亲眼“看到”交流电

我们知道,家庭照明电路使用的是电压为220V的交流电,而且电流的强弱和方向依正弦规律变化。交流电在灯丝中忽强忽弱,忽左忽右,那么电流强时,灯应该会变暗,可是怎么看不到灯有变暗的时候呢?     

函数信号发生器

本系统设计一个函数信号发生器,以MSP430为控制核心,结合DDS芯片AD9850,首先产生100HZ～200kHz(可扩展至1MHz)频率可调的正弦波信号,再经波形变换电路,产生方波和三角波信号;通过使用电压放大,可以在1KΩ负载电阻上使100Hz～200kHz范围内的正弦信号输出电压幅度峰峰值VP-P在0～5V内可调。系统采用LCD显示及语音提示,控制方便。     

断相短路保护交流接触器

目前通用的交流接触器主要供交流 5 0 HZ或 6 0 HZ、电压至5 0 0伏、电流至 15 0安电力电路的远距离接通与分断之用 ,并适宜于频繁地起动及控制交流电动机。该产品的缺陷在于当电路产生断相或短路故障时 ,不能有效地自动切断电源 ,从而造成烧坏电动机的严重事故。由国家提供的数据表明 :85 %的三相电动机烧坏是由于缺相运行造成的。以往人们常采用的方法是在三相电路中增加专用保护器 ,来对电动机进行各种保护。这些方法存在着保护装置控制线路复杂 ,占有较大的空间位置 ,增加设备费用等问题 ,因而在很多情况下得不到推广应用。本设计目的…     

正弦激励下的RL一阶电路暂态过程实验研究

研究RC、RL一阶电路的暂态过程响应及测定时间常数t是电路实验课程中的重要内容,而现今大部分高校在一阶电路实验中采用直流激励和RC实验,导致学生误认为无论何种激励RL一阶电路一定会经历暂态过程。本文重点研究实验改革后正弦激励下的RL一阶电路的暂态过程,分析了该电路在接通正弦交流电时,直接进入稳态响应的条件以及该电路产生过电压、过电流现象的原因。     

船用电池组管理系统设计

介绍了一种用STM32单片机为主控芯片,实现船用电池组管理系统的设计方案,给出了系统的工作原理、硬件设计和软件设计方案。该控制系统以逆变器和均衡电路为核心,实现正弦交流电输出,延长电池组的供电时间和电池的寿命。     

船用电池组管理系统设计

介绍了一种用STM32单片机为主控芯片,实现船用电池组管理系统的设计方案,给出了系统的工作原理、硬件设计和软件设计方案。该控制系统以逆变器和均衡电路为核心,实现正弦交流电输出,延长电池组的供电时间和电池的寿命。     

车载逆变电源设计

车载逆变器就是一种能把汽车上12V直流电转化为220V/50Hz交流电的电子装置,是常用的车用电子用品。本设计主要采用二次逆变实现逆变器的设计。运用TL494以及SG3525A等芯片,先将12V直流电源升压为高频交流电,再经过整流滤波将高频交流电整流为高压直流电,然后采用正弦波脉冲调制法,通过输出脉冲控制开关管的导通。最后经过LC工频滤波及保护电路后,输出稳定的正弦波。     

引入正弦脉宽调制控制三相逆变电路优化设计

以研制新型航空地面电源作为出发点,以逆变器为研究对象,研究了双BUCK组合式三相逆变电路加正弦脉宽调制控制技术,并对主电路的工作方式进行了详细分析,根据主电路的工作特点和工作状态选取了正弦脉宽调制控制(SPWM)方式,控制主电路IGBT的导通和关断,以达到控制输出电压的目的。最后介绍了输出为115 V/400 Hz的实验样机,给出了逆变器的实验数据,证明了该方案实现大功率逆变器的可行性。     

浅谈三相交流电相序判断

运用基尔霍夫定律和正弦交流电相量分析法对相序检测电路中灯泡端电压与相序的关系进行了理论推导和定量分析,以此为依据说明相序的判断方法。     

浅谈三相交流电相序判断

运用基尔霍夫定律和正弦交流电相量分析法对相序检测电路中灯泡端电压与相序的关系进行了理论推导和定量分析,以此为依据说明相序的判断方法。     

Students' Reactions to the Introduction of Videoconferencing for Classroom Instruction

This study examined how students who had no prior experience with videoconferencing would react to the use of videoconferencing as an instructional medium. Students enrolled in seven different courses completed a questionnaire at the beginning of the semester and again at the end of the semester. Students at the origination and remote sites did not differ in their reactions toward videoconferencing but there was a significant difference for gender. Women reacted less favorably to videoconferencing. Compared to the beginning of the semester, students reported significantly less positive attitudes toward taking a course through videoconferencing at the end of the semester. There were no significant differences in students' attitudes toward videoconferencing across courses at the beginning of the semester but there were significant differences across the courses at the end of the semester. The results suggest the need for better preparation for both students and instructors.     

Cooler temperatures inductive to locust outbreaks

Ajoint study by Prof. ZHANG Zhibin from the CAS Institute of Zoology and his co-workers from Norway, US and Swiss have indicated that historical outbreaks of migratory locusts in China were associated with cold spells, suggesting that China＇s projected climate warming could decrease the pest＇s numbers. The study was published in Proceedings of theNational Academy of Sciences on 17 September, 2007.     

