 and the series connected thermistor. Voltage proportional to the temperature is developed across the thermistor, which reaches the positive terminal of the operational amplifier. Op-Amp compares this voltage with the previous value and produces an amplified output in proportion to the change in temperature. An ADC followed by any user interface can be used to monitor this temperature value.
An LDR or Photoresistor is an electronic component that changes its resistance when light falls upon it. They are used in applications to detect the existence of light, or to calculate the intensity of the light. In the absence of light, LDRs have very high resistance about 100kΩ, but in the presence of light, the resistance of LDR reduces drastically to tens of kilo-ohms or even less. Resistance value varies depending on the intensity of the light.
Photoresistor is made of a high resistance semiconductor that can absorb photons. If these photons have energy greater than the band gap of the semiconductor, electrons in the valence band will get enough energy to jump into the conduction band. The resistance of photo resistor is reduced as the resulting free electrons conduct electricity. The number of free electrons thus formed, is dependent on the frequency and intensity of the photon incident on it.
Cadmium sulphide is the most common semiconductor used for making photo resistors. Photo resistors are in two types, that is, intrinsic and extrinsic based on the materials used.
Intrinsic semiconductor is the purest form of semiconductor. As the Photons fall on intrinsic photo resistors, the electrons are excited from the valence band to the conduction band. As a result, large numbers of free electrons evolve, which carries current, and therefore resistance of the device is reduced. In extrinsic devices, the light sensitivity is relatively low.
In extrinsic semiconductors, impurity atoms are intentionally added so as to increase its conductivity. A new energy band is created above the valence band by the dopants. The electrons in this new band need relatively less energy to excite into the conduction band. Therefore, photoresistors made of extrinsic semiconductors are more sensitive.
Figure shows a basic version of light sensor circuit using LDR. When LDR is subjected to light, resistance of the LDR is decreased and therefore more current reaches the base of the transistor. Transistor drives the output circuit, to which any device can be connected. This simple circuit can be used to detect the presence or absence of light or to measure its intensity.
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