An AC-to-DC power supply circuit has an AC capacitor, a half-wave rectifier, and a filter capacitor. Through the AC capacitor, the half-wave rectifier forms a power supply circuit with an AC power supply for converting AC power to half-wave DC power. The filer circuit further converts the half-wave DC power into low-voltage DC power. The AC-to-DC power supply circuit adjusts the ratio of the AC capacitor and the filter capacitor so that the capacitance ratio matches with the voltage ratio of the half-wave DC power and the lower-voltage DC power. As a consequence, the AC-to-DC power supply circuit does not need to use a large-size transformer and can still effectively convert AC power to low-voltage DC power. This can largely reduce the manufacturing cost.