JPH0556548A - Overload protector for power supply - Google Patents

Abstract

PURPOSE:To obtain a light and small overload protector for power supply in which a current interrupting switch can be reset automatically to conducting state. CONSTITUTION:A bistable transistor switch 2, interposed between a power supply 1 and a load 4, is turned OFF electrostatically when an overcurrent flows through the load 4 thus protecting the power supply 1 against overload. When the power supply is started, the bistable transistor switch 2 is turned ON by means of a differentiation signal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power supply overload protection device.

[0002]

2. Description of the Related Art Conventionally, there is a current interruption type overload protection device shown in FIG. In the figure, 1 is a power source,
2 is a current cutoff switch, 3 is a current detector, 4 is a load, 5
Is a control signal for turning off the current breaking switch 2, and 6 is a control signal for turning on the current breaking switch 2.

Next, the operation of the above configuration will be described. Power supply 1
The current supplied from is normally supplied to the load 4 via the current cutoff switch 2 and the current detector 3 which are in the conductive state.
When an excessive current flows through the load 4 due to a short circuit in the circuit or an abnormal operation of the device, the current detector 3 detects the excessive current and the control signal 5 turns off the switch 2. Specifically, a breaker is used to generate a magnetic force by the current flowing through the current detector 3, and the magnetic force turns off the switch 2, or a fuse is used as both the switch 2 and the current detector 3, and an excessive current flows. And melt it by self-heating,
Alternatively, a minute resistor is used for the current detector 3, and the relay is driven by the voltage proportional to the current across the current detector 3 to turn off the switch 2. Further, the switch 2 which has been once in the cut-off state is turned on by the control signal 6 or the control operation so that it can be reused.

[0004]

The conventional power supply overload protection device is constructed as described above, and when the protection operation is performed once, the breaker is used when the abnormal load operation is eliminated. In that case, a resetting operation for returning the switch 2 to the conductive state is required, and when a fuse is used, a replacing operation is required. Also, when a breaker or relay is used, the shape is large because it uses an electromagnet and a mechanical switch, making it difficult to use except for large equipment that uses a large current, and when using a fuse, a socket or a mounting bracket. It was necessary and it became larger.

The present invention has been made to solve the above problems, and provides a small and lightweight power supply overload protection device which can easily return a current cut-off switch to a conductive state. With the goal.

[0006]

A power supply overload protection device according to the present invention includes a bistable transistor switch which is connected between a power supply and a load and is turned off when an excessive current flows through a current detector, and a power supply. Is provided with a voltage differentiating circuit for turning on the bistable transistor switch by the differential output of the power supply voltage at the rise of the.

[0007]

According to the present invention, when an excessive current flows through the load, the bistable transistor switch is turned off and the load current stops flowing. When the power supply rises, the bistable transistor switch is turned on by the differential output of the power supply voltage.

[0008]

Embodiment 1 An embodiment of the present invention will be described below with reference to the drawings. Figure 1
Shows a configuration of a power supply overload protection device according to this embodiment, 1 is a power supply, 2 is a switch, 3 is a current detector, 4 is a load, 5 is a control signal for turning off the switch 2, and 6 is on. A control signal for turning on, 7 is a voltage differentiating circuit.

Next, the operation of the configuration shown in FIG. 1 will be described. The voltage differentiating circuit 7 is connected to the power supply 1 to differentiate the voltage, and when the power supply voltage rises, the switch 2 is turned on by the output 6 of the voltage differentiating circuit 7 and the voltage is supplied to the load 4. on the other hand,
The switch 2 and the current detector 3 are connected in series on the wiring from the power source 1 to the load 4, and when an excessive current flows through the load, the current detector 3 detects this and turns off the switch 2 by the control signal 5. , Prevent overload of the power supply 1.

FIG. 2 shows a concrete example of the configuration shown in FIG.
Q1 and Q2 are transistors that form a bistable transistor switch. R5 is a current detection resistor, C1 and R3 are capacitors and resistors forming the voltage differentiating circuit 7, R4 is a base bias resistor of the transistor Q1, and R1 and R2 are base bias resistors of the transistor Q2. Also, VC
C is the power supply voltage, VL is the load voltage, VB2 is the base voltage of the transistor Q2, VB1 is the base voltage of the transistor Q1, and VD is the voltage at the intersection of the capacitor C1 and the resistors R2 and R3. Further, FIG. 3 shows the voltage at each point in FIG. 2 when the power is turned on.

Next, the operation of the configuration shown in FIG. 2 will be described. First, when the positive power supply 1 is in the ON state and under normal load,
The resistors R4 and R5 are set so that the transistor Q1 is turned on and the transistor Q2 is turned off. That is, the load current is IL and the direct current amplification factor of the transistor Q1 is HFE.
1. If K is the overdrive coefficient for saturating the transistor Q1, IL / HFE1 <K (VCC-0.6) / R4 (1) Under the condition of IL.R5 << 1 (2), the resistance R4 R5 has been decided. Where K
Is the AC impedance of load 4 and transistor Q1
In consideration of the width of the DC current amplification factor HFE1 of the transistor Q1, the transistor Q1 is determined so as to maintain the saturation operation.
It should be 10 or less. Under this condition, the transistor Q1 becomes saturated and the collector-emitter voltage is V
It is represented by CESAT1. Therefore, the load voltage VL is VL = VCC-VCESAT1-IL.R5 (3).

