JPH0876213A - Power supply circuit, load device and camera provided with power supply circuit - Google Patents

Abstract

PURPOSE: To interrupt the short circuit or the overcurrent of a power source, and to rapidly restart the supply of power when such a state eliminated by providing a voltage monitoring means and a bias means impressing bias voltage on a power source line. CONSTITUTION: This circuit is provided with a switching means consisting of a transistor 12 and a resistance 14, the voltage monitoring means consisting of a transistor 17 and resistances 14, 16, 18 and 19, and the bias means consisting of a bias resistance 13. When the short circuit of a power source connector 15 or voltage drop by overload is detected, the transistor 12 is made in a non-conductive state so as to rapidly interrupt the overcurrent generated in a power source 11. Therefore, the fault of the power source 11 is avoided and safety is improved. When the short circuit or the overload of the connector 15 has been eliminated, the transistor 12 is rapidly made into conductive state by the bias resistance 13, thereby the power supply to an external load device is rapidly restarted.

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 cutting off power supply in a short-circuit state or an overload state, and a load device supplied with power by the power supply circuit.

[0002]

2. Description of the Related Art Conventionally, a large number of strobe devices, motor drive devices, and other external accessories have been prepared for camera devices in order to perform a variety of photographing operations.

Many of these external accessories require power supply from the camera side, and a circuit for supplying power to the camera side (hereinafter referred to as "power supply circuit").
Was provided.

In such a power supply circuit for a camera, the power supply on the camera side is connected to an external accessory via an electric contact terminal such as a power connector. In addition, in a power supply circuit provided in a general electric device, a small fuse is provided in a power supply line to protect the power supply from an overcurrent generated by a short circuit of the power supply.

[0005]

However, in the power supply circuit of the camera, the power supply on the camera side cannot be protected from the short circuit of the power supply line by the external accessory or the overcurrent due to the overload.

Therefore, the power source generates heat due to overcurrent,
In the worst case, there was a problem that an accident occurred. In addition, when an overcurrent flows through the external accessory, the voltage on the camera side also drops, which causes a problem that the operation of the camera becomes unstable or becomes inoperable.

Further, the power supply voltage is output to the power supply connector provided on the camera side with low impedance even when an external accessory is not connected. Therefore, there has been a problem in that a short circuit accident of the power supply connector occurs due to raindrops or water immersion.

In general electric equipment, when an overcurrent occurs in the power supply, the fuse of the power supply circuit is blown to protect the power supply. However, in order to supply power again,
There was a problem that it was necessary to replace the fuse anew and it took time to restore the power supply.

The present invention according to claims 1, 2, 3 and 6 cuts off an overcurrent due to a short circuit of a power source or an overload state,
Moreover, it is an object of the present invention to provide a power supply circuit that quickly restarts power supply when these conditions are resolved.

In addition to the above objects, the inventions according to claims 4 and 5 prevent the short circuit accident of the power supply connector in the unconnected state, and start the power supply in accordance with the connection of the load device (external accessory). An object of the present invention is to provide a power supply circuit and its load device.

[0011]

According to a first aspect of the present invention, there is provided a switch which is connected between an output end of a power supply and a power supply line and connects and disconnects a current supplied from the power supply to the load device through the power supply line. And a voltage monitoring means for comparing the voltage of the power supply line with a predetermined threshold value, and setting the switch means to the closed setting if the threshold value or more, and an open setting if the voltage value is less than the threshold value, and a switching means connected to the power supply line. And a bias means for applying a bias voltage equal to or higher than a threshold value to the power supply line in a state in which is set to open.

According to a second aspect of the present invention, in the power supply circuit according to the first aspect, the switch means is a first transistor having an emitter terminal connected to an output terminal of the power source and a collector terminal connected to the power source line. And a first resistor connected between the base and the emitter of the first transistor, the voltage monitoring means includes a second transistor whose emitter terminal is grounded, and an emitter terminal of the second transistor. A second resistor connected between the base terminal of the second transistor and the base terminal, a base resistor connected between the base terminal of the second transistor and the power supply line, and a base terminal of the first transistor and the second transistor. The bias means is composed of a collector resistance connected between the collector terminal and the collector terminal, and the bias means is composed of a bias resistance connected between the output terminal of the power supply and the power supply line. To.

