JP2003244961A - Power source circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a power source circuit in which a malfunction is prevented and a stable display can be performed when a power supply from an AC power source is eliminated. <P>SOLUTION: The power source circuit comprises a DC power source 2, for receiving a power from the AC power source 1 to generate DC power, a drive power source 4 for receiving the DC power from the power source 2 to supply DC power, an AC power source detecting means 12 for determining the presence or absence of the power from the AC power source 1, a discharge means 11 for discharging charge stored in the power source 4, a control means 5 for operating the discharge means 11 when it is determined that power from the AC power source 1 is not supplied, and a control means 5 for controlling an operation signal. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power supply circuit for a general-purpose electronic device.

[0002]

2. Description of the Related Art In a conventional power supply circuit, the voltage drop of a DC power supply when the power supply from an AC power supply is cut off is determined by the load connected to the DC power supply circuit. The load is generally set small in order to reduce the output capacity of the DC power supply circuit.

The structure of a conventional power supply circuit will be described below with reference to the drawings. FIG. 7 is a block diagram of a power supply circuit used for a board or the like of a conventional electronic device. Reference numeral 1 is an AC power source such as a commercial power source, 2 is a DC power source that receives power from the AC power source 1 to generate DC power, 3 is a display means for displaying the state of the device, such as a light emitting diode, and 4 is a DC power source. A drive power source for supplying DC power from the DC power supply 2 to the display means 3; a control means 5 for controlling the apparatus, such as a microcomputer; and 6 a DC power supply from the DC power supply 2. Is a control power supply for supplying DC power to the control means 5, and 7 is a backup power supply for supplying DC power to the control power supply 6 when there is no power supply from the AC power supply 1. Reference numeral 8 is a key input means for sending a signal to the control means 5 by key input.

FIG. 8 is a power supply circuit diagram showing the circuit example of FIG. The same functions as those in FIG. 7 are designated by the same reference numerals, and the description thereof will be omitted. The DC power supply 2 is composed of diodes 2a to 2d connected to the AC power supply 1 and an electrolytic capacitor 2e. The driving power supply 4 is composed of a diode 4a connected to the DC power supply 2 and an electrolytic capacitor 4b, and the control power supply 6 is composed of a diode 6a connected to the DC power supply 2 and an electrolytic capacitor 6b. Here, the driving power supply 4 is represented by Vcc and the control power supply 6 is represented by Vdd.
The backup power supply 7 comprises a diode 7a connected to the control power supply 6 and a battery 7b, and the battery 7b is connected to Vdd of the control power supply 6 via the diode 7a.

The key input means 8 comprises a series circuit of a resistor 8a and a key switch 8b which are connected in parallel to the electrolytic capacitor 4b of the driving power source 4. The display means 3 is a resistor 3a
And a light emitting diode 3b. The control means 5 comprises a micro coin computer, and the power terminal 5a of the micro coin computer is the control power source 6 and the output terminal 5b.
Is to the driving power source 4 via the resistor 3a of the display means 3 and the light emitting diode 3b, the input terminal 5c is to the driving power source 4 via the resistor 8a of the key input means 8, and to the ground via the key switch 8b. Connected.

In this structure, the AC power from the AC power supply 1 is rectified by the diodes 2a to 2d of the DC power supply 2 and smoothed by the electrolytic capacitor 2e to generate the DC power. The DC power generated by the DC power supply 2 is the driving power supply 4
Is rectified by the diode 4a, stabilized by the electrolytic capacitor 4b, and supplies DC power to the display means 3. On the other hand, the DC power rectified by the diode 6a of the control power supply 6 and stabilized by the electrolytic capacitor 6b is supplied to the control means 5. The backup power supply 7 supplies DC power to the control means 5 when the power supply from the AC power supply 1 is lost. The display means 3 receives the DC power from the driving power source 4 and displays the state of the device by the signal from the control means 5. The key input means 8 receives DC power from the driving power supply 4 and sends a signal to the control means 5 by operating the key switch 8b.

[0007]

In such a conventional power supply circuit, the voltage drop of the drive power supply 4 and the control power supply 6 is very gentle when the power supply from the AC power supply 1 is stopped. . Further, when the DC power supply is divided into two or more for the purpose of backup, etc., the respective voltage drop rates are different. Due to these factors, the following problems may occur.

