JPH06332588A - Power supply device - Google Patents

Abstract

PURPOSE:To reduce the size and cost of the device by charging a charge storage means when the commercially available power supply is normal, turning on a switching circuit which supplies the dc current to the system and turning of a switching circuit when the abnormal time exceeds a prescribed moment when the commercially available power supply is abnormal. CONSTITUTION:When the commercially available power supply is inputted to a rectifier circuit 1, the dc current is gradually outputted and a battery 4 is charged by the dc current and the battery backup power supply is supplied from a voltage adjustment circuit 5 to a memory. In a voltage detection circuit 6, the output voltage Ve is made to be at an H level when the voltage Va exceeds a threshold value Vth1 and the memory backup power is supplied from a line 2b to the system. As soon as the commercially available power supply is temporarily turned off, the voltage, Va of the output of the rectifier circuit 1 becmes lower. However, the charge stored in a capacitor C1 is discharged with a time constant and the voltage Vd is reduced gradually to prevent a transistor Tr1 from being turned off so that the power is supplied from the line 2b.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power supply device having a backup circuit which maintains power supply to a system against a momentary interruption of a commercial power source and retains memory contents when the commercial power source is momentarily interrupted or stopped.

[0002]

2. Description of the Related Art For example, in a fax machine, a copy machine, a multi-function telephone, etc., while the power supply to the system is maintained even when the commercial power source is interrupted, the memory contents are retained when the commercial power source is interrupted and the power source is turned off. Therefore, a power supply device having a backup circuit is provided.

FIG. 6 shows a circuit diagram of a conventional power supply device of this type.

In the figure, in a rectifier circuit 1, a commercial power source is rectified as a voltage Vin and a DC power source is output to a system power source side line 2a and a battery power source side line 3a. One end of a large-capacity capacitor C is connected to the line 2a on the system power supply side of the rectifier circuit 1, and the other end of the capacitor C is grounded.

On the other hand, the line 3a on the battery power source side has the (+) of the battery 4 through the resistor R1 and the diode D1.
Connected to the terminal, the (−) terminal of the battery 4 is grounded,
Further, a Zener diode ZD1 is connected in parallel to the battery 4, and a voltage adjusting circuit 5 is provided between the (+) terminal of the battery 4 and the line 3b on the memory backup power receiving side to provide a memory backup power circuit. Is formed.

In the voltage detection circuit 6, the input side in is connected to the line 2b on the system power supply side, the output side out is connected to the base of the transistor Tr1 via the resistor R2, and the transistor Tr1 is further connected. The collector is connected to the system power supply side line 2b, and the emitter is connected to the memory backup power supply side line 3b to form a switching circuit. The system power supply side line 2b and the memory backup power supply side line. A diode D2 is provided between the diode 3 and 3b.

When commercial power is input to the rectifier circuit 1, DC power is output to the system power supply side line 2a and the battery power supply side line 3a. As a result, the voltage of the capacitor C is boosted and the line 2 on the system power receiving side is supplied.
A predetermined voltage Vc is supplied to the system from b, and further,
The voltage Vb is supplied to the line 3b on the memory backup power receiving side via the diode D2.

On the other hand, the DC power supplied to the line 3a on the battery power source side is charged into the battery 4 through the resistor R1 and the diode D1, and the charging voltage of the battery 4 is adjusted by the voltage adjusting circuit 5 so that the memory backup power source is supplied. It is output to the line 3b on the receiving side.

In this case, since the voltage supplied via the diode D2 is higher than the output voltage of the voltage adjusting circuit 5 to some extent, the voltage supplied via the diode D2 is changed to the voltage V.
b is supplied to the memory from the line 3b on the memory backup power receiving side.

Next, when the voltage of the DC power source of the rectifier circuit 1 is momentarily dropped due to the momentary interruption of the commercial power source, the electric charge charged in the capacitor C is discharged, and the voltage is released to the line 2b on the system power receiving side. Vc is maintained at a predetermined value, and further, voltage Vb of line 3b on the memory backup power supply side is maintained at a predetermined value.

On the other hand, when the commercial power supply is turned off, the voltage of the capacitor C drops, so that the voltage Vc of the line 2b on the system power receiving side drops, and the voltage Vc becomes zero after a predetermined time. That is, the supply of power to the system is stopped when the commercial power supply is off for a longer time than the momentary interruption. At this time, in the voltage detection circuit 6, the voltage on the input side in falls to a predetermined threshold value. Therefore, the output side out of the voltage detection circuit 6 becomes L level, and the transistor Tr1 is turned off.

