JPH0718892B2 - Electronic switch overcurrent detection circuit - Google Patents

Description

The present invention relates to an overcurrent detection circuit for an electronic switch such as a proximity switch.

Conventional overcurrent detection circuit of this type is inserted the overcurrent detecting resistor R1 to the emitter of the output transistor Tr ₁ as shown in FIGS. 1 and 2, the transistors Tr for overcurrent detection voltage Vr ₁ of the opposite ends It is the simplest way to detect by ₂ and thus the overcurrent, and is used in numerous applications. This method has a drawback that the output terminal residual voltage becomes extremely large when half-side ON which can be easily realized. That is, the transistor Tr ₁ turns on
The time residual voltage V _on depends _on the output current value and the output current capacity of the transistor Tr ₁ , but it is assumed that Von is Von = 0.2 [V] and the output current It = 150 [mA].
When a current of 00 [mA] or more flows, this is an overcurrent. Then, the base-emitter voltage V of the transistor Tr ₂
When be exceeds 0.7 [V], the value of the overcurrent detection resistor R1 becomes R ₁ = 0.7 [V] /0.2 [A] = 3.5 [Ω]. When the output current It = 150 [mA], the voltage Vr ₁ across the overcurrent detection resistor R1 is Vr ₁ = 3.5 × 0.15 ≈ 0.525 [V]. Therefore, the transistor Tr ₁ and the overcurrent detection resistor R1
Voltage Vx of the first series circuit is _{Vx = Von = Vr 1 = 0.2} + 0.525 = 0.725 [V]. By inserting the overcurrent detection circuit in this way, the output residual voltage when ON as an electronic switch becomes three times or more, and as is clear from the calculation formula, the output transistor
By increasing the output power capacity of tr _1, ON including an overcurrent detection circuit be reduced much the ON time of the output remaining voltage Von
However, there is a drawback in that the remaining output voltage hardly decreases.

In the conventional overcurrent detection circuit, the transistor Tr ₁
It had a large negative temperature coefficient because its base-emitter voltage was an overcurrent detection voltage.

The present invention is intended to eliminate such conventional drawbacks, and an object of the present invention is to reduce a detection voltage as compared with a case where an overcurrent is detected by a base-emitter voltage of a transistor, thereby reducing an electronic switch. It is intended to reduce the overcurrent detection resistance of and reduce the remaining voltage when ON as an electronic switch.

An embodiment of the present invention will be described below.

FIG. 3 is a circuit diagram when the overcurrent detection circuit according to the present invention is applied to a proximity switch. This switch 10 is a current consisting of the proximity sensor IC circuit 1 and an output transistor for compensating the output current capacity of this IC circuit. Switching element 26
Composed of and. And this IC circuit has an oscillation circuit 2, a comparator 3, an integration circuit 4, a comparator 5,
It has an output circuit 6, a constant voltage circuit 7, a power supply reset circuit 8, an output controller 14, and an overcurrent detection circuit 16,
Electronic switch controller (controller) by the output circuit 6, the power supply reset circuit 8 and the output controller 14.
17 are composed. In addition, the detection coil L ₁ and the resonance capacitor
C ₁ , a sensitivity adjusting variable resistor R ₂₂ , a bypass capacitor C ₂₃ , an integrating capacitor C ₂₄ , a power reset capacitor C ₂₅ and a load 9 are externally attached.

Therefore, the oscillation circuit 2 starts or stops oscillation when the metal body approaches or separates from the detection coil L ₁ , and the output of the output circuit 6 is logic L or H.

The overcurrent detection circuit 16 is provided between the current switching element 26 and the power supply reset circuit 8.

FIG. 4 is a circuit diagram showing an essential part of the present invention. Basically, there are five transistors, that is, the first transistor 21, the second transistor 22, the third transistor 23, the fourth transistor 24 and the fifth transistor. Transistor 25 and resistors 31,32
And the current source 20 block
16 are composed. An overvoltage protection circuit 18 is configured by the diode 27 and the resistor 35, and protects the overcurrent detection circuit 16 when a large voltage is applied to the node 41. This overvoltage protection circuit is for a current sink. 19 is a power supply. The current switching element 26 only needs to control the current flowing through the load 9, and is composed of, for example, a transistor. Connection terminals a and b are drawn out at the connection points between the third and fifth transistors 23 and 25 and the resistors 31 and 32, respectively, and
The signal output terminal G is provided at the connection point between the transistor 24 and the fifth transistor 25, and this output terminal is further connected to the input terminal of the electronic switch controller 17 for controlling the current switching element 26, whereby the electronic switch controller It is designed to control 17 output states.
Further, in FIG. 4, the overvoltage protection circuit 18 is connected to the terminal b. In FIG. 4, the overcurrent detection circuit 16 and the overvoltage protection circuit 18 are integrated into an IC.

The resistors 31 and 32 for adjusting the constant current value have a function of adjusting the operation level of the current mirror circuit.

Even if the resistor 35 is 0Ω, that is, the same kind of operation is essentially performed without the resistor 35, the detection sensitivity is higher without the resistor 35, but if the resistor 35 and the diode 27 are not present, the circuit may be damaged.

That is, if an accident occurs in which the node 41 becomes almost the same potential as the power supply (Vcc), for example, when the load 9 is short-circuited, or when A and B are contacted while the power is on when the load is connected, A large voltage is generated and a large reverse bias is applied between the base and emitter of the fifth transistor 25, which may destroy the transistor. The resistor 35 and the diode 27 prevent this. Since there is the resistor 35, the voltage at the point b is divided by the resistor 35 and the resistor 32.
It is possible to avoid applying a high voltage to the point. A diode 27 is provided to make this operation more reliable.