According to a third aspect of the present invention, in the power supply circuit according to the first aspect, the switch means is a field effect transistor having a drain terminal connected to the output terminal of the power source and a source terminal connected to the power source line. A first transistor having an emitter terminal connected to the output terminal of the power supply and a collector terminal connected to the gate terminal of the field effect transistor; and a first resistor connected between the base and the emitter of the first transistor. The voltage monitoring means comprises a second transistor whose emitter terminal is grounded, and the second transistor.
A second resistor connected between the emitter terminal and the base terminal of the transistor, a base resistor connected between the base terminal of the second transistor and the power supply line, and a base terminal of the first transistor. And a collector resistor connected between the collector terminal of the second transistor and the collector terminal, and the bias means comprises a bias resistor connected between the output terminal of the power supply and the power supply line. To do.

The invention described in claim 4 is the same as in claim 1,
3. The power supply circuit according to any one of 3 above, wherein the switch means is set to open in an initial state, a voltage drop in the power supply line for a predetermined time width or less is detected, and the switch means is changed to closed setting. It is characterized by having.

According to a fifth aspect of the present invention, in the load device connected to the power supply line of the power supply circuit according to the fourth aspect, the load requesting means for causing the voltage of the power supply line to fall below the predetermined time width at the time of startup is provided. It is characterized by

According to a sixth aspect of the present invention, there is provided a camera, comprising the power supply circuit according to any one of the first to fourth aspects, wherein power is supplied to an external accessory through the power supply circuit. Characterize.

[0017]

In the power supply circuit of the first aspect, when an overcurrent flows in the power supply line due to a short circuit or an overload, the voltage of the power supply line drops due to the internal resistance of the power supply and the impedance of the power supply line.

When the voltage of the power supply line becomes less than the threshold value, the voltage monitoring means sets the switch means to the open setting to shut off the overcurrent. In such a state, when the short circuit or overload of the power supply line is eliminated, the bias means applies a voltage above the threshold value to the power supply line, and the voltage monitoring means returns the switch means to the closed setting again.

Therefore, the supply of electric power to the power supply line is promptly restored via the switch means. In the power supply circuit according to the second aspect, the current flowing through the power supply line is supplied to the load device via the first transistor.

By controlling the base current of the first transistor, the current supplied to the load device is connected / disconnected, and the switch means is constituted. A base voltage is applied to the second transistor from the power supply line through the base resistor. The second transistor performs switching with the base-emitter forward voltage V _BE as a threshold value by this base voltage, and opens and closes the switch means by connecting and disconnecting the base current of the first transistor. In this way, the voltage monitoring means is constructed.

The bias resistor constitutes a bias means by applying a bias voltage equal to or higher than the forward voltage V _BE to the base terminal of the second transistor when the first transistor is non-conductive.

According to another aspect of the power supply circuit of the present invention, the current flowing through the power supply line is supplied to the load device through the field effect transistor. The gate voltage of the field-effect transistor is controlled by the first transistor, so that the current supplied to the load device is connected and disconnected to form the switch means. In general, a field effect transistor has a high switching speed, so that a switching means that opens and closes at high speed can be realized.

The base voltage is applied to the second transistor from the power supply line through the base resistor. The second transistor performs switching with the forward-direction voltage V _BE between the base and the emitter as a threshold according to the base voltage, and connects and disconnects the base current of the first transistor to open and close the switch means. In this way, the voltage monitoring means is constructed.

The bias resistor constitutes a bias means by applying a bias voltage equal to or higher than the forward voltage V _BE to the base terminal of the second transistor when the field effect transistor is in the non-conducting state.

According to another aspect of the power supply circuit of the present invention, when the initial state of the switch means is set to open by the starting means, the power supply voltage is not directly applied to the power supply line but the voltage is supplied through the bias means. It

In such a state, if the power supply line is momentarily short-circuited by the electrolytic capacitor or other capacitance connected to the power supply connector on the load device side, a predetermined time
The voltage of the power supply line drops. The start-up means detects the voltage pulse of the predetermined time width or less and closes the switch means to start power supply to the load device.