As a result, line 2 on the system power supply receiving side
The DC voltage output from the voltage adjusting circuit 5 becomes higher than the voltage Vc of b, and this voltage supplies the memory backup power supply to the memory from the memory backup power supply receiving side line 3b as the voltage Vb.

In this way, when the commercial power supply is instantaneously cut off, power is supplied from the capacitor C to the system and the memory, and when the commercial power supply is off, only the memory is supplied from the memory backup power supply circuit so that the system can be operated even during the instantaneous interruption. It can be operated.

By the way, Japanese Patent Application Laid-Open No. 2-53352 discloses a technique for monitoring the commercial power supply and supplying the power from the secondary battery to the multifunction telephone at the time of power off or momentary interruption to guarantee the operation of the multifunction telephone. Is disclosed.

[0015]

However, in the conventional power supply device shown in FIG. 6, in addition to the memory backup power supply circuit having the battery 4, a circuit having a large-capacitance capacitor C is provided as a measure against instantaneous interruption of commercial power. Therefore, there has been a problem that the power supply device is increased in size and cost is increased.

That is, the battery 4 is used as a power supply device.
Since the charging efficiency is extremely poor as compared with the case of using one charging device, it is difficult to prevent the battery 4 and the large-capacity capacitor C from becoming large as a whole. Further, as compared with the case of one charging device, the battery 4 and the large-capacity capacitor C are separated, which causes a cost increase.

Further, in Japanese Patent Laid-Open No. 2-53352,
Regarding the power supply of the system, measures against instantaneous interruption and off of commercial power supply are disclosed, but they do not solve the above problems.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a power supply device which is small in size and can reduce the cost.

[0019]

According to the present invention, in order to maintain the supply of power to the system and to always retain the memory contents even if the commercial power source is interrupted, the power source is always supplied to the memory. In the power supply device that supplies power, a rectifier circuit that rectifies commercial power and converts it to direct current power, and a backup power supply circuit that charges the charge storage means when the commercial power is normal and uses it as backup power when the commercial power fails The switching circuit that supplies backup power to the system and the switching circuit that is turned on when the commercial power supply is normal, and the switching circuit that is turned off when the abnormal time of the commercial power supply exceeds the predetermined instantaneous interruption time. And a switching control circuit for switching.

According to a second aspect of the present invention, in a power supply device which constantly supplies power to the memory in order to maintain the supply of power to the system and to always retain the memory contents even if the commercial power supply is interrupted. A rectifier circuit that rectifies the commercial power source and converts it to a DC power source, and a normal power supply circuit that supplies the DC power source to the system and the memory when the commercial power source is normal,
A backup power supply circuit that charges the charge storage means when the commercial power supply is normal and uses it as a backup power supply when the commercial power supply is abnormal, a switching circuit that supplies backup power to the system, and a switching circuit that is turned on when the commercial power supply is normal On the other hand, a switching control circuit is provided which turns off the switching circuit when the abnormal time of the commercial power supply exceeds a predetermined instantaneous interruption time.

According to a third aspect of the present invention, in a power supply device that maintains the supply of power to the system and the memory even if the commercial power supply is momentarily cut off, a rectifier circuit that rectifies the commercial power supply and converts it to a DC power supply. While charging the charge storage means with a DC power source obtained from the rectifier circuit when the commercial power source is normal,
The power supply circuit that constantly supplies DC power to the memory, the switching circuit that supplies DC power to the system from the power supply circuit, and the voltage level of the DC power supply obtained from the rectifier circuit are monitored, and the normality of the commercial power supply is checked according to the voltage level. Alternatively, a voltage detection circuit that detects an abnormality and a switching control circuit that turns on the switching circuit when the commercial power supply is normal, and turns off the switching circuit after a predetermined time when the commercial power supply becomes abnormal should be provided. It was done.

According to a fourth aspect of the present invention, in a power supply device that maintains the supply of power to the system and the memory even when the commercial power supply is momentarily cut off, a rectifier circuit that rectifies the commercial power supply and converts it into a DC power supply. First supply of DC power obtained from the rectifier circuit to the system and memory when the commercial power supply is normal
A power supply circuit and a second power supply circuit for charging the charge storage means with a DC power supply obtained from the rectification circuit when the commercial power supply is normal, and for supplying the DC power supply to the memory when the commercial power supply is abnormal, and from the second power supply circuit to the system. A switching circuit that supplies a DC power supply, and a voltage detection circuit that monitors the voltage level of the DC power supply obtained from the rectifier circuit and detects whether the commercial power supply is normal or abnormal according to the voltage level, and a switching circuit when the commercial power supply is normal. And a switching control circuit for turning off the switching circuit after a predetermined time when the commercial power supply becomes abnormal.

