SCRs in AC Circuits
In the circuit shown in Figure 5-1, the gate of the SCR is connected through a resistor and diode directly to its anode. When the AC voltage applied to the anode rises in the positive direction, current flows through the gate-cathode section of the SCR. When it reaches the trigger point (assume 5 milliamperes for this SCR), the SCR fires and conducts through the anode-cathode section. It conducts as long as the AC voltage remains in positive direction and the current is above its holding current level.
When the AC voltage drops to zero and begins to increase in the negative direction, the SCR remains turned off. When the voltage applied to anode again becomes positive the gate will trigger the SCR on again. The output of SCR is half-wave rectified direct current.
Why use an SCR if it does the same thing as a simple junction diode? The SCR can be controlled as to when it turns on, Figure 5-2. A variable resistor has been added to the gate circuit of the figure. Assume that the gate current must reach a 5 mA level before the SCR will fire. By adjusting the variable resistor, it is possible to determine how much voltage must be applied to gate before a 5 mA current flows through the gate circuit. By adjusting the resistor to a higher value,it is possible to keep the SCR from firing until the AC voltage has reached its peak value in the positive direction. In this way the SCR will fire when the AC voltage is at its positive peak. With this setting of the resistor, the SCR will drop half the voltage and the load will drop the other half.
By reducing the resistance of gate-circuit, the gate current reaches 5 mA sooner, and the SCR fires earlier in the AC cycle. This causes less voltage to be dropped across the SCR and more to be dropped across the load.
If the gate resistance is reduced even more, the 5mA of gate current will be reached even sooner during the cycle and the SCR will fire earlier. Still less voltage is dropped across the SCR and more voltage is dropped across the load.
There is a problem with this type of control. The SCR controls half of the positive cycle of alternating current applied to it. The latest that the SCR can be fired is when the positive half cycle has reached 90°. This permits the SCR to control only half of the AC positive wave; half voltage is applied to the load when the SCR initially fires. If the load resistor is a light bulb, the bulb will burn at half brightness when it is first turned on.
在交流电路中可控硅
在图5-1中所示的电路中，可控硅的栅极通过一个电阻器和二极管直接将其阳极连接。当施加到阳极的交流电压上升，在正方向上，电流流经可控硅的栅极 - 阴极部分。当达到触发点（假定为5毫安这个SCR） ，可控硅触发，并进行通过阳极 - 阴极部分。它进行，只要在交流电压保持在正方向和电流高于其保持电流电平。
当AC电压下降到零，并开始增加在负方向上，这个SCR仍然关闭。当施加到阳极的电压再次变为正的栅极将再次触发可控硅上。可控硅的输出进行半波整流的直流电流。
所以，如果做同样的事情使用的SCR作为一个简单的结型二极管？可控硅可以当它打开时，图5-2进行控制，以。一种可变电阻器已被添加到图中的门电路。假设栅极电流必须达到5毫安级之前， SCR将闪光。通过调节可变电阻器，它能够确定多少电压如何，必须施加到栅极5 mA的电流流过栅极电路之前。通过调节电阻为较高的值，因此能够保持在SCR从击发直到AC电压达到其峰值的正方向。通过这种方式， SCR将火当交流电压为正峰值。与电阻器的此设置中，SCR将下降一半的电压和负载就会下降的另一半。
通过减小栅极电路的电阻，栅极电流达到5毫安越早，和SCR之前在AC周期闪光。这将导致较小的电压被穿过SCR下降，更要在负载两端丢弃。
如果栅极电阻更是降低了，门极电流的5毫安将会更早的周期内达到和SCR将提早火。还是少电压加在可控硅下降和更多电压加在负载下降。
存在与这种类型的控制的一个问题。可控硅控制的一半施加到其的交流电流的正周期。这个SCR可以发射最新的是当在正半周期已经达到了90℃ 。这使得SCR来控制交流正波的一半，另一半的电压施加到在SCR初始触发该负载。如果负载电阻是一个灯泡，灯泡会燃烧一半的亮度，当它第一次开启。