FIG. 9 shows a driving power supply 4 in the conventional power supply circuit when the power supply from the AC power supply 1 is stopped.
And changes in the power supply voltage of the control power supply 6.
In the figure, when the power supply from the AC power supply 1 is stopped, the power supply voltages of the driving power supply 4 and the control power supply 6 gradually decrease. The rate of decrease at this time is determined by the magnitude of the load connected to each of the drive power source 4 and the control power source 6. In the case of this example, the Vcc of the driving power supply 4 gradually decreases, and the Vdd of the control power supply 6 decreases more than that. The Vdd of the control power supply 6 is maintained at about 3V by the backup power supply 7.

The electrical rating of the input terminal 5c of the microcomputer constituting the control means 5 is given by the following equation.
Generally, the upper limit of the input voltage Vin is set to a value higher by 0.3 V than the power supply voltage Vdd. Vin ≦ Vdd + 0.3 [V] If this rating is exceeded, a through current flows from the input terminal 5c to the power supply terminal 5a, causing malfunctions due to abnormal heat generation of the microcomputer, malfunctions due to disturbances in the power supply voltage or noise, etc. There is a case.

As described above, in the conventional power supply circuit, as shown in FIG. 9, Vcc of the drive power supply 4 and Vd of the control power supply 6 are set.
The rate of decrease of d is different, and the Vcc of the driving power supply 4 may be higher than the Vdd of the control power supply 6 by 0.3 V or more, which may cause the above problem.

Further, the conventional power supply circuit may have the following problems. FIG. 10 is a block diagram when an LCD is used as the display means 3. The difference from FIG. 7 described above is that the DC power is supplied to the display means 3 from the drive power source 4 to the control power source 6. By using an LCD, the power consumption of the display means 3 can be reduced, and even if the AC power source 1 does not supply power, the backup power source 7 supplies power to display. .

FIG. 11 shows a circuit example of the display means 3 when an LCD is used as the display means 3. Other parts are the same as those in FIG. 8 described above. Components having the same functions as those described above are designated by the same reference numerals and the description thereof will be omitted. In the figure, 3c is an LCD which is a display means, and receives a signal from the control means 5 to perform display. The signal from the control means 5 to the LCD 3c is Vdd of the control power supply 6.
Is provided by the LCD power supply which is obtained by dividing the voltage with a resistor. Here, the LCD power supply is represented by Vlcd.

When power is supplied from the AC power supply 1, both the Vdd of the control power supply 6 and the Vcc of the drive power supply 4 are about 5V. At this time, the transistor 3d is biased from the driving power source 4 by the resistor 9 and the resistor 10, and thus is turned on. When the transistor 3d is turned on, the control power supply 6 is switched to the resistor 3e (10K
Ω) and a resistor 3f (15 KΩ) to divide the voltage to a ratio of 3/5, and the value of Vlcd of the LCD power supply becomes about 3V. Actually, the resistor 3f has a resistor 3g (390 KΩ) and the resistor 3h (3
90KΩ) and a resistor 3i (390KΩ) are connected in parallel, but compared to the value of the resistor 3f, the resistor 3f
The series resistance of g, the resistance 3h, and the resistance 3i is very large, and its influence is ignored.

When no power is supplied from the AC power supply 1, Vdd of the control power supply 6 is about 3V supplied from the backup power supply 7, and Vcc of the drive power supply 4 is about 0V. At this time, the transistor 3d is connected to the V of the driving power source 4.
cc is about 0V and is off. When the transistor 3d is off, Vdd of the control power supply 6 is divided by the series resistance of the resistors 3e and 3g, the resistor 3h, and the resistor 3i, and the value of Vlcd of the LCD power supply becomes the same as Vdd, but it is backed up. Since about 3V is supplied from the power supply 7, it becomes about 3V. In reality, a voltage drop also occurs in the resistor 3e, but the series resistance of the resistor 3g, the resistor 3h, and the resistor 3i is very large compared to the value of the resistor 3e, and its influence is ignored.

In the conventional power supply circuit having such a configuration, the operation when the power is supplied from the AC power supply 1 and when it is lost will be described with reference to FIG. The upper diagram (A) shows changes in Vcc of the driving power supply 4 and Vdd of the control power supply 6 when the power supply from the AC power supply 1 disappears. Lower figure (B)
Indicates the change in Vlcd of the LCD power supply at that time.

