可控硅(Silicon Controlled Rectifier)简称SCR,是一种大功率电器元件，也称晶闸管。它具有体积小、效率高、寿命长等优点。在自动控制系统中，可作为大功率驱动器件，实现用小功率控件控制大功率设备。它在交直流电机调速系统、调功系统及随动系统中得到了广泛的应用。可控硅分单向可控硅和双向可控硅两种。双向可控硅也叫三端双向可控硅，简称TRIAC。 双向可控硅在结构上相当于两个单向可控硅反向连接，这种可控硅具有双向导通功能。其通断状态由控制极G决定。在控制极G上加正脉冲(或负脉冲)可使其正向(或反向)导通。这种装置的优点是控制电路简单，没有反向耐压问题，因此特别适合做交流无触点开关使用。大家使用的是单向晶闸管，也就是人们常说的普通晶闸管,它是由四层半导体材料组成的，有三个PN结，对外有三个电极 :第一层P型半导体弓出的电极叫阳极A,第三层P型半导体引出的电极叫控制极G,第四层N型半导体引出的电极叫阴极K。
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从晶闸管的电路符号可以看到，它和二极管一样是一 种单方向导电的器件， 关键是多了一个控制极G,这就使它具有与二极管完全不同的工作特性。以硅单晶为基本材料的P1N1P2N2四层三端器件,起始于1957年，因为它的特性类似于真空闸流管,所以国际上通称为硅晶体闸流管,简称晶闸管T,因为晶闸管最初的在静止整流方面,所以又被称之为硅可控整流元件,简称为可控硅SCR。在性能上，可控硅不仅具有单向导电性,而且还具有比硅整流元件(俗称”死硅”)更为可贵的可控性。它只有导通和关断两种状态。可控硅能以安级电流控制大功率的机电设备，如果超过此功率，因元件开关损耗显著增加，允许通过的平均电流相降低，此时，标称电流应降级使用。
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可控硅的优点很多，例如:以小功率控制大功率，功率放大倍数高达几十万倍;反应极快，在微秒级内开通、关断;无触点运行,无火花、无噪音;效率高，成本低等等。
可控硅的弱点:静态及动态的过载能力较差;容易受干扰而误导通。
可控硅从外形上分类主要有:螺栓形、平板形和平底形。
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Silicon Controlled Rectifier (SCR), also known as thyristor, is a high-power electrical component. It has the advantages of small size, high efficiency, and long service life. In automatic control systems, it can be used as a high-power driver to control high-power devices with low-power controls. It has been widely used in AC and DC motor speed control systems, power regulation systems, and servo systems. There are two types of thyristor: unidirectional thyristor and bidirectional thyristor. Bidirectional thyristor, also known as three-terminal bidirectional thyristor, abbreviated as TRIAC. Bidirectional thyristor is structurally equivalent to two unidirectional thyristors connected in reverse, and this type of thyristor has bidirectional conduction function. Its on/off state is determined by the control pole G. Adding a positive (or negative) pulse to the control pole G can make it conduct in the forward (or reverse) direction. The advantage of this device is that the control circuit is simple and there is no reverse voltage withstand problem, so it is particularly suitable for use as an AC contactless switch. We are using unidirectional thyristors, also known as ordinary thyristors. They are composed of four layers of semiconductor material, with three PN junctions and three external electrodes: the electrode drawn from the first layer of P-type semiconductor is called anode A, the electrode drawn from the third layer of P-type semiconductor is called control electrode G, and the electrode drawn from the fourth layer of N-type semiconductor is called cathode K.
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