JPH0116087B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an overcurrent detection circuit, and more particularly to an overcurrent detection circuit having a current detection capability with specified hysteresis.

Generally, an overcurrent detection circuit is used to help prevent danger by displaying an external alarm or automatically stopping the current supply to the device when an excessive current flows through the device. , to detect its overcurrent. Furthermore, when the excessive current consumed by the device decreases and returns to within the permissible range, this may be detected and a request may be made to automatically start supplying current.

Points to consider when configuring this overcurrent detection circuit are that when detecting the current flowing to the equipment side, part of the current must not be consumed by the overcurrent detection circuit; If a comparator is used, the reference and input voltage must be set to a different level than the supply voltage, and if hysteresis is required, the hysteresis width must be increased. There are times when it is necessary to take a smaller value, and times when it is necessary to take a smaller value, and there are also cases where high precision is required. Additionally, it is generally desired that the current detection level and hysteresis be easily set or changed.

FIG. 1 is a diagram showing an example of a conventional overcurrent detection circuit. In this figure, 1 is a DC power supply, and a load 3 such as a device is connected between its two poles by inserting an overcurrent detection resistor 2 on the positive side.
4, 5, and 6 are resistors forming a reference voltage circuit. The resistors 4 and 5 are connected in series between both poles of the DC power supply 1. One end of the resistor 6 is connected to the connection point between the resistors 4 and 5. A series circuit of resistors 8 and 9 is connected between one end 7 of the resistor 2 on the load 3 side and the negative electrode of the DC power supply 1. This resistor 8 and 9
The connection point 10 is connected to the negative input side of the comparator 11. The other end of the resistor 6 is connected to the positive input side of the comparator 11. A resistor 12 is connected between the positive input of this comparator 11 and the negative terminal of the DC power supply 1.
is connected. Furthermore, the output of the comparator 11 is connected to the positive input of the comparator 11 via a resistor 13, and the output of the comparator 11 is connected to an output terminal 14.

In the overcurrent detection circuit configured in this way, the positive input voltage of the comparator 11 is used as the reference voltage V _r at the connection point 10 (comparator 1
An overcurrent is detected by comparing the voltage V ₁ (negative input of 1) with a comparator 11. That is,
When normal current is flowing through load 3, V ₁
>V _r , therefore, the output of the comparator 11 is V _L (“L” level). On the other hand, load 3
When an overcurrent flows through the overcurrent detection resistor 2, the current _I1 flowing through the overcurrent detection resistor 2 increases, the voltage at one end 7 decreases, and _the voltage _V1 at the connection point 10 decreases accordingly. ＜V _r . Therefore, the output of the comparator 11 is inverted to V _H (“H” level),
Detect overcurrent.

When the output of comparator 11 becomes _VH , the positive feedback loop of resistors 13 and 12 causes comparator 1 to
The positive input voltage (reference voltage) of No. 1 increases and has hysteresis. Therefore, when the current flowing through the load 3 decreases, the voltage at one end 7 of the overcurrent detection resistor 2 increases due to the decrease, and the voltage _V1 at the connection point 10 increases. Only when the voltage V ₁ becomes higher than the reference voltage V _r does the output of the comparator 11 change.
Re-flip to V _L.

However, in the conventional overcurrent detection circuit as described above, the reference voltage depends on the power supply voltage, as is clear from the configuration shown in FIG. 1, and the power supply voltage also affects the hysteresis width. Further, the reference voltage and hysteresis are also subject to change depending on the output voltage of the comparator 11. Furthermore, the current I ₁
flows into the series circuit of resistors 8 and 9. Therefore, the conventional method has a disadvantage in that highly accurate detection cannot be performed.

The present invention has been made in view of the above points, and it is an object of the present invention to provide an overcurrent detection circuit that solves the conventional drawbacks and also allows the use of the same power source as the load.