The time T0 is the time when the power supply from the AC power supply 1 is stopped. When the power supply from the AC power supply 1 is stopped at the time T0, the Vdd of the control power supply 6 is lowered to about 3V which is the voltage level maintained by the backup power supply 7 at the time T1. Vc of driving power source 4
Similarly, c also decreases and becomes about 0 V at time T2. In this example, the driving power source 4 and the control power source 6 have the same rate of voltage drop. With the decrease in Vcc of the driving power supply 4, the state of the transistor 3d changes from ON to OFF at time Ta. The Vlcd of the LCD power supply is V1c of the control power supply 6 because the transistor 3d is ON before the time Ta.
The voltage division ratio dd is divided into 3/5 by the resistors 3e and 3f. When the state of the transistor 3d changes from ON to OFF at time Ta, Vlcd of the LCD power supply becomes substantially equal to Vdd of the control power supply 6 as described above.

Vlcd of the LCD power supply is from T0 to T
In the period of “a”, it is lower than the original set value of about 3V. Generally, the contrast of LCD display segment is
It depends on the magnitude of the power supply voltage Vlcd. When Vlcd of the LCD power supply is lowered as in this example, the lightness and darkness of the display segment becomes light, and the visibility is sacrificed. In other words, the display flickers temporarily.

As described above, in the conventional power supply circuit, due to the difference in the rate of voltage drop between the two DC power supplies, the drive power supply 4 and the control power supply 6, the microcomputer constituting the control means 5 receives a through current. However, there is a problem in that a malfunction may occur due to heat generation of the microcomputer, noise, and the like. Further, in order to reduce the output capacity of the DC power supply 2, the load connected to the drive power supply 4 and the control power supply 6 is generally set to be small, and the voltage drop at this time is very gentle. Is. In such a case, the LCD of the display means consisting of the LCD
The switching timing of the voltage division ratio of the power supply is delayed, and the LCD
There is a problem that flicker such as lightness and darkness of the display of is displayed.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a power supply circuit capable of preventing malfunction and providing stable display when power supply from an AC power supply is cut off. To do.

[0020]

DISCLOSURE OF THE INVENTION A power supply circuit according to the present invention is a DC power supply which receives power from an AC power supply to generate DC power, and a DC power supply which receives DC power from the DC power supply. A driving power supply for supplying power, an AC power supply detecting means for determining whether or not power is supplied from the AC power supply, a discharging means for discharging electric charge stored in the driving power supply, and the AC power supply detecting means for Control means for operating the discharging means when it is determined that no power is supplied from the AC power supply.

Further, the key input means for inputting the operation signal to the control means is provided, and the key input means also serves as the discharge means.

Further, the control power supply for supplying the DC power from the DC power supply to the control means and the DC power for the control power supply when the AC power supply is not supplied. A backup power source and a display unit including an LCD for displaying an operating state are provided, and DC power is supplied from the control power source to the display unit.

[0023]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiment 1. 1 is a block diagram showing the configuration of a power supply circuit according to a first embodiment of the present invention, FIG.
3 is a power supply circuit diagram showing the circuit example of FIG. 1, and FIG. 3 is a diagram showing changes in the power supply voltage of the power supply circuit. Reference numeral 1 is an AC power source such as a commercial power source, 2 is a DC power source that receives power from the AC power source 1 to generate DC power, 3 is a display means for displaying the state of the device, such as a light emitting diode, and 4 is a DC power source. A drive power source for supplying DC power from the DC power supply 2 to the display means 3; a control means 5 for controlling the apparatus, such as a microcomputer; and 6 a DC power supply from the DC power supply 2. Is a control power supply for supplying DC power to the control means 5, and 7 is a backup power supply for supplying DC power to the control power supply 6 when there is no power supply from the AC power supply 1.
Reference numeral 8 is a key input means for sending a signal to the control means 5 by key input. Reference numeral 11 is a discharging means for discharging the electric charge stored in the driving power source 4 according to the signal of the control means 5. 12
Is an AC power supply detection means for determining whether or not power is supplied from the AC power supply 1.

In FIG. 2, the same parts as those in FIG. 1 are designated by the same reference numerals and the description thereof will be omitted. The DC power supply 2 is composed of diodes 2a to 2d connected to the AC power supply 1 and an electrolytic capacitor 2e. The driving power supply 4 is composed of a diode 4a connected to the DC power supply 2 and an electrolytic capacitor 4b, and the control power supply 6 is composed of a diode 6a connected to the DC power supply 2 and an electrolytic capacitor 6b. Here, the driving power supply 4 is represented by Vcc and the control power supply 6 is represented by Vdd. The backup power supply 7 is composed of a diode 7a connected to the control power supply 6 and a battery 7b, and the battery 7b is connected to Vdd of the control power supply 6 via the diode 7a.

