JPS5710821A - Constant voltage power supply circuit - Google Patents

Abstract

PURPOSE:To decrease a power loss in a constant voltage power supply circuit itself, by controlling the conducting phase of an AC input flowing into a low-frequency transformer, so that DC output voltage becomes constant even if AC supply voltage or load is varied. CONSTITUTION:When electric power supply is put to work T0, a transistor 32 is turned on immediately, a current flows into a primary winding, the potential of a point D rises, reaches V0 after t0, a pulse current flows into an LED 23, and a phototransistor 31 and a phototransistor 32 are turned on and off, respectively. The same operation is executed in the next half cycle, too, but in case of the first half cycle, since a smoothing capacitor 12 is charged from zero, it takes time, and in the next half cycle and thereafter, since the smoothing capacitor 12 is charged already, time t1 when a DC output reaches V0 is shorter than t0. In case when supply voltage has risen to E2 from E1 at T1, charging voltage of the capacitor 12 reaches V0 earlier, and on-time t2 of the transistor 32 is further shortened.