Embodiments of the present invention will be described below with reference to the drawings. FIG. 2 is a diagram showing an embodiment of the invention.
In this figure, 21 is a DC power supply, and a load 22 such as a device is connected between its two poles by inserting a current detection resistor _R1 on the positive side. R ₂ is the resistance,
23 is a constant voltage source, which is connected in series. Further, a circuit including a resistor R ₂ and a constant voltage source 23 is connected between both poles of the DC power source 21 . R ₃ is a resistor and D ₁ is a constant voltage diode, which are
They are connected in series to constitute a first level shift means. 24 is a first current source to which the output 25 of the constant voltage source 23 is connected. This first current source 2
4 is connected in series to the first level shift means (specifically, the anode of the constant voltage diode _D1 ). A circuit (first circuit) consisting of the first level shift means and the first current source 24 is connected between both poles of the DC power supply 21. R ₄ is resistance, D ₂
are constant voltage diodes, which are connected in series to constitute the second level shift means. A second current source 26 is connected in series to this second level shift means (specifically, to the anode of the constant voltage diode _D2 ). A circuit (second circuit) consisting of the second level shift means and the second current source 26 is connected between one end of the current detection resistor R ₁ on the load 22 side and the negative electrode of the DC power supply 21 . 27
is a comparator. The comparator 27 has a positive power terminal 28 connected to the positive electrode of the DC power source 21, and a negative power terminal connected to the load of the DC power source 21. Further, the comparator 27 has a reference side input 29, which is a first input, connected to a connection point between the voltage regulator diode _D1 and the first current source 24, and a second input 30, which is a voltage regulator diode.
It is connected to the connection point between D ₂ and the second current source 26 .
Further, the comparator 27 outputs the output from the output terminal 31.
and to the input of the switch circuit 32. This switch circuit 32 has an output 33 connected to the first current source 24 .

In such embodiments, the current sensing resistor R ₁
is usually chosen to have a low resistance value (for example, 10Ω). Therefore, by the time the voltage E ₁ of the DC power supply 21 reaches the power supply terminal of the load 22, the current detection resistor R ₁
The effect of the voltage drop caused by the voltage drop can be ignored.

Further, the constant current of the first current source 24 is set by the output of the constant voltage source 23 so as not to be affected by fluctuations in the DC power supply 21, and the output of the comparator 27 is set by the output of the switch circuit 32. It is switched in accordance with the output of the comparator 27 by being fed back via the . for example,
As will be described later, when a normal current is flowing through the load 22, the output V _O of the comparator 27 is V _H (“H”
level), but at this time, the constant current of the first current source 24 is set to _I2 . On the other hand, when an overcurrent is detected, the output V _O of the comparator 27 becomes V _L (“L”
At this time, the constant current of the first current source 24 is switched to I ₄ (I ₂ >I ₄ ).

Furthermore, a first current source 24 and a second current source 26
constitute Karen mirrors of each other, for example
I ₂ and the constant current I ₃ of the second current source 26 maintain a constant ratio. The constant current of the first current source 24 is switched to _I4 when the output of the comparator 27 becomes _VL . When the constant current of the first current source 24 is switched in this way, the first and second current sources 24, 26
Needless to say, the Cullen-Miller ratio is changed to maintain a constant ratio. Note that the second current source 26
The constant current I ₃ is selected to be 1 to 2% of the current flowing through the load 22 normally.

In addition, in the embodiment, the voltage of the DC power supply 21 is
A voltage that is lower than E ₁ by the voltage caused by the first level shift means (consisting of resistor R ₃ and constant voltage diode D ₁ ) is applied to reference side input 29 of comparator 27 . This voltage is called a reference voltage.

Expressing this reference voltage in concrete numerical value assuming that a constant current I ₂ is flowing through the first current source 24, V _r1 = E ₁ − R ₃ × I ₂ − V _D1 (1) Become. And the reference voltage V _r1 at this time
is the overcurrent detection level, and this level is R ₃ ,
I ₂ and V _D1 can be set arbitrarily by selecting them. Note that V _D1 is the voltage of the constant voltage diode _D1 .

On the other hand, when a constant current I ₄ (I ₂ >I ₄ ) is flowing through the first current source 24, the reference voltage becomes V _r2 =E ₁ −R ₃ I ₄ −V _D1 (2). And this reference voltage V _r2 (V _r2
>V _r1 ) is the current detection level when the excess current decreases, and this level can be arbitrarily set by arbitrarily selecting the constant current I ₄ .