The key input means 8 comprises a series circuit of a resistor 8a and a key switch 8b which are connected in parallel to the electrolytic capacitor 4b of the driving power source 4. The display means 3 is a resistor 3a
And a light emitting diode 3b. The control means 5 is composed of a micro coin computer, the power supply terminal 5a of the microcomputer is for the control power supply 6, the output terminal 5b is for the drive power supply 4 via the resistor 3a and the light emitting diode 3b of the display means 3, and the input terminal 5c. Is the resistance 8 of the key input means 8.
It is connected to the driving power source 4 via a and to the ground via the key switch 8b.

The discharging means 11 comprises a transistor 11b whose base is connected to the control means 5, whose collector is connected to the positive electrode of the driving power source 4 via the resistor 11a, and whose emitter is connected to the negative electrode of the driving power source 4. Has been done. The input side of the AC power supply detection means 12 is connected to the AC power supply 1 and the output side is connected to the control means 5.

Next, the operation of the power supply circuit of the first embodiment configured as described above will be described with reference to FIGS.
Normally, the AC power from the AC power supply 1 is rectified by the diodes 2a to 2d of the DC power supply 2 and smoothed by the electrolytic capacitor 2e to generate DC power. The DC power generated by the DC power supply 2 is rectified by the diode 4a of the driving power supply 4 and stabilized by the electrolytic capacitor 4b to supply the DC power to the display means 3. On the other hand, the diode 6 of the control power supply 6
DC power rectified by a and stabilized by the electrolytic capacitor 6b is supplied to the control means 5. The display means 3 receives the DC power from the driving power source 4 and displays the state of the device by the signal from the control means 5. The key input means 8 receives DC power from the driving power supply 4 and sends a signal to the control means 5 by operating the key switch 8b.

When the power supply from the AC power supply 1 is lost, the backup power supply 7 supplies DC power to the control means 5. At the same time, the output of the AC power supply detection means 12 is OF
At F, the control means 5 outputs a signal to the discharging means 11 to turn on the transistor 11b of the discharging means 11 to discharge the electric charge stored in the driving power source 4.

Now, changes in the power supply voltages of the driving power supply 4 and the control power supply 6 when the power supply from the AC power supply 1 is stopped will be described with reference to FIG. In the figure, time T0 is the time when the power supply from the AC power supply 1 is stopped.
When the power supply from the AC power supply 1 is stopped at time T0, Vdd of the control power supply 6 is lowered, and the voltage level maintained by the backup power supply 7 at time T1 is about 3
It becomes V. Similarly, Vcc of the driving power source 4 starts to decrease, but since the electric charge stored in the driving power source 4 is discharged from the discharging means 11, the rate of decrease is fast and about 0 V at time T2.
Becomes

The rate of voltage drop of the driving power source 4 at this time is determined by setting the value of the resistor 11a of the discharging means 11. Further, the rate of voltage drop of the control power supply 6 is determined by the size of the load connected to the control power supply 6.
In the present embodiment, the rate of voltage drop of Vcc of drive power supply 4 is larger than that shown in FIG. 9 showing the case of the conventional power supply circuit, and the power supply voltage is lower than Vdd of control power supply 6. ing.

The electrical rating of the input terminal 5c of the microcomputer constituting the control means 5 is as shown in the following equation.
Generally, the upper limit of the input voltage Vin is set to a value higher by 0.3 V than the power supply voltage Vdd. Vin ≦ Vdd + 0.3 [V] If this rating is exceeded, a through current flows from the input terminal 5c to the power supply terminal 5a, causing malfunctions due to abnormal heat generation of the microcomputer, malfunctions due to disturbances in the power supply voltage or noise, etc. There is a case.

However, as described above, in the power supply circuit according to the first embodiment of the present invention, Vcc of the drive power supply 4 is set to be lower than Vdd of the control power supply 6, and the drive power supply 4 is set. Vcc never becomes higher than Vdd of the control power source 6 by 0.3V.

As described above, a malfunction can be prevented when the power supply from the AC power supply is cut off.

Embodiment 2. FIG. 4 is a block diagram of a power supply circuit showing a second embodiment of the present invention, and FIG. 5 is a view showing changes in the power supply voltage and the power supply voltage of the display means. In the present embodiment, an LCD is used as the display means, and direct-current power is supplied to the display means from the driving power source 4 in the first embodiment, but is controlled from the control power source 6.