Exploring Multiplexity: Social Network Structures in a Computer-Supported Distance Learning Class

A computer-mediated group is a complex entity whose members exchange many types of information via multiple means of communication in pursuit of goals specific to their environment. Over time, they coordinate technical features of media with locally enacted use to achieve a viable working arrangement. To explore this complex interaction, a case study is presented of the social networks of interactions and media use among members of a class of computer-supported distance learners. Results show how group structures associated with project teams dominated who communicated with whom, about what, and via which media over the term, and how media came to occupy their own communication niches: Webboard for diffuse class-wide communication; Internet Relay Chat more to named others but still for general communication across the class; and e-mail primarily for intrateam communication. Face-to-face interaction, occurring only during a short on-campus session, appears to have had a catalytic effect on social and emotional exchanges. Results suggest the need to structure exchanges to balance class-wide sharing of ideas with subgroup interactions that facilitate project completion, and to provide media that support these two modes of interaction.     

To serve and promote science development via innovation

CAS should stick to the principle of rendering service to, and giving impetus for, the development of China＇s science enterprise by making S＆T innovations, said CAS President LU Yongxiang. The CAS president made the remarks in a recent talk to communicate the gist of the winter session of the Party＇s Leading-member Group at CAS, which was held from 7 to 11 January in Beijing.     

The Role of Intermediating Institutions in the Diffusion of Electronic Data Interchange (EDI): How Industry Associations Intervened in Denmark, Finland, and Hong Kong

Electronic data interchange (EDI) provides means for interorganizational communication, creates network externalities, requires an advanced information technology (IT) infrastructure, and relies on standards. In the diffusion of such innovations, institutional involvement is imperative. Such institutions contain governmental agencies, national and global standardization organizations, local government, and nonprofit private organizations like industry associations. The last type of organizations we call intermediating institutions. They intermediate or coordinate ("inscribe") the activities of a group of would-be adopters. Unfortunately, little is known of how these organizations shape the EDI diffusion trajectory. In this article we examine one specific type of intermediating organizations?industry associations?and how they advanced the EDI diffusion process in the grocery sectors of Hong Kong, Denmark and Finland. We identify six institutional measures, placed into a matrix formed by the mode of involvement (influence vs. regulation) and the type of diffusion force (supply push vs. demand pull), that can be mobilized to further the EDI diffusion. Industry associations were found to be active users of all these measures to varying degrees. Their role was critical especially in knowledge building, knowledge deployment, and standard setting. Furthermore, institutional involvement varied due to policy and cultural contingencies and power dependencies.     

Tracing the origin and early radiation of birds——Paleornithologist ZHOU Zhonghe

With great care, Dr. ZHOU Zhonghe takes out a package wrapped by cotton tissue from a drawer and says: "This is the gem of our collections: the fossil of a bird that lived 125 million years ago!" Then, pointing at a tiny mound, he explains: "Look, this is the claw and that is the head. It was in the egg shell and ready to hatch ... The species fell into a family of waterside inhabitants."     

How Electronic Publishers are Protecting Against Piracy: Doubts About Technical Systems of Protection

The increasing prospects of digital piracy has prompted the perceived need by electronic publishers to adopt technical systems of protection, and governments to reform their copyright laws. This article is a preliminary study of the management of intellectual property by electronic publishers, defined as those involved in the production of online databases, and CD-ROMs. It focuses on three main issues: (1) how electronic publishers view the increasing threat of piracy; (2) the methods of protection employed to protect intellectual property in digital format; and (3) the importance of technological protection of intellectual property in electronic publications. The analysis is based on a sample of 23 UK electronic publishers. The interviews revealed an interesting assortment of protection methods and did not show that technological protection was a preferred way. Instead, the means of protection, in addition to copyright law, comprised niche markets, pricing, trust, bad publicity, and nontechnical and technical means.     

Rough Justice: A Statistical Assessment of ICANN's Uniform Dispute Resolution Policy

Long-standing conflict between domain name registrants and trademark holders prompted the Internet Corporation for Assigned Names and Numbers (ICANN) to create a global, mandatory arbitration procedure known as the Uniform Dispute Resolution Policy (UDRP). The UDRP has been used in 2166 cases involving 3938 domain names as of 1 November 2000. The policy gives the initiator of a complaint, generally a trademark holder, the right to choose which ICANN-accredited dispute resolution service provider (RSP) will handle the case. During the preparation of the UDRP,some feared that complainant selection would lead to "forum shopping" that might bias the results. This article performs a statistical assessment of the forum-shopping thesis and finds support for it. There are statistically significant differences in the various RSPs' propensity to take away names from defendants; there are also major differences in the number of cases brought to each RSP. RSPs who take away names have the larger share of cases. The study examines other variables that might explain differences in market share, such as price, the plaintiff's nationality, or the time taken to decide a case. It finds that nationality and time also are correlated with market share. The study concludes that shopping for a favorable outcome is an important factor in the UDRP.     

Prevalence of soft neurological signs in Chinese people

Prof. Raymond C.K. Chen, a neuropsychologist with the CAS Institute of Psychology, has made novel progress in his studies of schizophrenia. His work has been reported by a recent issue of Behavioural Neurology.     

Monoculture Rubber Plantations Have Negative Ef ects on Controlling Splash Erosion

<正>In Xishuangbanna,one of China’s most biodiverse regions,landscape has changed dramatically during the past three decades due to the conversion of tropical rainforest to rubber plantations.In steep areas,terraces are often constructed before planting rubber trees,which causes two important changes in the soil:the destabilization of soil in the bench terraces and the increased vulnerability of unvegetated riser faces to erosion.Few studies have documented the nature and intensity of erosion on bench terraces.Prof.LIU Wenjie and his colleagues from the Xishuangbanna Tropical Botanical Garden(XTBG)conducted a study in Menglun County(21°5′39″N,101°15′55″E),Xishuangbanna to evaluate the influence