A power supply device is provided with a diode rectifier circuit having an input connected to an AC power source, the output of the diode rectifier circuit producing a rectified voltage; a capacitor connected to the diode rectifier to be charged by the rectified voltage; and a controller configured to receive as input a detection signal representing a charging current flowing into the capacitor, the controller being further configured to calculate a voltage frequency, a cycle, or a power voltage phase of the AC power on the basis of the detection signal. At least one of the following is used to detect the charging current flowing into the capacitor and produce the detection signal: a photocoupler; an amplifier circuit including a shunt resistor and an operational amplifier; and a current transformer.