The invention of claim 5 is the invention of claims 1 to 4.
In the power supply device described above, a secondary battery or a large capacity capacitor is used as the charge storage means.

[0024]

According to the invention of claim 1, when the commercial power supply is normal, the charge storage means of the backup power supply circuit is charged as the backup power supply. At this time, the ON signal is input to the switching circuit. Therefore, backup power is being supplied to the system and memory. When the commercial power supply is instantaneously disconnected, the switch control circuit outputs the ON signal as it is, assuming that the abnormal voltage time is within the predetermined instantaneous disconnection time. As a result, the backup power is continuously supplied from the switching circuit to the system when the commercial power is interrupted. When the commercial power supply is off, in the switching control circuit, the abnormal voltage time exceeds the predetermined instantaneous disconnection time, so the switching circuit is turned off and the power supply of the system from the backup power supply is stopped. As a result, the backup power supply is used even when the commercial power supply is interrupted, so that the power supply for the instantaneous power failure and the power supply for the memory backup are unified. Therefore, the device can be made smaller than the conventional device, and the cost can be reduced.

According to the second aspect of the present invention, when the commercial power supply is at a normal voltage level, the normal-time power supply circuit supplies the DC power from the rectifier circuit to the system and the memory. At this time, the charge storage means of the backup power supply circuit is charged as a backup power supply and an ON signal is input to the switching circuit. However, since the voltage of the DC power supply from the rectifier circuit is higher than the voltage of the memory backup power supply, DC power is supplied to the system and memory. When the commercial power supply is instantaneously disconnected, the switch control circuit outputs the ON signal as it is, assuming that the abnormal voltage time is within the predetermined instantaneous disconnection time. As a result, the backup power is supplied from the switching circuit to the system when the commercial power is interrupted. When the commercial power supply is off, the switching control circuit outputs an off signal because the abnormal voltage time exceeds the predetermined instantaneous interruption time. Therefore, the system power supply from the backup power supply is stopped. As a result, the backup power supply is used even when the commercial power supply is interrupted, so the power supply for the instantaneous power failure and the memory backup power supply are shared, and when the commercial power supply is normal, the DC power from the rectifier circuit is used. It is directly supplied to the system and memory. Therefore, the load when the commercial power supply is normal is reduced, the size is made smaller than the conventional device, and the cost is further reduced.

According to a third aspect of the invention, in addition to the first aspect of the invention, the voltage level of the DC power supply is monitored by a voltage detection circuit, and when the voltage is normal, the switching circuit is turned on, while when the voltage is abnormal. When the time exceeds a predetermined instantaneous interruption time, the switching circuit is turned off. As a result, the voltage level of the DC power supply is monitored by the voltage detection circuit and the switching circuit is operated, so that the operation can be performed accurately, and the device can be made smaller than the conventional device and the cost can be reduced.

According to a fourth aspect of the present invention, in addition to the second aspect of the invention, the voltage level of the DC power supply is monitored by a voltage detection circuit, and when the voltage is normal, the switching circuit is turned on, while when the voltage is abnormal. When the time exceeds a predetermined instantaneous interruption time, the switching circuit is turned off. As a result, the voltage level of the DC power supply is monitored by the voltage detection circuit and the switching circuit is operated, so that the operation can be performed accurately, and the device can be made smaller than the conventional device and the cost can be reduced.

The invention of claim 5 is the first to fourth aspects of the invention.
In the invention described above, since the secondary battery or the large-capacity capacitor is used as the charge storage means, the life can be extended.

[0029]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a circuit diagram of a power supply device showing a first embodiment of the present invention.

The rectifier circuit 1 has a commercial power source of voltage V on the input side.
It is input as in, rectifies alternating current and outputs direct current.
The power supply output side line 2c of the rectifier circuit 1 is branched and connected to the voltage detection circuit 6, and the resistor R1 and the diode D are connected.
1 is connected to the (+) terminal of the battery 4, the (−) terminal of the battery 4 is grounded, the zener diode ZD1 is connected in parallel to the battery 4, and the power supply output side line 2c is connected to the main body side. Main unit side line 2d that supplies power to
It is connected to the.

