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The most common type of thyristor is the silicon-controlled rectifier. The SCR is a semiconductor that can be thought off as a controlled diode. It has three terminales called anode, cathode and gate.
Schematic buildup:

       |   |   |   |   |  
  A ---| P | N | P | N |--- C
A  Anode
C  Cathode
G  Gate

P  Positively doped material
N  Negatively doped material

The SCR is usually used in series with the load like this:
             | |
             | | RL
             -+-  SCR
   G _______/ | C
The resistor RL represents the resistance of the load. When the power to the circuit is turned on the SCR will not conduct, until a triggering pulse is applied to the gate. When the SCR first is triggered it will stay in the conducting state until the current flow falls under the minimal holding current(called Ih). The SCR can also be triggered if the voltage over it rises high enough, a characteristic used in the diac.

Another type of thyristor is the Gate Turn Off(GTO) thyristor. This component can also be turned off by using the gate.

The thyristor is used in power switching, power supply and dimming circuits.