新型电子课件第5章.ppt

CHAPTER 5 SCR in DC and AC Circuit 5.1 SCR introduction The silicon-controlled rectifier (SCR) is made by bonding four layers of semiconductor material together to form a p-n—p-n junction. 1. the construction 2. Schematic symbol and working principles Ⅰ. Identifying the SCR 1) SCRs are known as the workhorses of industrial electronics because of their ability to control hundreds of volts and amps. 2) SCRs are also found in smaller sizes with ratings of less than 1amp at only a few volts 3) They are also found in many different sizes and styles of cases. SCRs has many same styles as transistors. Ⅱ. Schematic symbol and working principles 3. Operating Characteristics SCR operates very similar to a relay which is controlled by a set of pushbuttons and is latched with holding contacts. In this circuit the start button is used to turn relay coil CR on. Once the coil is turned on, the holding contacts, operated by coil CR, close and maintain a current path around the normally open pushbutton. Once the holding contacts have closed, current continues to flow to coil CR even if the start button is released and the pushbutton is again opened. The start button can control when the relay coil is turned on, but once the relay is energized, the start button has no more control over the circuit. In order to turn the relay off, the normally closed stop button must to be pushed to open the circuit to the coil. After the relay coil has been de-energized, CR contacts open and the stop button can be released. 继电器是一种电子控制器件，它具有控制系统（又称输入回路）和被控制系统（又称输出回路），通常应用于自动控制电路中，它实际上是用较小的电流去控制较大电流的一种“自动开关”。故在电路中起着自动调节、安全保护、转换电路等作用。 电磁式继电器一般由铁芯、线圈、衔铁、触点簧片等组成的。 只要在线圈两端加上一定的电压，线圈中就会流过一定的电流，从而产生电磁效应，衔铁就会在电磁力吸引的作用下克服返回弹簧的拉力吸向铁芯，从而带动衔铁的动触点与静触点（常开触点）吸合。 当线圈断电后，电磁的吸力也随之消失，衔铁就会在弹簧的反作用力返回原来的位置，使动触点与原来的静触点（常闭触点）吸合。这样吸合、释放，从而达到了在电路中的导通、切断的目的。 4. Operation of the SCR The SCR operates in a manner similar to this relay circuit