FIG. 4 is a diagram in which an LCD is used as the display means 3 in FIG. 1 of the first embodiment, and direct-current power is supplied to the display means 3 from the control power supply 6. The circuit of the display means 3 is shown in FIG. An example is shown in FIG. 11 which is a circuit example of a conventional power supply circuit.
Is the same as The other parts are the same as those in FIG. 2 of the first embodiment, and therefore the description of the configuration will be omitted.

Next, the operation of the power supply circuit according to the second embodiment having the above-described structure when the power is supplied from the AC power supply 1 to when the power is not supplied will be described with reference to FIG. The upper diagram (A) shows Vc of the driving power source 4 when the power is supplied from the AC power source 1 to when it is lost.
c represents the change in Vdd of the control power supply 6.
The lower diagram (B) shows the change of Vlcd of the LCD power supply at that time.

The time T0 is the time when the power supply from the AC power supply 1 is stopped. When the power supply from the AC power supply 1 is stopped at time T0, the output of the AC power supply detection means 12 is turned off, and the control means 5 outputs a signal to the discharge means 11 to turn on the transistor 11b of the discharge means 11, The electric charge stored in the driving power source 4 is discharged.

The rate of the voltage drop of the driving power source 4 at this time is determined by the setting of the resistor 11a of the discharging means 11. Further, the rate of voltage drop of the control power supply 6 is determined by the size of the load connected to the control power supply 6. Figure 5
As shown in (A), it can be seen that the rate of the voltage drop of Vcc of the driving power supply 4 is larger than that shown in FIG. 12 showing the case of the conventional power supply circuit.

The Vdd of the control power source 6 also decreases when the power supply from the AC power source 1 is stopped, and is maintained at about 3V by the backup power source 7 at time T2. The Vcc of the driving power source 4 also drops and becomes about 0 V at time T3. With the decrease in Vcc of the driving power source 4, the state of the transistor 3d of the display means 3 changes from ON to OFF at time Tb. The Vlcd of the LCD power supply is Vdd of the control power supply 6 because the transistor 3d is ON before the time Tb.
Is divided into a voltage division ratio of 3/5 by the resistors 3e and 3f. At time Tb, the state of the transistor 3d is ON
When it changes from OFF to OFF, Vlcd of the LCD power supply becomes about 3V which is almost equal to Vdd of the control power supply 6.

Vlcd of the LCD power supply is from time T0 to T
In the period of b, it is lower than the original set value.
Generally, the lightness and darkness of the display segment of the LCD depends on the magnitude of Vlcd which is the power supply voltage. As in this example, L
In the case where the Vlcd of the CD power source is lowered, the lightness and darkness of the display segment becomes light and the visibility is lowered.
In other words, the display flickers temporarily.

However, in the present embodiment, the driving power source 4
The rate of voltage drop of Vcc of
FIG. 12 showing the case of a conventional power supply circuit by setting a.
Since it is larger than that shown in FIG. 12, the time from time T0 to time Tb shown in FIG. 5 is much shorter than the time from time T0 to time Ta in FIG. . In this way, the time from the time T0 to the time Tb can be made as short as possible so that there is almost no effect on the visibility, and it is possible to prevent the occurrence of flicker such as light and shade of the display segment of the LCD.

As described above, the malfunction can be prevented, the power consumption of the display means 3 can be reduced, and even if the AC power source 1 does not supply the power, the backup power source 7 stabilizes the power supply. The displayed information can be displayed.

Embodiment 3. 6 is a power supply circuit diagram showing a third embodiment of the present invention. In the first embodiment, the discharging means is provided, but in the present embodiment, the key input means also serves as the discharging means. FIG. 6 is a diagram in which the discharging means 11 is omitted from FIG. 2 of the first embodiment, and 5d of the control means 5 is an input / output terminal for performing input and output. The other configuration is the same as that of FIG. 2 and its explanation is omitted.

The operation of the power supply circuit according to the third embodiment thus configured will be described with reference to FIG. When the key input means 8 functions as the key input means, the control means 5
The input / output terminal 5d functions as an input terminal. Terminal 5d
Is connected to the driving power source 4 via the resistor 8a. When the key switch 8b is OFF, the state of the input / output terminal 5d is "High" (about 5V), and the key switch 8b is O.
When it is N, the state of the input / output terminal 5d is "Low" (about 0
V). The control means 5 determines the input level and starts the operation of the device.