Furthermore, the difference between the reference voltage V _r2 and the reference voltage V _r1 is the hysteresis width, which is determined by the difference between the constant currents I ₂ and I ₄ and can be arbitrarily set by arbitrarily selecting I ₂ and I ₄ . .

On the other hand, the input 30 of the comparator 27 has a second
Resistor R ₄ and voltage regulator diode D ₂ (the voltage of regulator diode D ₂
V _D2 ), V ₁ = E ₁ − R ₁ × I ₁ − R ₄ × I ₃ − V _D2 …(3) (where, in equation (2), I ₁ is the current detection resistor R ₄
and the current flowing through the load 22). This voltage V ₁ is called an input voltage.

Now, in the embodiment, when the current I ₁ flowing through the load 22 is normal, the voltage drop due to R ₃ ×I ₂ is greater than R ₁
If the voltage drop is set to be larger than the voltage drop due to ×I ₁ +R ₄ ×I ₃ , V _r1 <V ₁ and the output V _p of the comparator 27 is held at V _H (“H” level). At this time,
The voltage at the input 29 of the comparator 27 (which is the threshold level of the comparator 27 and also the overcurrent detection level) is V _r1 = E ₁ − R ₃ × I ₂ − V _D1
It is.

Next, an overcurrent flows through the load 22 (current I ₁ increases), and the voltage drop due to R ₃ × I ₂ becomes R ₁ × I ₁ + R ₄ ×
When the voltage drop of I ₃ becomes larger, V _r1 >V ₁ , and at that moment the comparator 27 starts to function in an inverting manner, and the output V _p becomes L _L (“L” level).
Then, when the output V _p becomes V _L , the switch circuit 3
By the second operation, the constant current of the first current source 24 is switched to _I4 . Therefore, input 2 of comparator 27
The voltage at point 9 (threshold level of comparator 27) is V _r2 =E ₁ −R ₃ ×I ₄ −V _D1 . here,
I ₂ > I ₄ and V _r2 > _r1 . That is, when the constant current of the first current source 24 is switched to _I4 , the threshold level of the comparator 27 becomes higher than before. Moreover, since this is a kind of positive feedback effect, the inversion operation of the comparator 27 is strengthened while the comparator 27 is in the middle of inversion.

After that, when the current flowing through the load 22 decreases, V ₁ increases as the current decreases, and only when V ₁ exceeds the raised threshold level V _r2 does the comparator 27 invert again. ,output
V _p becomes V _H. Furthermore, as the output of the comparator 27 becomes _VH , the constant current of the first current source 24 is switched to _I2 again, and therefore the voltage (threshold voltage) at the input 29 of the comparator 27 becomes
Return from V _r2 to V _r1 .

The above operation is shown in FIG. In FIG. 3, the route is →→→→→.

FIG. 4 is a circuit diagram specifically showing the portions shown in FIG. 3 by blocks or symbols.
As shown in this figure, the constant voltage source 23 is realized by a Zener diode _DZ1 , and the first current source 24 is composed of resistors _R5 , _R6 , _R7 , _R11 and transistors _Q1 , _Q2. . Further, the switch circuit 32 is composed of a transistor Q ₃ and a resistor R ₈ , the comparator 27 is composed of a third current source 41 and transistors Q ₄ to Q ₇ , and the second current source 26 is composed of a transistor Q ₉ and a resistor R ₁₀ . be done. Further, in the figure, 42 is a fourth current source, which is connected between the base of the transistor Q ₃ and the positive electrode of the DC power supply 21 . Furthermore, Q ₈ is a transistor connected to the output of the comparator 27 , and the collector of this transistor Q ₈ is connected to the output terminal 31 and to the input of the switch circuit 32 . R ₉ is a resistor connected between the collector of transistor Q ₈ and the positive electrode of DC power supply 21 .

In FIG. 4, when the collector of the transistor Q ₃ is at "H" level and the transistor Q ₃ is on, a constant current I ₂ flows through the first current source 24 . This constant current I ₂ becomes I ₂ ≒V _D −V _BE /R ₅ +R ₁₁ ×R ₁₁ /R ₆ (4). However, V _D is the voltage of the Zener diode DZ ₁ , and V _BE is the base-emitter voltage of the transistors Q ₁ and Q ₂ .