The voltage detection circuit 6 is constructed, for example, as shown in FIG. 2 and has a resistor R connected to the power source when the power source is normal.
When the voltage across 7 is higher than a predetermined threshold voltage Vth1, the AMP 6a outputs L level and the transistor 6
When b is turned off, a current is supplied from the power source to the output side via the resistor R7, and the capacitor C is charged via the diode D2. On the other hand, when the power supply is abnormal, the voltage across the resistor R7 drops to a predetermined threshold voltage Vth1 or less and becomes H level. As a result, the transistor 6b becomes O
The charge voltage of the capacitor C becomes N and is discharged to the ground side through the resistor R3 and the transistor 6b.

The output side of the voltage detection circuit 6 is connected to the base of the transistor Tr1 via the resistors R3 and R4, the diode D2 is connected in parallel to the resistor R3, and the connection point between the resistors R3 and R4. The capacitor C is grounded to form a switching control circuit for turning on / off a switching circuit described later with a switching signal.

The collector of the transistor Tr1 is connected to the line 3b on the memory backup power supply side, and the emitter is connected to the line 2b on the system power supply side.
It forms a switching circuit that is turned on by a switching signal.

In the voltage adjusting circuit 5, for example, as shown in FIG. 3, the voltage across the resistor R8 and the reference voltage Vref of the reference voltage circuit 5c are connected to the error amplifier 5b in Darlington connection according to the voltage difference between them. A base voltage is applied to the transistor 5a so that a predetermined current is applied to reduce the voltage difference.
From the side to the out side. The voltage adjusting circuit 5 is provided with a temperature protection circuit 5d and an overcurrent protection circuit 5f.

On the other hand, the output side of the voltage adjusting circuit 5 is connected to the line 3b on the memory backup power supply side, and supplies power to the line 2b on the system power supply side when the transistor Tr1 is on.

Next, the operation of the circuit diagram shown in FIG. 1 will be described with reference to the operation waveform diagram shown in FIG. 4. First, when commercial power is input to the rectifier circuit 1 as the voltage Vin at time t0, rectification is performed. Then, the DC power source is gradually output as the voltage Va. After that, the battery 4 is charged by the DC power source, and the battery backup power source is supplied from the voltage adjusting circuit 5 to the memory as the voltage Vb.

In the voltage detection circuit 6, when the voltage Va exceeds the preset threshold value Vth1, the voltage Ve output from the voltage detection circuit 6 is set to the H level. As a result, the voltage V charged in the capacitor C1 via the diode D2
d becomes the H level, the transistor Tr1 is turned on by the switching signal, and the memory backup power supply, which is the output of the voltage adjustment circuit 5, is the line 2 on the system power supply receiving side.
b to the system. In this state, for example, when the input of the commercial power supply is temporarily turned off due to a momentary interruption at time t1, the voltage Va of the output of the rectifier circuit 1 drops. The voltage detection circuit 6 outputs the voltage Ve as the L level because it becomes smaller than the threshold value Vth1.

As a result, the electric charge accumulated in the capacitor C1 of the switching control circuit is discharged through the resistor R3 with a time constant, and the voltage Vd of the capacitor C1 gradually drops. In this case, the voltage Vd drops, but does not reach the threshold value Vth2 for turning on / off the transistor Tr1, so that the transistor Tr1 is not turned off (time t in the figure).
1 and time t2). Therefore, even if the commercial power supply is cut off instantaneously, the power is supplied to the system from the system power supply side line 2b through the transistor Tr1 as in the case where the commercial power supply is normal.

For example, when the commercial power supply is turned off at time t3, the output voltage Va of the rectifier circuit 1 decreases, so the voltage detection circuit 6 sets the voltage Ve to the L level. As a result, the electric charge accumulated in the capacitor C1 is discharged through the resistor R3, the voltage Vd of the capacitor C1 gradually decreases, and at a time t4 after a predetermined time, the voltage Vd changes to the threshold value Vt.
It becomes h2 or less, and the switching signal is not output.

Therefore, the transistor Tr1 is turned off, the voltage Vc of the line 2b on the system power receiving side becomes zero, and power is not supplied to the system.

