Power circuit

Claim 2

A power circuit comprising an AC power supply, a DC power supply, a rectifying circuit, and active filter circuit connected to an output terminal of the rectifying circuit, switching means for connecting, when the input power is an AC power, the AC power to the input terminals of the rectifying circuit and for connecting, when the input power is a DC power, the DC power to the input terminals of the active filter, first and second resistors connected in series for setting the inter-terminal voltage of the active filter, and a transistor connected in parallel to the first resistor, the switching means turning the transistor off when the input power is the AC power and turning the transistor on when the input power is the DC power to vary the inter-terminal voltage of the active filter depending on whether the input power is the AC power or the DC power.

Claim 3

A power circuit comprising an AC power supply, a DC power supply, a rectifying circuit having first and second input terminals and first and second output terminals, an active filter having first and second input terminals and first and second output terminals, the first input terminal of the active filter being connected to the first input terminal of the rectifying circuit, a load connected between the first and second output terminals of the active filter, switching means for connecting, when the input power is an AC power, the AC power between the first and second input terminals of the rectifying circuit and connecting the second output terminal of the rectifying circuit to the second input terminal of the active filter and for connecting, when the input power is a DC power, the DC power between the first and second input terminals of the active filter, series connected first and second resistors provided to the active filter for setting the inter-terminal voltage of the active filter, a transistor having an input terminal, an output terminal, and a control terminal, the input terminal of the transistor being connected to one end of the first resistor, the output terminal of the transistor being connected to the other end of the first resistor, and fourth and fifth resistors connected in series between the first input terminal of the active filter and the second output terminal of the rectifying circuit, the control terminal of the transistor being connected to the junction point between the fourth and fifth resistors, whereby the transistor is turned off when the input power is the AC power and turned on when the input power is the DC power to vary the inter-terminal voltage of the active filter depending on whether the input power is the AC power or the DC power.

Claim 4

A power circuit comprising an AC power supply, a DC power supply, a rectifying circuit having first and second input terminals and first and second output terminals, an active filter having first and second input terminals and first and second output terminals, the first input terminal of the active filter being connected to the first input terminal of the rectifying circuit, a load connected between the first and second output terminals of the active filter, switching means for connecting, when the input power is an AC power, the AC power between the first and second input terminals of the rectifying circuit and connecting the second output terminal of the rectifying circuit to the second input terminal of the active filter and for connecting, when the input power is a DC power, the DC power between the first and second input terminals of the active filter, series connected first and second resistors provided to the active filter for setting the inter-terminal voltage of the active filter, a transistor having an input terminal, an output terminal, and a control terminal, the input terminal of the transistor being connected to one end of the first resistor, the output terminal of the transistor being connected to the other end of the first resistor, and fourth and fifth resistors connected in series between the first input terminal of the active filter and the second output terminal of the rectifying circuit, the control terminal of the transistor being connected to the junction point between the fourth and fifth resistors, whereby the transistor is turned off when the input power is the AC power and turned on when the input power is the DC power to vary the inter-terminal voltage of the active filter depending on whether the input power is the AC power or the DC power.

Claim 5

A power circuit according to claim 2, in which the active filter comprises a transistor having an input terminal, an output terminal, and a Control terminal, the input terminal of the transistor being connected to the first input terminal of the active filter, the output terminal of the transistor being connected to the first output terminal of the active filter, a capacitor one terminal of which is connected to the control terminal of the transistor and the other terminal of which is connected to the second input terminal and the second output terminal of the active filter, and first and second resistors connected between the input terminal and the control terminal of the transistor.

Claim 6

A power circuit according to claim 2, in which the active filter comprises a power processing circuit which operates as a constant voltage circuit when the input power is higher than a predetermined value and operates as a filter when the input power is lower than a predetermined value.

Claim 7

A power circuit according to claim 4, in which the active filter compreses a control transistor connected in series between the first input terminal and the first output terminal of the active filter, an error detector and amplifier circuit including detecting means for detecting the variation in the output voltage and connected between the first and second output terminals of the active filter for detecting and amplifying the variation in the output voltage to supply it to the control transistor, an input/output voltage difference detecting transistor which becomes conductive, when the voltage between the input and the output of the control transistor becomes lower than a predetermined value, to produce an output current corresponding to the voltage between the input and output of the control transistor, and a transistor driven in response to the output of the input/output voltage difference detecting transistor to vary the impedance of the output voltage detecting means of the error detector and amplifier circuit, and operates as a ripple filter when the input voltage becomes lower than a predetermined value.

Claim 8

A power circuit according to claim 2, in which the active filter comprises a control transistor whose emitter is connected to the first input terminal of the active filter and whose collector is connected to the first output terminal of the active filter, an error detector and amplifier circuit including fifth and sixth resistors for output voltage detection and connected between the first and second output terminals of the active filter for detecting and amplifying the variation in the voltage between the first and second output terminals of the active filter, a series circuit of the first and second resistors and a constant current source connected between both ends of the power supply, an input/output voltage difference detecting transistor whose base is connected to the junction point of the second resistor and the constant current source, whose emitter is connected to the collector of the control transistor, and which becomes conductive, when the emitter-collector voltage of the control transistor becomes lower than a predetermined value, to produce an output current corresponding to the emitter-collector voltage of the control transistor, and a transistor whose collector is connected to one end of the fifth resistor, whose emitter is connected to the other end of the fifth resistor, and whose base is connected to the collector of the input/output voltage difference detecting transistor.