In the load device of the fifth aspect, the activation requesting means causes the voltage of the power supply line to drop to a predetermined time width or less when the load device is activated. Therefore, the power supply circuit of the fourth aspect detects this voltage drop. Then, the power supply to the load device is started.

In the camera of claim 6, the power source of the camera is
It is supplied to an external accessory via a power supply circuit.

[0029]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below with reference to the drawings. FIG. 1 is a diagram showing an embodiment corresponding to the invention described in claims 1 and 2.

In the figure, a ground wire is connected to the negative terminal of the power source 11, and the emitter of the transistor 12, one terminal of the bias resistor 13 and the resistor 1 are connected to the positive terminal.
One of the four terminals is connected.

The collector of the transistor 12 is connected to the power connector 15, the other terminal of the bias resistor 13 and one terminal of the resistor 16, and the other terminal of the resistor 16 is connected to the base of the transistor 17 and one of the resistors 18. The terminals are connected.

A ground line is connected to the other terminal of the resistor 18 and the emitter of the transistor 17, and the transistor 1
One terminal of the resistor 19 is connected to the collector of 7.
The other terminal of the resistor 19 is connected to the base of the transistor 12 and the other terminal of the resistor 14.

Regarding the correspondence relationship between this embodiment and the invention of claim 1, the transistor 12 and the resistor 14 correspond to the switching means, and the transistor 17 and the resistor 1
Reference numerals 4, 16, 18, and 19 correspond to voltage monitoring means, and the bias resistor 13 corresponds to bias means.

In the power supply circuit having such a structure, the base current of the transistor 17 is supplied to the base terminal of the transistor 17 through the resistor 16, so that the collector-emitter is electrically connected and the transistor 19 of the transistor 12 is connected through the resistor 19. Base current flows.

Therefore, the collector and the emitter of the transistor 12 are electrically connected, and the power supply 11 to the power supply connector 15 are connected.
Power is supplied to the outside via the. When an overcurrent occurs in the power supply 11 due to a short circuit or overload condition of the power supply connector 15, the voltage of the power supply connector 15 drops due to the internal resistance of the power supply 11. Such a power connector 15
When the base voltage of the transistor 17 becomes equal to or lower than the forward voltage V _BE between the base and the emitter due to the voltage drop at, the collector and the emitter become non-conductive.

Therefore, the base current is not supplied to the transistor 12, and the collector-emitter of the transistor 12 is also non-conductive. In this way, the transistor 12
Is turned off, the overcurrent generated in the power supply 11 due to a short circuit or overload is cut off.

In this state, when the short circuit or overload in the power supply connector 15 is eliminated, the current flowing through the bias resistor 13 decreases, and the base voltage of the transistor 17 becomes the forward voltage V _BE between the base and the emitter. As described above, the transistor 17 becomes conductive.

Therefore, the transistor 17 supplies the base current to the transistor 12 and makes the transistor 12 conductive. In this way, the transistor 12 is turned on, so that the power supply from the power supply 11 is quickly restarted.

As described above, in the power supply circuit of this embodiment, the transistor 12 is brought into the non-conducting state by detecting the voltage drop due to the short circuit or overload of the power connector 15, so that the overcurrent generated in the power supply 11 is prevented. Shut off immediately. Therefore, the failure of the power supply 11 can be avoided and the safety can be improved.

When the short circuit or overload condition of the power supply connector 15 is eliminated, the bias resistor 13 causes the transistor 12 to quickly become conductive, so that the power supply to the external load device can be promptly restarted. You can

Next, another embodiment will be described. FIG. 2 is a diagram showing an embodiment corresponding to the invention described in claims 1 and 3. In the figure, the drain of the field effect transistor 22, one terminal of the bias resistor 23 and the power supply terminal of the voltage conversion circuit 24 are connected to the positive terminal of the power supply 21,
The ground line and the ground terminal of the voltage conversion circuit 24 are connected to the negative terminal.