Assuming that the values of VCESAT1 and IL.R5 are each 0.1 volt, VL = VCC-0.
It becomes 2. Base voltage VB2> VL of transistor Q2
Therefore, the base current of the transistor Q2 does not flow,
It is turned off. Next, an excessive current flows in the load 4 due to some cause such as a short circuit or a failure of the circuit, and the load voltage V
The resistors R1 to R3 are determined so that the transistor Q2 is turned on when L is lowered to the threshold voltage VS. That is, when the direct current amplification factor of the transistor Q2 is HFE2, it is represented by (VCC-0.6) /R4.HFE2= (VCC-0.6-VS) / (R2 + R3) -0.6 / R1 (4) Be done. Under this condition, the transistor Q2 turns on and the base voltage VB1 rises to near the power supply voltage VCC, so the transistor Q1 turns off. Therefore, for the load 4, the resistances R1 to R from the power supply voltage VCC
3 is only connected in series, and only a maximum current of (VCC-0.6V) / (R2 + R3) flows. Therefore, the power supply 1 and the load 4 are protected from destruction and abnormal heat generation.

Once the transistor Q2 is turned on, the current through the resistors R1 to R3 is small even if the load 4 becomes normal, the load voltage VL does not rise to the specified value, and the protection state continues. In the conventional device, at this time, it is necessary to perform a resetting operation such as a switch operation and a part replacement, but this operation is automatically performed in this embodiment.

That is, FIG. 3 shows voltage waveforms of respective parts in the process of turning on the power supply 1 again after turning off the power supply 1, 8 shows a change in which the power supply voltage VCC rises from 0 to a specified value, and 9 shows the base of the transistor Q1 at this time. The voltage VB1 is shown, and 10 is the load voltage VL. Reference numeral 11 indicates a change in the load voltage VL when the load 4 is capacitive, and reference numeral 12 indicates an on / off threshold voltage VS of the transistor Q2. Also, 13 is VC
A differential waveform of C is shown, and 14 shows a change of the charge / discharge waveform VD, that is, VD-VL obtained from the capacitor C1 and the resistors R1 to R3 when the load voltage VL is assumed to be 0.

T1 is the rise start time of the power supply voltage VCC,
t2 is the rising start time of the threshold voltage VS, t3
Indicates the last time when VL <VS is established. However, V
L <VS is satisfied in the case of the characteristic 11, and is not satisfied in the case of the characteristic 10. That is, when the load 4 is capacitive, the rise of VL is delayed, and a period of VL <VS occurs. During this period, the transistor Q2 is turned on and the transistor Q1 is turned off, so that the load 4 is not applied with voltage.

Here, the peak value of the VD-VL curve 14 is set by the rise time of the power source 1, the discharge time constant is set by the capacitor C1, and VD is the curve 11 and V of VL.
Since the D-VL curve 14 is added, t2, t
Power supply 1 to satisfy VD> VCC-0.6V during 3
The rise time and the capacitor C1 can be set. Under the condition of VD> VCC-0.6V, the transistor Q2 is off, and the transistor Q1 and the transistor Q2 can be stably kept on when the power supply 1 rises.

Embodiment 2 In FIG. 2, a positive power source is used as the power source 1, but a negative power source may be used. In this case, the transistor Q is used.
NPN type transistors may be used for 1 and Q2. Third Embodiment Although the resistor R5 is used as the current detector 3, it may be omitted and the on-resistance of the transistor Q1 may be used instead.

Fourth Embodiment Although the resistor R5 is used as the current detector 3, the resistor R5 is omitted and the base current of the transistor Q1 is set to a small value to limit the collector current of the transistor Q1 at the time of overload. It is also possible to detect a decrease in the collector voltage that accompanies this. Fifth Embodiment Further, although the circuit is composed of the individual components such as the transistors Q1 and Q2 and the resistors R1 to R5, it may be an integrated circuit. Embodiment 6 The integrated circuit as described above may be added to an existing integrated circuit.

[0019]

As described above, according to the present invention, the bistable transistor switch is turned on by the differential signal of the power supply voltage when the power supply voltage rises, and the work of returning the switch to the conductive state is automatically and easily performed. be able to. Further, the off operation of the bistable transistor switch at the time of overload can be easily performed by the voltage fluctuation due to the overload, and the breaker, the fuse, the relay, etc. are not required, and the size and weight can be reduced.

[Brief description of drawings]

FIG. 1 is a block diagram of an apparatus according to the present invention.

FIG. 2 is a specific configuration diagram of the device of the present invention.

FIG. 3 is a voltage waveform diagram at each point after power-on of the device of the present invention.

FIG. 4 is a configuration diagram of a conventional device.

[Explanation of symbols] 1 power supply 2 switch 3 current detector 4 load 5, 6 control signal 7 voltage differentiating circuit Q1, Q2 transistor C1 capacitors R1 to R5 resistance

Claims (1)

[Claims] 1. A current detector for detecting a current flowing from a power source to a load, a bistable transistor switch connected between the power source and the load and turned off when an excessive current flows through the current detector, An overload protection device for a power supply, comprising a voltage differentiating circuit that turns on a bistable transistor switch by a differential output of the power supply voltage when rising.