The firing angle of a thyristor refers to the delay in degrees or radians between the instant the thyristor begins to conduct current after a gate signal is applied and the point in the AC waveform where the voltage across the thyristor becomes positive enough to trigger conduction. It determines when during each AC cycle the thyristor switches from its non-conducting state (off state) to its conducting state (on state). Controlling the firing angle allows for the regulation of power delivered to loads in AC circuits, such as in phase control of AC voltage and power control applications.
The maximum firing angle of a Silicon Controlled Rectifier (SCR), which is a type of thyristor, depends on the specific device but typically ranges from 0 to 180 degrees. At 0 degrees firing angle, the SCR conducts immediately when the gate signal is applied, allowing the maximum possible current through the device. As the firing angle increases towards 180 degrees, conduction occurs later in each AC cycle, resulting in less average power being delivered to the load.
The firing angle of a TRIAC, a bidirectional semiconductor device similar to two SCRs connected in reverse parallel, also refers to the phase delay between the application of a gate signal and the initiation of conduction. However, TRIACs can conduct in both directions, allowing for control of AC power in both positive and negative half-cycles of the AC waveform. The firing angle of a TRIAC is typically adjustable across the entire AC cycle, ranging from 0 to 180 degrees.
The extinction angle of a thyristor refers to the phase angle at which the thyristor stops conducting after it has been triggered into conduction. It is the phase angle at which the current through the thyristor drops to zero and the device returns to its non-conducting state. Extinction angle is critical in AC circuits where precise control of current flow and timing of thyristor turn-off is required to avoid damaging components or disrupting operation.
Firing of an SCR refers to the process by which the device is turned on or triggered into conduction. When a positive gate current or voltage signal is applied to the gate terminal of an SCR, it causes the device to switch from its non-conducting state to its conducting state, allowing current to flow through it. Firing is essential for controlling the timing and amount of current delivered to loads in AC circuits, enabling applications such as dimming lights, controlling motor speed, or regulating power in heaters and other devices.