On the other hand, the collector of the transistor _Q8 goes to "L" level and the transistor _Q3 is turned off, so that the constant current of the first current source 24 is switched to _I4 .
The constant current I ₄ is I ₄ ≒V _D −V _BE /R ₅ +R ₁₁ ×R ₁₁ /R ₆ +R ₇ (5).

On the other hand, a constant current I ₃ as shown in the following equation (6) flows through the second current source 26.

I ₃ =V _D −V _BE /R ₅ +R ₁₁ ×R ₁₁ /R ₁₀ ...(6) FIG. 5 shows the third and fourth current sources 41 in FIG.
42 is a diagram illustrating a circuit in actual use, in which the circuit 42 and the like are made more specific. In this figure, the third and fourth current sources 41 and 42 are transistors.
It consists of Q ₁₅ and Q ₁₃ . Also, in the figure, Q ₁₀ to Q ₁₂ ,
Q ₁₄ , Q ₁₆ to _{Q 18} are newly added transistors, and R ₁₂ is a newly added resistor.

In FIG. 6, specific constants of each part are determined as follows.

E ₁ = 15V, V _D = 5.6V, R ₅ = 18KΩ, R ₁₁ = R ₆ =
R ₇ = R ₁₀ = 2KΩ, R ₃ = 9KΩ, R ₄ = 4KΩ, R ₁ =
10Ω, V _D1 = V _D2 = 7V And when each constant is set like this, I ₂ =
I ₃ = 200μA, I ₄ = 100μA, V _r1 = 6.2V, V _r2 = 7.1V,
A hysteresis voltage of 0.9V is calculated. Therefore, in terms of circuit operation, when the current _I1 flowing into the load 22 starts from 0 and reaches 90mA, the output changes from the "H" level to the "L" level, an alarm is displayed, and conversely, the current I1 flowing into the load 22 reaches 90mA. When the current decreases to 10mA, the "L" level returns to the "H" level.

As is clear from the above description, in the overcurrent detection circuit of the present invention, level shift means are provided on both input sides of the comparator. Therefore, the same power source used for the device can be used.

Further, a first current source is connected in series to the first level shift means, and a constant current is caused to flow through the series circuit.
Since the reference voltage is obtained from the connection point of the current source, the influence of the power supply voltage on the reference voltage and hysteresis width is reduced.

Furthermore, the second level shift means side connected to the output side of the current detection resistor, that is, the load side, obtains an input voltage from the connection point of the second level shift means and the second current source in the same way. The circuit that obtains this input voltage no longer affects the current flowing to the load.

In addition, the constant current value of the first current source is switched by operating a switch circuit according to the output level of the comparator, thereby switching the reference voltage, that is, providing hysteresis. As a result, the reference voltage and hysteresis are not subject to change, and furthermore, it is possible to set a wide hysteresis width.

Due to this point and the second and third effects described above, the detection accuracy can be improved in the circuit of the present invention.

[Brief explanation of drawings]

Figure 1 is a circuit diagram showing a conventional overcurrent detection circuit.
Fig. 2 is a circuit diagram showing an embodiment of the overcurrent detection circuit of the present invention, Fig. 3 is a diagram for explaining the operation of the embodiment, and Figs. 4 and 5 show the circuit of the embodiment in more detail. FIG. 21... DC power supply, 22... Load, 24... First current source, 26... Second current source, 27... Comparator, 32... Switch circuit.

Claims (1)

[Claims] 1 A current detecting resistor connected between a load connected to a DC power source and one pole of the DC power source, a first level shift means, and a first current source connected in series, and the DC power source A first circuit connected between both poles of the circuit, a second level shift means, and a second current source are connected in series, and is connected between one end of the load side of the current detection resistor and the other pole of the DC power supply. a second circuit connected to the second circuit, a connection point between the first level shift means and the first current source is connected to the first input as a reference input; An overcurrent detection circuit comprising: a comparator whose connection point with a second current source is connected to a second input; and a switch circuit that controls the first current source based on the output potential of the comparator.