Thyristors are solid state semiconductor devices. Thyristors have four layers of alternating N and P type material. This enables thyristors to act as bistable switches, they conduct when thier gate recieves a current pulse, and they continue to conduct for as long as they are forward biased. Thyristors are commonly employed when high currents and high voltages are used, and thyristors are very often empoyed to regulate alternating currents. Thyristors are also found in power supplies for digital circuits, where they are used as a circuit breaker. Thyristors are used as control elements for triggered controllers, also known as phase fired controllers. There are many types of thyristors of which these are some: ASCR Asymmetrical SCR, GTO Gate Turn Off Thyristor, SITh Static Induction Thyristor and IGCT Integrated Gate Commutated Thyristor.