The source of the field effect transistor 22 is
The other terminal of the bias resistor 23, the power supply connector 25 and one terminal of the resistor 26 are connected, and the other terminal of the resistor 26 is connected to the base of the transistor 27 and one terminal of the resistor 28.

A resistor 28 is provided at the emitter of the transistor 27.
Of the transistor 29 and the collector of the transistor 29 are connected, and the emitter of the transistor 29 is connected to the ground line.

At the gate of the field effect transistor 22,
One terminal of the resistor 30 is connected to the collector of the transistor 31, and the other terminal of the resistor 30 is connected to the ground line.

The 12V terminal of the voltage conversion circuit 24 and one terminal of the resistor 32 are connected to the emitter of the transistor 31, and the transistor 3 is connected to the other terminal of the resistor 32.
1 and one terminal of the resistor 33 are connected.

The diode 3 is connected to the other terminal of the resistor 33.
The collector of the transistor 27 is connected through 4 in the forward direction. The microcomputer 3 is connected to the 5V terminal of the voltage conversion circuit 24.
5 is connected to the power supply terminal, and the control terminal of the voltage conversion circuit 24 is connected to the first output terminal of the microcomputer 35. A ground wire is connected to the ground terminal of the microcomputer 35, and one terminal of the resistor 36 is connected to the second output terminal of the microcomputer 35.

To the other terminal of the resistor 36, the base of the transistor 29 and one terminal of the resistor 37 are connected,
A ground line is connected to the other terminal of the resistor 37. A ground wire is connected to the input terminal of the microcomputer 35 via the switch 38.

Regarding the correspondence relationship between the present embodiment and the invention of claim 1, the field effect transistor 22, the transistor 31 and the resistor 32 correspond to the switching means, and the transistor 27, the resistors 33, 26 and 28 monitor the voltage. The bias resistor 23 corresponds to the bias means.

In the power supply circuit having such a configuration, the base current of the transistor 27 is supplied to the base terminal of the transistor 27 through the resistor 26, so that the collector-emitter is electrically connected to the transistor 31 via the resistor 33. Base current is supplied.

Therefore, the collector-emitter of the transistor 31 becomes conductive, and the voltage is applied to the gate of the field-effect transistor 22, so that the field-effect transistor 22 becomes conductive and the power supply 21 is connected to the outside via the power supply connector 25. Power is supplied.

In such a state, when an overcurrent occurs in the power supply 21 due to a short circuit or overload in the power supply connector 25, the internal resistance of the power supply 21 and the field effect transistor 2 are increased.
The on resistance of 2 causes the voltage of the power supply connector 25 to drop. Due to such a voltage drop of the power supply connector 25, the base voltage of the transistor 27 becomes equal to or lower than the base-emitter forward voltage V _BE , and the collector-emitter of the transistor 27 becomes non-conductive.

Therefore, no voltage is applied to the gate of the field effect transistor 22, and the drain-source of the field effect transistor 22 is also non-conductive, so that an overcurrent generated in the power supply 21 due to a short circuit or an overload is quickly interrupted. To be done.

When the short circuit or overload in the power supply connector 25 is eliminated in such a state, the current flowing through the bias resistor 23 is supplied to the base terminal of the transistor 27, the transistor 27 becomes conductive, and the field effect transistor 22. A voltage is applied to the gate of.

Therefore, the field effect transistor 22 becomes conductive, and the power supply from the power supply 21 is promptly restarted.
Further, the microcomputer 35 lowers the voltage of the second output terminal to make the transistor 29 non-conductive, cut off the emitter current of the transistor 27, and make the field-effect transistor 22 non-conductive.

Therefore, the microcomputer 35 can control the switching of the power supply to the power connector 25. As described above, in this embodiment, the same effect as that of the above-described embodiments can be obtained.

Further, in the power supply circuit of the present embodiment, since the field effect transistor 22 is used as the switch means, the switching speed is fast and the short-circuit current can be cut off quickly when the power supply is short-circuited.

Further, in the power supply circuit of this embodiment, the circuit connected to the gate side of the field effect transistor 22 is
Since power is supplied from the voltage output of the voltage conversion circuit 24, no power is supplied to the power supply connector 25 when the voltage conversion circuit 24 is in a non-operating state.

