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Proceedings of the Technical Session of Institute of Physics, Sri Lanka 19 (2003) 1-11 Breakdown characteristics of a rod plane gap with dual spaced secondary gap system The behaviour of a complex spark gap containing a set of floating electrodes has been studied. The high-voltage electrode, the earthed electrode and the floating electrodes formed the electrode system. In these experiments, the floating electrodes were located at a distance of a few millimeters from the earthed rod electrode and this was referred to as the secondary gaps. Under the application of an impulse waveform of 1.2 µs/50 µs, the 50% breakdown voltage and the time to breakdown were studied as a function of both gap distances from the complex gap containing the floating electrode. Corrections for atmospheric conditions were done according to IEC standards. The present results of this experiment on floating electrodes have revealed of a minimum 50% breakdown voltage and a maximum time to breakdown corresponds to pints located in the secondary gaps of 0.5 mm and 1.0 mm respectively. This also showed a low standard deviation for both time to breakdown and 50% breakdown voltage. Our results include that a complex floating electrode system can be used to trigger the streamer initiation of a high voltage environment in a controlled manner. Back to content page |
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Proceedings of the Technical Session of Institute of Physics, Sri Lanka 19 (2003) 13-16 Time variation of
resistance of coir fibre with different moisture conditions In this study, the electrical resistance of individual and bundled coir fibre is investigated. A type of coir fibres of export quality named Bristle fibre are used in this study. In the first part of the study the time taken for the resistance of a single fibre to vary from starting value to 100 MW (the limiting value of the emasuring equipment) was taken. Fibres were kept in water for 3 hours and 24 hours respectively. The results show that the resistance of individual coir fibres is significantly reduced when soaked in water. The time period of recovering the 100MW limiting resistance is independent of the time of soaking. In the second part of the experiment, a bundle of fibres of mass 10 g was investigated for the variation of resistance with time. The bundle is soaked in water and shaken well until the dripping of water stops. Then the mass of the wet bundle is measured. At the two ends, the bundle was tied by two circular metal electrodes with a seperation of 5 cm. The resistance between the electrodes was taken at regular intervals. The wetting of the fibre bundle reduces its resistance down to 25% of the dry state value. The resistance increases with time in an approximately linear manner.
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Proceedings of the Technical Session of Institute of Physics, Sri Lanka 19 (2003) 17-25 Construction of
an electronic device for monitoring the voltage standing wave ratio of a
transmission antenna A special electronic system, which prevents the destruction of delicate amplifiers due to sudden impedance mismatches raised in telecommunication antenna systems has been designed and constructed. These impedance mismatches occur due to bad weather and other failures and result in a reflection of high portion of power back to the amplifier of the transmitter. Although there are expensive systems which saefguard transmitters from such reflection of power available in the market, developing of a low cost system is the objective of the project. An electronic device that measures the magnitude of forward and reflected power of the transmitter and converts them to digital signals has been constructed. Signal was then fed to a computer and the computer software has the capability of calculating Voltage Standing Wave Ratio (VSWR). Return Loss, Mismatch Loss and the percentage of the power reflection. When the value of VSWR reaches a critical level, the system creates an alarming signal warning the user. In addition the system will automatically isolate the antenna system from the amplifier to prevent further reflections and which would otherwise destroy the amplifier. The user has provisions to set the cut-off level at a desired and suitable value. The developed measuring circuitry and the software can be used for similar requirements of other systems.
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Proceedings of the Technical Session of Institute of Physics, Sri Lanka 19 (2003) 27-34 Construction and design of
a digital thickness gauge and supporting software Micrometer is a well known device, which uses the pitch of a screw thread to measure small distances. Usually the sensitivity of this type of a device is 0.01 mm. The method was designed that boosts sensitivity of a micrometer up to 0.0025 mm. It can be used to measure 0.0025 mm to 20 mm thickness. Full computer control of the device is possible by clicking the mouse. To achieve this, a stepper motor was used to rotate spindle that has 0.5 mm csrew pitch, instead of manual rotation of the conventional micrometer. The screw can accurately move 1/200 part of complete revolution on incorporation of the stepper motor. Measureing probe of this device simply touches on the sample surface that was needed to measure, without any rotary motion. Computer that counts all steps drove the stepper motor until touched measuring probe and sample under predetermine pressure. Crossing mechanism of an IR beam which incident on detector, was used for precisely determines this touching position. The setup can be connected to computer via printer port without installing hardware. Two kinds of software, compatible with DOS and Windows operating systems have been produced using QuickBasic and VisualBasic. It can be easily upgradable for any other operating systems. This setup can be upgraded to measure thickness variation along the one axis of the sample (x-y profilemetry) or to plot thickness variation on the sample surface (surface profilemetry) by using another stepper motor with moving mechanisms.
