Switching power supply having output terminal disconnection detecting circuit

A switching power supply for supplying electric current to a load located away from the switching power supply, comprising:

• a transformer;
• main switching means for switching on and off an electric current of a direct current source supplied thereto, an electric current switched on and off by the main switching means being supplied to said transformer;
• rectifying and smoothing means for rectifying and smoothing an electric current from said transformer, an output of the rectifying and smoothing means being supplied to terminals of the load;
• remote detecting terminals connected to the terminals of the load;
• remote detecting means connected to the remote detecting terminals for detecting a voltage between the terminals of the load, said remote detecting means including a differential amplifier with an output terminal, and a comparator connected to the output terminal of the differential amplifier, said differential amplifier having an inverse terminal and a non-inverse terminal connected to said remote detecting terminals, said comparator comparing an output voltage of said differential amplifier with a reference voltage and outputting an alarm signal when the output voltage of said differential amplifier is lower than the reference voltage;
• first reference voltage supply means for supplying a first reference voltage;
• control means connected to the main switching means for changing ON-OFF duty ratio of the main switching means by comparing the output voltage of said differential amplifier of the remote detecting means with the first reference voltage to control an output voltage of said main switching means at a predetermined value;
• first overvoltage protecting means to be connected to the control means for protecting said load from overvoltage by monitoring the output of said rectifying and smoothing means;
• output voltage detecting means connected to the rectifying and smoothing means for detecting the output of the rectifying and smoothing means;
• second reference voltage generating means for outputting a second reference voltage higher than the first reference voltage;
• second overvoltage protecting means connected to the remote detecting terminal;
• and
• first, second and third switching means electrically connected to the remote detecting means to be changed over by the alarm signal outputted from the remote detecting means;
• said first switching means normally feeding the output voltage of the differential amplifier to the control means and being changed over by the alarm signal to feed an output of said output voltage detecting means to said control means;
• said second switching means normally feeding the first reference voltage to said control means and being changed over by the alarm signal to feed the second reference voltage to said control means;
• and
• said third switching means normally feeding an output of said second overvoltage protecting means to said control means and being changed over by the alarm signal to feed an output of said first overvoltage protecting means to said control means.