In this way, after the power is supplied from the voltage conversion circuit 24 to the microcomputer 35, the power connector 25
Since the power supply is started, the microcomputer 35 can be surely activated before the power supply to the load device connected to the power connector 25. Therefore, the control of the load device by the microcomputer 35 can be stably started.

Further, the diode 34 is connected to the transistor 27.
Since the voltage converter circuit 24 is in the non-operating state, the reverse current flowing from the power supply 21 to the PN junction between the base and collector of the transistor 27 via the resistors 23 and 26 can be cut off by providing the collector of the transistor. it can.

Next, another embodiment will be described. FIG.
FIG. 6 is a diagram showing an embodiment corresponding to the inventions described in claims 1, 3, 4, 5, and 6.

In the figure, the positive terminal of the power supply 41 provided in the power supply circuit 40 provided in the camera (not shown) is connected to the drain of the field effect transistor 42, one terminal of the bias resistor 43, and the power supply terminal of the voltage conversion circuit 44. Further, the anode of the diode 45 is connected, and the negative terminal is connected to the ground line and the ground terminal of the voltage conversion circuit 44.

The source of the field effect transistor 42 is
The other terminal of the bias resistor 43, the power supply connector 46, one terminal of the resistor 47 and the first input terminal of the microcomputer 48 are connected, and the other terminal of the resistor 47 is connected to the base of the transistor 49 and one of the resistors 50. The terminals are connected.

The emitter of the transistor 49 is connected to the first output terminal of the microcomputer 48 and the other terminal of the resistor 50. In the gate of the field effect transistor 42,
One terminal of the resistor 51 and the collector of the transistor 52 are connected, and the other terminal of the resistor 51 is connected to the ground line.

The 12V terminal of the voltage conversion circuit 44 and one terminal of the resistor 53 are connected to the emitter of the transistor 52, and the other terminal of the resistor 53 and one terminal of the resistor 54 are connected to the base of the transistor 52. It

The diode 5 is connected to the other terminal of the resistor 54.
The collector of the transistor 49 is connected through 5 in the forward direction. The control terminal of the voltage conversion circuit 44 is connected to the second output terminal of the microcomputer 48, and the ground line is connected to the second input terminal of the microcomputer 48 via the switch 56.

A ground wire is connected to the ground terminal of the microcomputer 48, and the cathode of the diode 57 and the cathode of the diode 58 are connected to the power supply terminal. Diode 57
The 5V terminal of the voltage conversion circuit 44 is connected to the anode of, and the cathode of the diode 45 is connected to the anode of the diode 58.

On the other hand, the power supply terminal of the load circuit 61 provided in the external accessory 60 is connected to the power supply connector 46, one terminal of the capacitor 62 and one terminal of the resistor 63, and the other terminal of the capacitor 62 is connected to the other terminal. The other terminal of the resistor 63 and one terminal of the switch 64 are connected. The other terminal of the switch 64 has a ground connector 65.
And the ground terminal of the load circuit 61 is connected.

The signal terminal of the load circuit 61 is connected to the signal terminal of the microcomputer 48 via the signal connector 66. Regarding the correspondence relationship between this embodiment and the inventions of claims 4 and 5, the field effect transistor 42 and the transistor 52 correspond to the switching means, the transistor 49 and the resistors 47 and 54 correspond to the voltage monitoring means, and the bias. The resistor 43 corresponds to biasing means, the microcomputer 48 corresponds to starting means, and the capacitor 62 and the switch 64 correspond to starting requesting means.

FIG. 4 is a flow chart showing the operation of this embodiment. FIG. 5 is a timing chart showing the operation of this embodiment. The operation of this embodiment will be described below.

In the power supply circuit 40 having such a configuration, the microcomputer 48 sets the base of the transistor 52 by setting the first output terminal to the non-grounded state when the switch 56 is set to open (step S1). The current is cut off and the transistor 52 is turned off (step S
2).

Therefore, no voltage is applied to the gate of the field effect transistor 42 through the transistor 52,
The field effect transistor 42 is turned off. In such a state, the load circuit 61 is set by the bias resistor 43.
The current supplied to is limited.

