A high voltage AC power supply circuit for a capacitive load C_L, such as an electroluminescent lamp, includes a low voltage DC supply, an inductor L and a FET S in series. The FET S can be pulsed so that the inductor L generates a voltage to charge the capacitive load C_L via an H-bridge H, which is in parallel with the FET S. A diode D prevents current discharging from the capacitive load C_L while the FET S is closed. The total capacitance downstream of the diode D and in parallel with the capacitive load C_L is less than the capacitive load C_L, so that when the polarity of the H-bridge is reversed, the voltage across the H-bridge collapses to earth and the capacitive load C_L is discharged via the low voltage DC supply. The circuits which a employ a large smoothing capacitor in parallel with the H-bridge.