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晶体三极管驱动继电器电路及其抗干扰的一些建议

1、连接方法

用晶体三极管驱动继电器时，建议连接集电极使用。另外，继电器为 ON 时外加额定电压、继电器为 OFF 时使电压为零是一种避免故障的使用方法。并且，在低电压线路（5V 以下）中，请选择考虑到晶体三极管电压降落的电源电压、继电器品种。（在 5V 线路中，建议使用线圈额定电压为 4.5 V型） 继续阅读“晶体三极管驱动继电器电路及其抗干扰的一些建议”

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Relational Databases ~ Introduction to SQL I

There are a number of database query languages in use, either commercially or experimentally. In this series we study the most widely used query language, SQL. Although we refer to the SQL language as a “query language”, it can do much more than just query a database. It can define the structure of the data, modify data in the database, and specify security constraints.

It is not our intention to provide a complete users’ guide for SQL. Rather, we present SQL’s fundamental constructs and concepts. Individual implementations of SQL may differ in details, or may support only a subset of the full language. 继续阅读“Relational Databases ~ Introduction to SQL I”

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动态图解晶体管开关电路及结型场效应管原理

场效应管和晶体管（三极管）皆为最常用的元件，弄清楚其原理和功能十分有必要。晶体管的功能之一即为开关，利用其截止特性，实现开关功能。文中动态图片生动的阐述了三极管作为开关的功能以及结型场效应管工作原理，值得闲暇之余淡然回味。 继续阅读“动态图解晶体管开关电路及结型场效应管原理”

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Relational Databases ~ Introduction to the Relational Model

A data model is a collection of conceptual tools for describing data, data relationships, data semantics, and consistency constraints. In this part, we focus on the relational model.

The relational model uses a collection of tables to represent both data and the relationships among the data. Its conceptual simplicity has led to its widespread adoption; today a vast majority of database products are based on the relational model. The relational model describes data at the logical and view levels, abstracting away low-level details of data storage. 继续阅读“Relational Databases ~ Introduction to the Relational Model”