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Proceedings of the Technical Session of Institute of Physics, Sri Lanka 19 (2003) 35-44 Beam forming for adaptive
antennas Adaptive antenna arrays at the base station of mobile communication systems can locate and track users and can dynamically adjust the radiation pattern to enhance the quality of reception while increasing the capacity of the system. It is a smart antenna that can detect the direction of arrival of a desired signal and form the antenna beam towards the user and form a null towards the interferer. This paper presents the results of a computer simulation of a neural network based beamforming carried out with one user and one interferer, when the directions of arrival are known. A Generalised Regression Neural Network was for the simulation. The network consists of 2-layers of neurons between the inputs and the output. The inputs were the directions of arrival of a desired signal and an interfering signal. The output is the phase difference required for the feeding circuit of a linear phased array. the network was trained using a data set determined theoretically. The trained network was used to determine the phase angles, which in turn was used to draw the radiation pattern that showed the desired beamforming. Simulation was carried out with a variet of inputs. For a 10-element array the rms error of the major lobe direction for a given direction of arrival of a signal was 3.11o and the rms error of the direction of the null for a given interfering signal was 2.55o. The results are promising.
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Proceedings of the Technical Session of Institute of Physics, Sri Lanka 19 (2003) 45-49 LED matrix display system
for Sinhala characters and animated graphics LED matrix displays are becoming more and more popular as character displays. Attractiveness of the display and low power consumption are two main reasons for this popularity. But commercially available LED matrix displays cost from morethan Rs. 15,000 and a display with special animation capabilities cost much more. Left to right, right to left scrolling character type displays are commo but graphically animated LED display boards are not that much being used due totheir high cost. Further, Sinhala displayable LED matrix displays are currently not available in the market and this work has been carried out as a solution to these problems. The device discussed in this paper, which is capable of any kind of graphic, costs around Rs. 5000.
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Proceedings of the Technical Session of Institute of Physics, Sri Lanka 19 (2003) 51-57 Surface trap mediated
recombinations in dye-sensitized solid-state solar cell with CuI as
hole collector Surface states at dye and semiconductor interface induce recombinations in dye-sensitized solid-state solar cells that greatly reduce the efficiency. CuI is a one of the p-type semiconductor that has been identified suitable for fabricating dye-sensitized solid-state solar cells giving reasonably high photocurrents and voltages. In this work we have observed that the iodine adsorbed at the surface of CuI film act as hole traps affecting the performance of the cell. Fluorescencespectroscopic measurements revealed that presence of iodine at the surface of the CuI films create surface trap 0.2 eV above the valence band edge. Exposure of CuI films to radiations with energies higher than the band gap excitation energy released iodine in the bulk and capping the CuI crystallites with thin layer of triethylamine thiocyanate seems to prevent the coming of this liberated iodine to the surface.
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Proceedings of the Technical Session of Institute of Physics, Sri Lanka 19 (2003) 59-64 Ti/CU2O photoelectrodes in
photoelectrolytic solar cell Cuprous Oxide thin films deposited on Ti substrates were investigated as photoelectrodes in a photoelectrochemical cell. Electrodeposition was carried out in an electrochemical cell containing aqueous solutions of cupric acetate and sodium acetate.X-ray diffraction (XRD) and scanning electronmicrographs (SEM) confirmed that the films are polycrystaline Cu2O films. X-ray photoelectron spectroscopy (XPS) revealed that the films are pure CU2O and there are noanyother phases. The photoresponse of the films in a PEC produced a zero bias photocurrent (XBPC) with an n-type photoresponse. Comparing with the thermally grown Cu2O films, an enhanced spectral response in the long wavelength region could be obtained with electrodeposited Cu2O on Ti substrate. A charge separation mechanism at the Ti/Cu2O interface isproposed as the possible reason for the observed spectral response enhancement.
