The SCR is connected with a diode bridge rectifier. The rectifier circuit converts AC to a unidirectional current through the thyristor. The voltage on the capacitor follows the diode rectifier waveform because RC circuit is connected to diode rectifier. During each half cycle when the voltage across the capacitor reaches trigger level which is just enough to supply gate trigger current to thyristor, then thyristor starts conducting. Firing angle
α = \((\frac{x}{y})_{180}\)