For this reason the LDR cannot be used where rapid fluctuations of light are to be recorded or used to actuate control equipment, but this same property is exploited in some other devices, such as audio compressors, where the function of the light dependent resistor is to smooth the response. design constraints. The medium light resistance was 4.5kΩ while the data supplied stated 48kΩ to 140kΩ. light must shine into the LDR for the circuit to be activated. If a constant voltage is applied to it and the intensity of light is increased the current starts increasing. Typical MDR resistor consists of four perm-alloy films connected like a Wheatstone bridge. If the light level is 600 Lux, the resistance is about 10Ω which is close to zero. This may result in a short circuit and may lead to a fire hazard. LDRs are also called Photoresistors. Sometime the resistor symbol is circled, with the arrows outside the circle. The following symbol is used to depict light dependent or photo resistors according to the IEC standard. These resistors are constructed from ceramics or polymers like nickel, cobalt, manganese, platinum, iron, titanium, etc. Therefore, the performance indicators of LDR are properties like sensitivity to intensity of light, wavelength dependency and latency. voltage drops. These are also called Silistors. CLICK HERE FOR ELECTRONICS INDEX There are many special types of resistors which work as transducers too and so, they can be used as sensors in the circuits. Magnetic Dependent Resistor or Magneto Resistor is a variable resistor of which resistance varies relative to the strength of an applied magnetic field. These resistors are used in the detection of the magnetic field, measurement of magnetic strength and polarity, electronic compass and position sensors. the LDR. The thermistors have the following IEC standard symbol: NTC thermistors 4) Magnetic Dependent Resistors The applications like current limiting, surge protection, time delay, and flow measurement are based on heat capacity and dissipation constant of thermal resistors. Gauge factor is defined as a ratio of relative change in resistance of strain gauge to the mechanical strain. PTC Thermistors come in two types – Linear PTC Thermistors and Switching Type PTC Thermistors. A major drawback of these varistors is their standby current which leads to large power dissipation. The carriers are electrons. Learn how your comment data is processed. The sensitivity of a photo resistor varies with the light wavelength. These resistors are made up of a metallic foil enclosed in an insulating case where deformation in foil varies the resistance. The output impedance is actually the parallel resistance of the series and shunt LDRs. Operating temperature range _____ -60 C to +75 C Electrical characteristics T A = 25 C. 2854 K tungsten light source 1. The LDRs are generally designed to have a sensitivity to light wavelengths visible to humans. LDR gamma characteristic is a measure of the rate of change of resistance over range of light variation. LDR versus photodiodes and phototransistors The latency of LDR may be indicated by Dark Resistance at different time instants in a dark situation. These are made up of a sintered matrix of silicon carbide. Dark to 110% R L 2. The time required by LDR in dropping or rising its resistance relative to incident light is called latency. The semiconductors commonly used to make photoresistors include cadmium sulfide, cadmium selenide, lead sulfide, lead selenide, and indium antimonide. transistor. The figure shown here represents the spectral response of photoconductive detectors made of different materials, with the operating temperature expressed in K and written in the parentheses. PTC Thermistors are specially designed thermistors that have a positive temperature coefficient. Extrinsic photo resistors are made of materials doped with impurities, also called dopants. Characteristics of LDR: Max power dissipation. To gain access to revision questions, please sign up and log in. ?. It conducts very small leakage current until the voltage reaches clamping level. The variation in resistance of strain gauge is indicated by gauge factor. This is a typical examination question. • The resistance range and sensitivity of a photoresistor … In a 12 Volt circuit, if the maximum transistors. google_color_url = "008000"; 2) Silicon Carbide Varistors – Theses varistors are suitable for high power and high voltage applications. The delay in the response time in these applications is on the order of 0.1s. So, a varistor of high clamping voltage must be used in any application and a thermal fuse must be connected in series with it. The NTC Thermistors have the following important properties: Operating Temperature Range – Most of the NTC Thermistors (which come available in bead, disc or chip packages) have an operating temperature range between -55°C and 200°C.

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