Here, when the switch 64 on the external accessory 60 side is turned on, the voltage of the power supply connector 46 drops in a pulse shape due to the charging current of the capacitor 62 (FIGS. 5a and 5b).

When the microcomputer 48 detects this voltage pulse by monitoring the voltage of the power supply connector 46 (step S3), it activates the voltage conversion circuit 44 (step S4).

In this state, the microcomputer 48 lowers the voltage of the second output terminal and makes the transistor 49 conductive, thereby applying a gate voltage to the field effect transistor 42 to make it conductive (step S4).

In this way, the power supply 41 and the external accessory 60 are connected with low impedance via the field effect transistor 42, and power supply is started (FIG. 5).
c).

In this state, if an excessive current is generated in the power source 41 due to a short circuit or overload of the power source line,
The voltage of the power supply connector 46 drops due to the internal resistance of the power supply 41 and the impedance of the power supply line (FIG. 5d).

When the base voltage of the transistor 49 drops due to this voltage drop, the transistor 49 becomes non-conductive, the gate voltage is not applied to the field effect transistor 42, and the field effect transistor 42 becomes non-conductive.

Therefore, the overcurrent generated in the power supply 41 is cut off (FIG. 5e), and the power supply 41 and the external accessory 60 are connected.
Are connected with a high impedance via a bias resistor 43.

Here, when the short circuit or the overload is eliminated, the current flowing through the bias resistor 43 decreases and the voltage of the power supply connector 46 rises (FIG. 5f). Therefore, the transistor 49 becomes conductive again, the gate voltage is applied to the field effect transistor 42 via the transistor 52, and the field effect transistor 42 becomes conductive.

In this way, the power supply 41 and the external accessory 60 are connected with low impedance through the field effect transistor 42, so that the power supply is restarted quickly (FIG. 5g).

As described above, also in this embodiment, the same effect as that of the above-described embodiment can be obtained. further,
In the power supply circuit of this embodiment, the microcomputer 48
Since the output impedance of the power supply connector 46 in the initial state is set to a high impedance, it is possible to prevent a short circuit accident or the like due to the contact of the power supply connector 46.
It is also possible to prevent a short circuit accident of the power supply connector 46 due to raindrops, water immersion, or the like.

When the external accessory 60 causes a momentary voltage drop in the power supply line, the microcomputer 48 starts power supply, so that power supply can be started in accordance with the connection of the external accessory.

In addition, in the external accessory 60 of this embodiment, the switch 64 can instantaneously drop the voltage of the power supply connector to request the power supply circuit 40 to supply power.

By thus requesting the power supply, it is not necessary to separately transmit the power supply request signal via the signal line, and the number of signals of the signal connector 66 can be reduced.

In this embodiment, the external accessory 60
Although the power supply is requested by opening / closing the switch 64 provided in, the microcomputer 48 detects the voltage drop at the time of connection due to the smoothing capacitor of the power supply unit provided in the external accessory 60 and supplies the power. You may start it.

[0086]

As described above, in the power supply circuit according to the first aspect of the present invention, the voltage monitoring means detects the voltage drop due to the short circuit of the power supply line or the overload, and the switch means is set to the open state. Overcurrent can be cut off.

When the short circuit or overload condition of the power supply line is eliminated, the bias means closes the switch means, so that the power supply to the load device can be restarted quickly.

Therefore, it is not necessary to replace the fuse as in the conventional case, and the labor for restarting the power supply can be greatly reduced. In the power supply circuit according to claim 2, since the power supply circuit is realized by the circuit configuration including the two transistors, the circuit is provided while reliably connecting and disconnecting the power supply in accordance with a short circuit or overload state of the power supply line. It is possible to reduce the size and cost.

In the power supply circuit according to the third aspect, since the power supply circuit is realized by the circuit configuration including the three transistors, the power supply is surely connected / disconnected according to the short circuit of the power supply line or the state of overload. At the same time, it is possible to reduce the size and cost of the circuit.