As described above, even if the commercial power supply is instantaneously cut off, the system is supplied with power from the line 2b on the system power supply side and operates normally. Further, when the commercial power supply is turned off, power is continuously supplied to the memory from the memory backup power supply by the battery 4 through the line 3b on the memory backup power supply side, and after a certain period of time, from the line 2b on the system power supply side to the system. Turn off the power.

In the first embodiment, the transistor Tr1 is turned on / off based on the output signal of the voltage detection circuit 6, but the voltage detection circuit 6 is not essential.

That is, even if the voltage detection circuit 6 is omitted in FIG. 1, if the resistors R2, R3, R4 and the capacitor C1 are appropriately set, the DC power source is connected to the capacitor C1 via the resistor R2 and the diode D2 when the commercial power source is normal. Is charged and the transistor Tr can be turned on by the charging. Furthermore, when the commercial power supply is abnormal, the charge stored in the capacitor C1 is discharged to the output side of the rectifier circuit having a low impedance through the resistors R2 and R3 due to a predetermined time constant. The transistor Tr is turned off when the commercial power supply is off for a predetermined time without being turned off. By doing so, the present invention can be implemented without providing the voltage detection circuit 6.

FIG. 5 is a circuit diagram of a power supply device showing a second embodiment of the present invention.

The difference from the first embodiment shown in FIG. 1 is that the power supply output side line 2c is connected to the emitter of the transistor Tr1 and the system power supply receiving side line 2b through a diode D3 for backflow prevention, and the transistor Tr1 is connected. That is, the diode D4 is connected in parallel between the emitter and the collector so that the DC power from the rectifier circuit 1 is supplied to the system and the memory when the commercial power is normal.

When the commercial power source is on, power is supplied to the system from the system power supply line 2b via the diode D3 connected to the power output line 2c, and the memory backup power supply side is supplied via the diode D4. Power is supplied to the memory from the line 3b.

In the voltage detection circuit 6, the transistor Tr1 is turned on because the switching signal is input to the transistor Tr1 from the switching control circuit as in the first embodiment.

At this time, power is supplied from the battery 4 to the voltage adjusting circuit 5, but the voltage Vc of the line 2b on the system power receiving side is higher than the voltage Vb of the line 3b on the memory backup power receiving side. Power is not supplied to the line 2b on the system power receiving side and the line 3b on the memory backup power receiving side.

In this state, if the commercial power supply is momentarily cut off, the voltage Vc of the power supply output side line 2c decreases, so that power is supplied from the voltage adjusting circuit 5 to the system power supply receiving line 2b via the transistor Tr1. At the same time, it is supplied to the line 3b on the memory backup power receiving side.

In this case, similarly to the first embodiment shown in FIG. 1, the voltage Vd gradually drops while the capacitor C1 discharges, but the switching signal is not turned off within a predetermined time when there is a momentary interruption. Therefore, the switching signal is still input to the white Tr1, and the transistor T does not enter during a momentary interruption.
Never turn off r1. As a result, even if the commercial power supply is momentarily cut off, the power is supplied to the system, so that the system operates normally.

When the commercial power supply is turned off, the output of the voltage detection circuit 6 becomes L level and the switching control circuit turns off the switching signal after a predetermined time, whereby the transistor Tr1 is turned off. As a result, the supply of power to the system from the line 2b on the system power supply side is stopped. In this case, power is continuously supplied from the voltage adjusting circuit 5 to the memory on the line 3b on the memory backup power receiving side.

As described above, according to the present embodiment, the system and memory are supplied with power from the rectifier circuit 1 when the commercial power source is normal, and the power from the battery 4 is supplied from the battery 4 when the commercial power source is interrupted. Since the power is supplied to the power supply device, the load on the power supply device is smaller than that in the first embodiment shown in FIG.
As a result, the temperature rise of the transistor Tr1 and the voltage adjusting circuit 5 is small and the entire device can be downsized.

In the second embodiment, like the first embodiment, the present invention can be implemented in the same manner even if the voltage detection circuit 6 is omitted.

[0055]

According to the first aspect of the present invention, the backup power supply is used even when the commercial power supply is instantaneously cut off, so that the power supply for the measure against instantaneous interruption and the power supply for the memory backup are unified. Therefore, the device can be made smaller than the conventional device, and the cost can be reduced.

According to the second aspect of the present invention, the backup power supply is used even when the commercial power supply is momentarily cut off, so that the power supply for the measure against instantaneous interruption and the memory backup power supply are made common, and when the commercial power supply is normal, DC power from the rectifier circuit is supplied to the system and memory as it is. Therefore, the load when the commercial power supply is normal is reduced, the size is made smaller than the conventional device, and the cost is further reduced.

