A thyristor is a solid-state component used to switch and control electric current flow. Also known as a silicon controlled rectifier (SCR), a thyristor is a robust electronic component used in high current flow applications. They consist of four layers of alternating n and p type semiconductor materials equipped with anode, cathode, and gate terminals. Thyristors start to conduct when they receive a preset voltage on their gate terminal and will, subject to several variables, continue to conduct even if the gate voltage is removed. These operational variables and a wide power rating range make thyristors extremely useful current controllers.
Although thyristors may be broadly classified as simple current switching devices, the range of operational variables they posses makes them very useful in a number of control applications. Essentially thyristors are high current switching devices made up of four alternating p and n layers. An anode is located on the first p layer, a gate terminal on the second p layer, and a cathode on the last n layer. When idle, there is no current transfer across the anode/cathode path. The component requires a voltage of set value applied to the gate layer to switch it on and cause it to conduct current.
The fact that the component will not become active if the gate voltage falls short of its rated threshold value is one of the useful variables a thyristor possesses. This allows for precise control over the switching conditions of the component. Once the thyristor has been turned on, it will remain active even if the gate voltage is removed and current it passes does not drop below the holding value of the component. This known holding voltage is another handy characteristic of thyristors. If the anode voltage value is below the holding level, the thyristor will not switch on even if it receives a gate pulse.
Thyristors can comfortably handle extremely high voltage and current ratings. They are commonly used in zero cross alternating current (AC) controllers, power supplies, phase fired controllers, and long distance power transmission facilities. This last application features huge thyristor banks arranged in Graetz bridge configurations which are capable of reliably switching power values of several megawatts (1,000,000 watts). On the other end of the scale, small AC/DC power supplies may use thyristors rated at 20 watts or less. This flexibility and range of operating power ratings makes the thyristor one of the most useful current flow controllers in the circuit designer's arsenal.