Thyristor

Thyristor is an NPNP four layer or three junction semiconductor device. Thyristor is used switch. Thyristor is big family having several devices performing different tasks. One of them is SCR. SCR stands for silicon controlled rectifier. An SCR has three terminals Anode, Cathode and gate. The anode is connected to the positive supply and normally load is taken in series of anode. Cathode is connected to the negative supply and gate is used to trigger the device. A small voltage short duration pulse at gate with respect to ground or cathode can switch on this device. On DC supply if SCR is once switched on, it cannot be off through gate. It will remain on even if the gate pulse is removed. The only way to switch off an SCR from on to off is to remove the anode voltage. But in AC supply the SCR remains on in corresponding half cycle the device is switched on. For AC two SCRs are connected back to back to work for both half cycles or one Triac is connected which is usually the combination of two SCRs.

Silicon control rectifier

SCRs are available in different current rating and sizes. Also PIV is another consideration to be taken into account. PIV is peak inverse voltage. Normally used scrs have a high piv rating of 1200v. The current rating is from one ampere to thousands of amperes. If a low PIV SCR is used at high voltage, it will breakdown and destroyed.

Thyristors are very commonly used in industrial control systems and motor controls. They are used in motors soft starters. In motor soft starters they start and stop the motors softly by controlling input energy to the motor. Meaning they controls the motor three phase power input and starting the motors from some low voltage of say 70V and bring the motor up to line voltage 380 or 480v in certain amount of time. This time is known as ramp up time. Also same is applied in reverse to stop the motor softly. The motor stopping is known as ramp down. This way motor starts and stops softly and it is not giving mechanical and electrical stress to the system.

SCR or silicon control rectifier is a sort of rectifier with control. It means you can control the energy passes through scr by triggering its gats at certain time. That time is angle of half cycle. This angle is known as alfa angle. The value of alfa angle can be from zero to 180 degrees. The less the angle, the more the power passes through the scr. The bigger the angle, less the power passes through the scr. This is a sort of inverse proportion.