On the other hand, when the key input means 8 functions as a discharge means, the input / output terminal 5d of the control means 5 functions as an output terminal. That is, when power is supplied from the AC power supply 1, the output of the input / output terminal 5d of the control means 5 is controlled to be "high", and the terminal 5d has substantially the same potential as Vcc of the driving power supply 4. Has become. Then, when the power supply from the AC power supply 1 is stopped, the output of the AC power supply detection means 12 is turned off and the output of the input / output terminal 5d of the control means 5 is controlled to be "Low" (about 0 V). When the output of the input / output terminal 5d becomes "Low", the electric charge stored in the driving power source 4 is discharged through the resistor 8a of the key input means 8.

Therefore, as in the first embodiment (FIG. 3),
The rate of voltage drop of Vcc of the driving power supply 4 is larger than that of the conventional power supply circuit, and the power supply voltage is lower than Vdd of the control power supply 6. Vcc of the driving power source 4 is
It is never higher than Vdd of the control power source 6 by 0.3V.

As described above, with a simple structure, malfunctions can be prevented and the device can be made inexpensive.

[0048]

As described above, according to the present invention, a DC power supply which receives power supplied from an AC power supply to generate DC power,
A driving power supply that receives DC power from the DC power supply to supply DC power, an AC power supply detection unit that determines whether or not power is supplied from the AC power supply, and an electric charge stored in the driving power supply. And a control means for operating the discharging means when it is determined by the AC power supply detection means that no power is supplied from the AC power supply. It is possible to prevent malfunction when the supply is exhausted.

Further, since the key input means for inputting the operation signal to the control means is provided, and the key input means also serves as the discharge means, malfunction can be prevented with a simple structure.

Further, the control power supply for receiving the DC power from the DC power supply to supply the DC power to the control means, and the DC power for the control power supply when the AC power supply does not supply the power. A backup power source and a display unit including an LCD for displaying an operation state are provided, and since DC power is supplied from the control power source to the display unit, a malfunction occurs when the power supply from the AC power source is lost. Prevents and provides stable display.

[Brief description of drawings]

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

FIG. 2 is a circuit diagram of a power supply circuit showing the first embodiment of the present invention.

FIG. 3 is a graph illustrating changes in the power supply voltage of the power supply circuit according to the first embodiment of the present invention.

FIG. 4 is a block diagram of a power supply circuit showing a second embodiment of the present invention.

FIG. 5 is a graph illustrating changes in the power supply voltage of the power supply circuit according to the second embodiment of the present invention.

FIG. 6 is a circuit diagram of a power supply circuit showing a third embodiment of the present invention.

FIG. 7 is a block diagram showing a conventional power supply circuit.

FIG. 8 is a circuit diagram showing a conventional power supply circuit.

FIG. 9 is a graph illustrating changes in the power supply voltage of the conventional power supply circuit.

FIG. 10 is a block diagram showing another conventional power supply circuit.

FIG. 11 is a circuit diagram showing a display means of another conventional power supply circuit.

FIG. 12 is a graph illustrating changes in the power supply voltage of another conventional power supply circuit.

[Explanation of symbols]

1 AC power supply, 2 DC power supply, 3 display means, 4 drive power supply, 5 control means, 6 control power supply, 7 backup power supply, 8 key input means, 11 discharge means, 12
AC power supply detection means.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Wataru Fujimoto             1728 Omaeda, Hanazono-cho, Osato-gun, Saitama 1728               Within Mitsubishi Electric Home Equipment Co., Ltd. F-term (reference) 5H006 AA04 BB06 CA01 CA07 CB01                       CC08 DA04 DB01 DC05

Claims (3)

[Claims] 1. A direct-current power supply for receiving direct-current power from an alternating-current power supply to generate direct-current power; a drive power supply for receiving direct-current power from the direct-current power supply to supply direct-current power; AC power supply detection means for determining the presence or absence of power supply, a discharge means for discharging the electric charge stored in the driving power supply, and the AC power supply detection means determines that there is no power supply from the AC power supply. A power supply circuit comprising: a control unit that operates the discharge unit when 2. The power supply circuit according to claim 1, further comprising a key input unit for inputting an operation signal to the control unit, wherein the key input unit also serves as a discharge unit. 3. A control power supply for supplying DC power from a DC power supply to the control means, and a DC power supply for the control power supply when no power is supplied from the AC power supply. 3. The power supply circuit according to claim 1, further comprising: a backup power supply; and a display unit including an LCD that displays an operating state, wherein direct-current power is supplied from the control power supply to the display unit.