The thyristor is a four-layer semiconducting device, with each layer consisting of an alternately N or P-type material, for example N-P-N-P. The main terminals, labeled anode and cathode, are across the full four layers, and the control terminal, called the gate, is attached to one of the middle layers. The operation of a thyristor can be understood in terms of a pair of tightly coupled transistors, arranged to cause the self-latching action.
Thyristors are mainly used where high currents and voltages are involved, and are often used to control alternating currents, where the change of polarity of the current causes the device to automatically switch off; refered to as Zero Cross operation. The device can also be said to be in synchronous operation as, once the device is open, it conducts in phase with the voltage applied over its cathode to anode junction. This is not to be confused with symmetrical operation, as the output is unidirectional, flowing only from cathode to anode, and so is asymmetrical in nature.
These properties are used control the desired load regulation by adjusting the frequency of the trigger signal at the gate. The load regulation possible is broad as semiconductor based devices are capable of switching at extremely high speeds over extremely large numbers of switching cycles; something difficult to achieve with mechanical switching elements. This is a Frequency domain method of control. With Phase angle control a thyristor is turned on at a specific and adjustable portion of the cycle of the controlling sinusoidal input. Moving the point at which the thyristor is turned on regulates power output. An example of this method of control is a dimmer switch for lights. The turn on point of a thyristor is controlled to occur at a particular point on the sine curve of the AC supply. The thyristor stays on for the remainder of that cycle and the longer the thyristor stays on, the brighter the light. Fine adjustment of the system's output is possible with this method. Phase angle control is particularly suitable for slow-responding loads such as Tungsten filament lamps or temperature variable resistive loads, with the method often being used to control the voltage across the resistive elements found in electrical ovens and furnaces. Phase angle is also utilised in a large number of inductive load controllers.
Phase angle triggered controllers, also known as phase fired controllers, can be used to control resistive and inductive loads directly from the AC mains, substantially reducing the cost and weight of systems previously requiring transformers.
The functional drawback of a thyristor is that, like a diode, it only conducts in one direction. A similar self-latching 5-layer device, called a Triac, is able to work in both directions. This added capability, though, also can become a shortfall. Because the triac can conduct in both directions, reactive loads can cause the triac to fail to turn off during the zero-voltage instants of the ac power cycle. Because of this, use of triacs with (for example) heavily-inductive motor loads usually requires the use of a "Snubber" circuit around the triac to assure that it will turn off with each half-cycle of mains power. Inverse-parallel SCRs can also be used in place of the triac; because each SCR in the pair has an entire half-cycle of reverse polarity applied to it, the SCRs, unlike triacs, are sure to turn off.
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An earlier Gas filled tube device called a Thyratron provided a similar electronic switching capability, where a small control voltage could switch a large current. It is from a combination of "thyratron" and "Transistor" that the term "thyristor" is derived.
Modern thyristors can switch large amounts of power (up to megawatts). In the realm of very high power applications, they are still the primary choice. However, in low and medium power (from few tens of watts to few tens of kilowatts) they have almost been replaced by other devices with superior switching characteristics like MOSFETs or IGBTs. One major problem associated with the thyristor is that it is not a fully controllable switch in the sense that triggering current direction needs to be reversed to switch it off. GTO (Gate Turn-off Thyristor) is another related device which addresses this problem. In high-frequency applications, thyristors are poor candidates due to large switching times arising out of bipolar conduction. MOSFETs, on the other hand, have much faster switching capability because of their unipolar conduction (only Majority carriers carry the current).