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Proceedings of the Technical Session of Institute of Physics, Sri Lanka 19 (2003) 65-72 Construction and
commissioning of a large area solt gradient solar pond at Palatupana
salterns in southern Sri Lanka,, Solar ponds are large-scale solar energy collectors with integrated thermal energy storage. The concept of collecting and storing solar energy using non-convective salt gradient solar ponds has been known for some time. We report here the details of construction and commissioning of a large-area solar pond built at Palatupana salterns in Southern Sri Lanka. A rectangular pond with dimensions 70 m x 35 m could retain a maximum water capacity of ~ 5000 m3. The pond can be filled to a maximum height of 2 m. The initial controlled filling of the pond to a total depth of 60 cm with a 30 cm layer of high-density brine led to formation of the three-layer stability with a maximum bottom temperature of 52 oC. Destruction of the pond stability was observed with the increase of wind speed over the pond. About three months later with the monsoon rain, the pond bottom recorded a maximum temperature of 69 oC with the reestablishment of three-layer stability at a total depth of 110 cm. The observed salinity and temperature profiles show the usual three-layer configuration, but with relatively thick upper convective zone. This situation could arise due to wind mixing of layers to a depth of about 30 cm. It is remarkable to observe that the uncontrolled natural conditions raised the bottom temperature of the pond to a maximum of 69 oC. Our results indicate wind is the major cause for mixing of layers which prevents storing of solar energy at the bottom layers of the pond. The commissioned solar pond could be used for wind control mechanism studies and controlled filling mechanisms to optimize different layers of the pond to obtain maximum bottom temperatures. Back to content page |
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Proceedings of the Technical Session of Institute of Physics, Sri Lanka 19 (2003) 73-81 Random weights in neural
network models for chaos in spontaneously bursting Hippocampal slices A Neural Network model was used to simulate stochastic population bursting by introducing random weights at different stages of the simulation. It was found that maps of interspike intervals (ISI) exhibit recurrent unstable-periodic-orbit (UPO) like trajectories similar to those found in experiments on hippocampal slices. The network behavior is found to be very sensitive to the network parameters and hence choice of parameter values for simulating the model is very narrow. To simulate the effects of high Potassium medium, the network is "chemically kindled" through a Hebbian learning mechanics switched on throughout the simulation under random inhibition. Generation of these new excitatory synapses creates a new high-activity (epileptic) attractor of the network dynamics. It was observed the kindled network makes occasional, spontaneous, short-lived excursions to the high activity attractor, similar to the spontaneous population bursting observed in slice experiment. Periodic Pacing (PP) control method was used to control chaos from the network. Back to content page |
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Proceedings of the Technical Session of Institute of Physics, Sri Lanka 19 (2003) 83 Interactive simulations
for physics teaching Physics is believed to be a difficult subject for the students all over the world. In many countries, students are compelled to avoid taking Physics as a subject unless it is a compulsory for their course path. It has been shown that traditional teaching methods such as ?chalk & talk? and ?lecture plus laboratory experiments? do little to improve the conceptual understanding and more student-centered methods need to be utilized for effective teaching. With the development of computer technology, computers play vital role in Physics education and today there are many software packages, having potential to provide a better understanding of the subject. Further computer technology could be used to provide more information attractively with the aid of pictures, applets, video clips and many simulation programs etc. The Sri Lankan students following Physics as a subject are not getting exposed to such environment due to the lack of computer facilities and the cost of Physics education software. However, opportunities to Sri Lankan students to use computers are growing fast. Therefore computer technology could be effectively used for education in the future. The objective of this project is to develop Physics education simulations for tertiary level Physics education. Macromedia DirectorÒ technology has been used to develop the simulation and they have been embedded in the web pages with the view of uploading into the Internet. The CD version can be prepared for the users who do not have access to the Internet. In this presentation, simulations developed for the Motion under gravity, Electrostatics and Photoelectric effect are presented.
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