In the power supply circuit according to the fourth aspect, since the power supply line in the initial state is set to a high impedance by the starting means, it is possible to prevent a short circuit accident due to contact of the power supply connector, and further to prevent raindrops. It is possible to prevent a short-circuit accident due to flooding or flooding.

Further, when the load device causes a voltage drop of a predetermined time width or less in the power supply line, the power supply is started by the starting means, so that the power supply can be performed according to the connection of the load device.

In the load apparatus according to the fifth aspect, the activation requesting means requests the power supply circuit of the fourth aspect to supply power to the load apparatus by lowering the voltage of the power supply line to a predetermined time width or less. can do.

According to the sixth aspect of the present invention, by providing the power supply circuit, it is possible to prevent a short circuit of the battery built in the camera, prevent the battery from being consumed, and, in the worst case, the battery. It is possible to prevent a fire accident.

As described above, in the electric equipment to which the present invention is applied, power supply is promptly restarted when these conditions are eliminated while the overcurrent of the power supply line is promptly cut off to avoid the failure of the power supply. Therefore, safe and reliable transmission and reception of electric power can be performed according to the state of the power supply line.

[Brief description of drawings]

FIG. 1 is a diagram showing an embodiment corresponding to the invention described in claims 1 and 2.

FIG. 2 is a diagram showing an embodiment corresponding to the invention described in claims 1 and 3.

FIG. 3 is a diagram showing an embodiment corresponding to the invention described in claims 1, 3, 4, and 5.

FIG. 4 is a flowchart showing the operation of this embodiment.

FIG. 5 is a timing chart showing the operation of the present embodiment.

[Explanation of symbols]

 11 power supply 12,17 transistor 15 power supply connector 13 bias resistance 14, 16, 17, 18, 19 resistance

Claims (6)

[Claims] 1. A switch means for connecting and disconnecting a current supplied from an output end of a power supply to a load device through a power supply line, and a voltage of the power supply line is compared with a predetermined threshold value. Voltage monitoring means for setting the means to a closed setting and opening setting if less than the threshold value, and bias means for applying a bias voltage equal to or higher than the threshold value to the power supply line when the switch means is set to the open setting. Power supply circuit characterized by. 2. The power supply circuit according to claim 1, wherein the switch means has a first transistor and a first transistor having an emitter terminal connected to an output terminal of the power supply and a collector terminal connected to the power supply line. And a first resistor connected between the base and the emitter of the second transistor, the voltage monitoring means includes a second transistor whose emitter terminal is grounded, and a second transistor whose emitter terminal is grounded, and between the emitter terminal and the base terminal of the second transistor. A second resistor to be connected, a base resistor connected between the base terminal of the second transistor and a power supply line, a base terminal of the first transistor and a collector terminal of the second transistor. The bias means is composed of a bias resistor connected between the output terminal of the power supply and the power supply line. Power supply circuit that. 3. The power supply circuit according to claim 1, wherein the switch means includes a field effect transistor having a drain terminal connected to the output terminal of the power supply and a source terminal connected to the power supply line, and an output terminal of the power supply. An emitter terminal is connected to the field effect transistor, and a collector terminal is connected to the gate terminal of the field effect transistor; and a first resistor connected between the base and the emitter of the first transistor. The monitoring means includes a second transistor whose emitter terminal is grounded, a second resistor connected between the emitter terminal and the base terminal of the second transistor, a base terminal of the second transistor, and a power supply. And a base resistor connected between the line and a base terminal of the first transistor and a collector terminal of the second transistor. It is composed of a collector resistance which is, biasing means, the power supply circuit, characterized in that it is constituted from a connected bias resistor between the output terminal and the power supply line of the power supply. 4. The power supply circuit according to any one of claims 1, 2 and 3, wherein the switch means is set to open in an initial state, and a voltage drop within a predetermined time width of the power supply line is detected, A power supply circuit comprising a starting means for changing the switch means to a closed setting. 5. A load device which is connected to a power supply line of the power supply circuit according to claim 4 and has a start requesting means for lowering the voltage of the power supply line to the predetermined time width or less at the time of start-up. 6. A camera comprising the power supply circuit according to claim 1, and supplying power to an external accessory via the power supply circuit.