abstract an overcurrent output protector is electrically connected to a constant voltage switching power supply provided with a switching transistor which converts a dc voltage obtained by smoothing an ac voltage supplied from an ac power source into a cyclic pulse signal in the overcurrent output protector duty ratio monitor judges whether an on duty ratio of the cyclic pulse signal is a predetermined ratio or more a deactivator turns off the switching transistor in a case where the duty ratio monitor judges that the on duty ratio is the predetermined ratio or more application_number US 43157203 A description one preferred embodiment of the invention will be described referring to fig1 to 3 components similar to those in the related art configuration will be designated by the same reference numerals and detailed explanations for those will be omitted here in this embodiment the switching transistor q 1 is a field effect transistor the field effect transistor can operate on a smaller current than that for a bipolar transistor thus allowing more stable switching operation and outputting a more stable dc voltage the constant voltage switching power supply according to the embodiment comprises a duty ratio monitor 5 and a switching transistor deactivator 6 to constitute an overcurrent output protecting circuit the duty ratio monitor 5 monitors the duty ratio of a high frequency pulse output from the switching transistor q 1 and outputs a predetermined dc voltage to the switching transistor deactivator 6 when the on duty ratio has exceeded a.