To provide a constant direct-current power supply wherein it is possible to make easier control for maintaining the output of designated constant current regardless of whether a load is positive or negative.

The constant direct-current power supply includes: a direct-current power source 100; an inverter 11 having multiple switches 111 to 114 (first bridge) and connected to the direct-current power source 100; an inverter driving means 70 that controls turn-on/off of each switch of the inverter 11 and causes the inverter 11 to output rectangular-wave alternating-current voltage or current; a converter 12 having multiple switches 121 to 124 (second bridge), connected to the inverter 11, and coupled to output terminals X, Y; and a constant-current controlling means 80 that controls the phase of current relative to the voltage of rectangular-wave alternating current of the inverter 11 to control output voltage that occurs between output terminals by on/off control on each switch of the converter 12 so that constant direct current is maintained as output current regardless of load electromotive force.

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