![]() Its name comes from the tri-ode for alternating current. another similar to the thyristor device, is the triac. The thyristor as we have already mentioned is sometimes referred at by the initials scr and its electronic symbol is the one you see in diagram 1. Thyristors are used today in many devices such as ac rectifiers, dimmers, electronic car ignition circuits, overload protection devices, logic circuits, and also in industrial applications where they are used to control heavy currents with very much lower control currents. From the moment the thyristor becomes conductive it behaves as a silicon rectifier and only allows current flow in one direction. As it has already been mentioned the thyristor consists of a series of pn junctions and this gives it some of the characteristics of a silicon diode. In general if there is a voltage across the anode and the cathode of a thyristor and we apply a low positive voltage at the gate the thyristor will turn on and even if we remove the trigger voltage it will stay on till the current which flows through it, is somehow interrupted. The gate is what makes the thyristor so special because a relatively small current through its gate can control much heavier currents which pass through the device itself. There are many different types of thyristors but the most common of all is the one which has three leads, the anode (a) the cathode (k) and the gate (g). The thyristor or as it is otherwise called scr (silicon controlled rectifier) is a silicon diode which consists of three or more pn junctions and can either be conductive (on) or not (off). While they have many similarities, important differences exist between their operation and use. Because of their switching and latching actions, both devices are called thyristors. Unlike some switches, which return to a stable "off" state, SCRs and triacs "latch" on or off, and remain that way until certain conditions change. The silicon controlled rectifier and the triac are both solid-state electronic components that turn electrical currents on and off. The tester operates to identify the anode and cathode terminals of the conducting device, as well as to determine without destructive failure of the device under test whether the device is open, shorted or exhibiting leakage. In the case of an SCR and TRIAC device under test, for example, pulses of a predetermined energy level and polarity are applied to the gate electrode to trigger the SCR or TRIAC into conduction. A tester is provided in which a test signal having a selected polarity and magnitude is applied across the main electrodes of a device under test. ![]()
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