Formulas: V_peak = V_RMS×√2. Half-wave and full-wave bridge rectifiers: each diode only ever sees the peak supply voltage in reverse, so PIV = V_peak. Full-wave center-tap: each diode also sees the OTHER half-winding's peak added to its own during the off half-cycle, so PIV = 2×V_peak — a commonly overlooked reason center-tap designs need much higher-rated diodes than bridge designs for the same transformer. As standard practice, pick a diode rated for at least 1.5–2× the calculated PIV to allow for supply variation and switching transients.