João Miranda

A Single-Stage High-Power-Factor Electronic Ballast Based on Integrated Buck Flyback Converter to Supply Metal Halide Lamps

In this paper, a novel single-stage electronic ballast with a high power factor is presented. The ballast circuit is based on the integration of a buck converter to provide the power factor correction, and a flyback converter to control the lamp power and to supply the lamp with a low-frequency square-waveform current. Both converters work in discontinuous conduction mode, which simplifies the control. In spite of being an integrated topology, the circuit does not present additional stress of voltage or current in the main switch, which handles only the flyback or buck current, depending on the operation mode. To supply the lamp with a low-frequency square-wave current to avoid acoustic resonances, the flyback has two secondary windings that operate complementarily at a low frequency. The design procedure of the converters is also detailed. Experimental results from a 35-W metal halide lamp are presented, where the proposed ballast reached a power factor of 0.95, a total harmonic distortion of 30% (complying with IEC 61000-3-2), and an efficiency of 90%.

A survey on web archiving initiatives

Web archiving has been gaining interest and recognized importance for modern societies around the world. However, for web archivists it is frequently difficult to demonstrate this fact, for instance, to funders. This study provides an updated and global overview of web archiving. The obtained results showed that the number of web archiving initiatives significantly grew after 2003 and they are concentrated on developed countries. We statistically analyzed metrics, such as, the volume of archived data, archive file formats or number of people engaged. Web archives all together must process more data than any web search engine. Considering the complexity and large amounts of data involved in web archiving, the results showed that the assigned resources are scarce. A Wikipedia page was created to complement the presented work and be collaboratively kept up-to-date by the community.

Creating a billion-scale searchable web archive

Web information is ephemeral. Several organizations around the world are struggling to archive information from the web before it vanishes. However, users demand efficient and effective search mechanisms to access the already vast collections of historical information held by web archives. The Portuguese Web Archive is the largest full-text searchable web archive publicly available. It supports search over 1.2 billion files archived from the web since 1996. This study contributes with an overview of the lessons learned while developing the Portuguese Web Archive, focusing on web data acquisition, ranking search results and user interface design. The developed software is freely available as an open source project. We believe that sharing our experience obtained while developing and operating a running service will enable other organizations to start or improve their web archives.

Trends in Web Characteristics

The Web is permanently changing, with new technologies and publishing behaviors emerging everyday. It is important to track trends on the evolution of the Web to develop efficient tools to process its data. For instance, Web trends influence the design of browsers, crawlers and search engines. This study presents trends on the evolution of the Web derived from the analysis of 3 characterizations performed within an interval of 5 years. The Web portion used as a case study was the Portuguese Web. Several metrics regarding site and content characteristics were analyzed.

Search the past with the portuguese web archive

The web was invented to quickly exchange data between scientists, but it became a crucial communication tool to connect the world. However, the web is extremely ephemeral. Most of the information published online becomes quickly unavailable and is lost forever. There are several initiatives worldwide that struggle to archive information from the web before it vanishes. However, search mechanisms to access this information are still limited and do not satisfy their users who demand performance similar to live-web search engines. This demo presents the Portuguese Web Archive, which enables search over 1.2 billion files archived from 1996 to 2012. It is the largest full-text searchable web archive publicly available [17]. The software developed to support this service is also publicly available as a free open source project at Google Code, so that it can be reused and enhanced by other web archivists. A short video about the Portuguese Web Archive is available at vimeo.com/59507267. The service can be tried live at archive.pt.

How are web characteristics evolving

The Web is a hypertextual environment in permanent evolution. There are new technologies and Web publishing behaviors emerging everyday. This study presents trends on the evolution of the Web, derived from the comparison of two characterizations of a web portion performed within a 5 year interval. The Portuguese Web was used as a case study. Several metrics regarding content and site characteristics were analyzed.