Now for the key question - how do thyristors work? They typically have three leads or electrodes (points at which electricity enters or exits). These are called the anode, the cathode and the gate (or control gate). The first is the positive terminal and the second the negative one, while the gate controls the main anode-to-cathode current, triggering this via an external pulse. Some models feature two or four electrodes.
Within a typical SCR, there are two alternating layers of N-type (negative) semiconductor and P-type (positive) semiconductor. This makes four layers altogether, with three junctions between these. The four-layered silicon in each is electrically treated to increase the number of either negative or positive charge-carrying electrons. They are also known as NPN and PNP semiconductors, referring to the pattern of positive and negative electrons in each.
If no current is flowing into the device via the gate, it will rest in an off state, with the central junction (of three) reversed, opposite to the anode and cathode, so current cannot pass through whatever the direction of the device. This is referred to as forward blocking mode or negative blocking mode, depending on the direction.
For current to flow as required, the anode must be positive, and the cathode must be negative. Once the gate current launches, positive and negative charge flows into the four layers of silicon, activating each in turn as it moves from one layer of the semiconductor to the next through the device. Once all four layers have been activated, the current can flow freely through the device. The thyristor is now forward conducting; it has latched on (moved into an on state) and it will remain latched until the current is switched off outside the device - usually the current to the entire circuit. The gate current is not required to maintain the current between the anode and the cathode.
SCR Circuit with AC
Thyristors have slightly differing circuitry, depending on whether they are intended for use with an AC or DC current.
The diagram illustrates an SCR circuit for use with AC.