WHAT ARE THYRISTORS? INFORMATION ABOUT THYRISTORS
Thyristors (or silicon controlled rectifiers) are three-terminal devices which can be used for switching and a.c. power control. Thyristors can switch very rapidly from a conducting to a nonconducting state.
In the off state, the thyristor exhibits negligible leakage current, while in the on state the device exhibits very low resistance. This results in very little power loss within the thyristor even when appreciable power levels are being controlled.
Once switched into the conducting state, the thyristor will remain conducting (i.e. it is latched in the on state) until the forward current is removed from the device. In d.c. applications this necessitates the interruption (or disconnection) of the supply before the device can be reset into its non-conducting state.
Where the device is used with an alternating supply, the device will automatically become reset whenever the main supply reverses. The device can then be triggered on the next half-cycle having correct polarity to permit conduction.
Like their conventional silicon diode counterparts, thyristors have anode and cathode connections; control is applied by means of a gate terminal (see Fig. 5.13).
The device is triggered into the conducting (on state) by means of the application of a current pulse to this terminal. The effective triggering of a thyristor requires a gate trigger pulse having a fast rise time derived from a low-resistance source.
Triggering can become erratic when insufficient gate current is available or when the gate current changes slowly. Table 5.4 summarizes the characteristics of several common thyristors.
The semiconductor devices which are mainly used in the industry are Thyristors The other heating elements like Silicon Carbide are also used.
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