While the four-layer, Shockley diode is useful as a switching  device, a slight modification to its design creates a silicon- controlled rectifier (SCR). To construct a SCR, an additional  terminal known as a gate is added. It provides more control  and utility. In the four-layer semiconductor construction,  there are always two junctions forward biased and one  junction reversed biased. The added terminal allows the  momentary application of voltage to the reversed biased  junction. All three junctions then become forward biased and  current at the anode flows through the device. Once voltage  is applied to the gate, the SCR become latched or locked on.  Current continues to flow through it until the level drops off  significantly, usually to zero. Then, another applied voltage  through the gate is needed to reactivate the current flow.  [Figures 11-29  and  11-30] SCRs are often used in high voltage situations, such as power  switching, phase controls, battery chargers, and inverter  circuits. They can be used to produce variable DC voltages  for motors and are found in welding power supplies. Often,  lighting dimmer systems use SCRs to reduce the average  voltage applied to the lights by only allowing current flow  during part of the AC cycle. This is controlled by controlling  the pulses to the SCR gate and eliminating the massive heat  dissipation caused when using resistors to reduce voltage.  Figure 11-31  graphically depicts the timing of the gate pulse  that limits full cycle voltage to the load. By controlling the  phase during which, time the SCR is latched, a reduced  average voltage is applied.  SCRs are limited to allowing current flow in one direction  only. In AC circuitry, this means only half of the voltage  cycle can be used and controlled. To access the voltage in  Figure 11-32  illustrates a triac and shows a triac in a simple  circuit. It can be triggered with a pulse of either polarity and  remains latched until the voltage declines, such as when the  AC cycles. Then, it needs to be triggered again. In many  ways, the triac acts as though it is two SCRs connected side  by side only in opposite directions. Like an SCR, the timing  of gate pulses determines the amount of the total voltage that  the reverse cycle from an AC power source, a triac can be  used. A triac is also a four-layer semiconductor device. It  differs from an SCR in that it allows current flow in both  directions. A triac has a gate that works the same way as  in a SCR; however, a positive or negative pulse to the gate  triggers current flow in a triac. The pulse polarity determines  the direction of the current flow through the device. Gate Anode base-plate Mounting stud Cathode Anode (case) N-type (cathode) P-type (gate) N-type P-type (anode) P P N N Gate Gate Gate Gate P P N P N N Anode Cathode Silicon Controlled Rectifier Transistor Equivalent Equivalent Schematic Schematic Symbol Anode Anode Cathode Cathode 11-14 Figure 11-29.  A silicon controlled rectifier (SCR) allows current to pass in one direction when the gate receives a positive pulse to latch  the device in the on position. Current ceases to flow when it drops below holding current, such as when AC current reverses cycle.  Triacs Figure 11-30.  Cross-section of a medium-power SCR. 