According to the third aspect of the present invention, the voltage level of the DC power supply is monitored by the voltage detection circuit and the switching circuit is operated, so that the operation can be performed accurately, and the device can be made smaller than the conventional device and the cost can be reduced. .

According to the fourth aspect of the invention, the voltage level of the DC power supply is monitored by the voltage detection circuit and the switching circuit is operated, so that the circuit can be operated accurately and can be made smaller than the conventional device and the cost can be reduced. .

According to the invention of claim 5, since the secondary battery or the large-capacity capacitor is used as the charge storage means, the life can be extended.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a power supply device showing a first embodiment of the present invention.

FIG. 2 is a configuration diagram showing an example of the voltage detection circuit of FIG.

3 is a configuration diagram showing an example of the voltage adjustment circuit of FIG.

FIG. 4 is an explanatory diagram showing an operation of the power supply device of FIG. 1.

FIG. 5 is a circuit diagram of a power supply device showing a second embodiment of the present invention.

FIG. 6 is a circuit diagram of a power supply device showing a conventional example.

[Explanation of symbols]

 1 Rectifier circuit 2b System power supply receiving side line 3b Memory backup power supply receiving side line 4 Battery 5 Voltage adjustment circuit 6 Voltage detection circuit

Claims (5)

[Claims] 1. A power supply device for maintaining the supply of power to a system and a memory even if a commercial power supply is momentarily cut off, and a rectifier circuit for rectifying the commercial power supply to convert it into a direct-current power supply; A power supply circuit which charges the charge storage means with a DC power supply obtained from the rectifier circuit at a normal time and constantly supplies a DC power supply to the memory; a switching circuit which supplies a DC power supply from the power supply circuit to the system; A switching control circuit that turns on the switching circuit when the power supply is normal, and turns off the switching circuit when the abnormal period exceeds a predetermined instantaneous interruption time when the commercial power supply is abnormal. Characteristic power supply device. 2. A power supply device for maintaining the supply of power to a system and a memory even if the commercial power supply is momentarily cut off, and a rectifying circuit for rectifying the commercial power supply to convert it into a DC power supply, A first power supply circuit that supplies a DC power supply obtained from the rectifier circuit to the system and the memory when the commercial power supply is normal; A second power supply circuit that supplies DC power to the memory when the power supply is abnormal, a switching circuit that supplies DC power to the system from the second power supply circuit, and an ON operation of the switching circuit when the commercial power supply is normal On the other hand, when the commercial power supply is abnormal and the abnormal period exceeds a predetermined instantaneous interruption time, the switching circuit turns off the switching circuit. Power supply apparatus characterized by comprising a control circuit. 3. A power supply device for maintaining the supply of power to a system and a memory even if the commercial power supply is momentarily cut off, and a rectifier circuit for rectifying the commercial power supply to convert it into a DC power supply, A power supply circuit which charges the charge storage means with a DC power supply obtained from the rectification circuit at a normal time and constantly supplies a DC power supply to the memory; a switching circuit which supplies a DC power supply from the power supply circuit to the system; A voltage detection circuit that monitors the voltage level of the DC power source obtained from the circuit and detects whether the commercial power source is normal or abnormal according to the voltage level and the switching circuit is turned on when the commercial power source is normal, while And a switching control circuit for turning off the switching circuit after a predetermined time when the commercial power supply becomes abnormal. Power supply device to be. 4. A power supply device for maintaining the supply of power to a system and a memory even when the commercial power supply is momentarily cut off, and a rectifying circuit for rectifying the commercial power supply to convert it into a DC power supply, A first power supply circuit that supplies a DC power supply obtained from the rectifier circuit to the system and the memory when the commercial power supply is normal; A second power supply circuit that supplies a DC power supply to the memory when the power supply is abnormal, a switching circuit that supplies a DC power supply from the second power supply circuit to the system, and a voltage level of the DC power supply obtained from the rectifier circuit is monitored. The voltage detection circuit that detects whether the commercial power supply is normal or abnormal according to the voltage level and the switching circuit is turned on when the commercial power supply is normal. To one, the power supply apparatus characterized by comprising a switching control circuit for turning off operation of the switching circuit after a predetermined time when the commercial power source becomes abnormal. 5. The power supply device according to claim 1, wherein the charge storage means is a secondary battery or a large-capacity capacitor.