In particular, when the ICs up to the overvoltage protection circuit 18 are integrated, the IC circuit is vulnerable to overvoltage, so that the resistor 35 and the diode 27 are necessary.

Now, in FIG. 4, the current I ₀ from the current source 20 is I ₀ = 5 [μ
A], and the resistance values R ₃₁ and R ₃₂ of the resistors ₃₁ and ₃₂ are set to R ₃₁ = R ₃₂ = 10 [K
Ω], the voltage V _{41 of the} node 41 becomes an overcurrent detection voltage, and its value becomes V ₄₁ = I ₀ × R ₃₁ = I ₀ × R ₃₂ = 5 × 10 = 50 [mV]. This is because when the node 41 is open, the collector currents of the fourth transistor 24 and the fifth transistor 25 become equal, and the output node 46 is in a critical state where there is no inflow or outflow of current.

Now, if node 41 is connected to overcurrent detection resistor R ₁ ,
The voltage generated across the overcurrent detection resistor R ₁ is 50 [mV].
If it is smaller, a current flows from the node 41 toward the overcurrent detection resistor R1 through the resistor 35, so that the current flowing through the resistor 32 decreases, the emitter voltage of the fifth transistor 25 decreases, and the fourth transistor 24 and The fifth transistor 25 becomes unbalanced, the collector current of the fifth transistor 25 becomes larger than that of the fourth transistor 24, and current flows from the electronic switch controller 17 to the output node 46. At this time, the output of the electronic switch controller 17 becomes H, for example, and the current switching element 26 is in the ON state.

When the current flowing through the current switching element 26 exceeds a predetermined value for some reason, that is, when the voltage generated across the overcurrent detection resistor R1 exceeds 50 [mV], the fifth voltage is reversed.
The emitter voltage of the transistor 25 rises, the collector current of the fifth transistor 25 becomes smaller than that of the fourth transistor 24, and current flows from the output node 46 to the electronic switch controller 17. At this time, the output of the electronic switch control 17 becomes L, for example, and the current switching element 26 is turned off.

In this way, the overcurrent detection circuit 16 outputs a current from the output node 46 when the voltage generated across the overcurrent detection resistor R1 is less than 50 [mV], and when the voltage exceeds 50 [mV], the output node 46 Current flows in from.

That is, when the voltage of the overcurrent detection resistor R1 is 50 [mV], no current flows through the resistor 35, which becomes the threshold voltage.

Although the above is described with reference to FIG. 4, when the output of the overcurrent detection circuit 16 requires reverse logic, such as when the logic at the input end of the electronic switch controller 17 is reverse, You can do it like this.

That is, the circuit shown in FIG. 5 is largely different from that shown in FIG. 4 in that the connection between the terminal a and the terminal b is reverse, and the flow of current into and out of the output node 46 at the time of overcurrent detection is opposite. It is a point.

Therefore, the case where the current in each part of the circuit is the same as that in FIG. 4 will be described.

That is, the voltage generated across the overcurrent detection resistor R ₁ is 50
When it is smaller than [mV], a current flows from the node 41 to the overcurrent detection resistor R1 through the resistor 35, so that the current flowing through the resistor 31 decreases, the emitter voltage of the third transistor 23 decreases, and the fourth The balance between the transistor 24 and the fifth transistor 25 is lost, the collector current of the fifth transistor 25 is smaller than that of the fourth transistor 24, and current flows from the electronic switch controller 17 to the output node 46.

When the current flowing through the electronic switching element 26 exceeds a predetermined value, that is, the voltage generated across the overcurrent detection resistor R1
When it exceeds 50 [mV], the emitter of the third transistor 23 rises contrary to the former, the collector current of the fifth transistor 25 becomes larger than that of the fourth transistor 24, and the output node
A current flows from 46 to the electronic switch controller 17.

Although the description of FIGS. 4 and 5 is an example of the output format of the current sink, the output format of the current source may be as shown in FIG.

The circuit shown in FIG. 6 is a circuit for a Karen source, which is the reverse of the circuit shown in FIGS. 4 and 5, and the emitter of the second transistor 22 is connected to one terminal of the power supply 19 via the resistor 33. The emitter of the fourth transistor 24 is connected to one terminal of the power source 19 via the resistor 34. Further, the collector of the first transistor 21 is connected to one terminal of the power supply 19 via the current source 20, and the emitter thereof is connected to the other terminal of the power supply 19. Also diode
An overvoltage protection circuit 18 is constituted by 28 and the resistor 36, and this circuit is connected to a terminal d provided at a connection point between the emitter of the fourth transistor 24 and the resistor 34. The overvoltage protection circuit may be connected to the terminal c provided at the connection point between the emitter of the second transistor 22 and the resistor 33. A PNP type transistor is used as the electronic switching element 26.

That is, the circuit shown in FIG. 6 differs greatly from the circuits shown in FIGS. 4 and 5 in that the connection direction of the overcurrent detection circuit 16 to the power supply 19 is opposite, and that the electronic switching element 26 is a PNP transistor. Is the point that is used.

Note that FIG. 7 shows a modification of the current mirror circuit, and it is known that there is no modification shown in FIGS. 7B and 7C with respect to FIG.

In Fig. 4, the detection voltage is 50 [mV] and the overcurrent is 200
[MA] to the overcurrent detecting resistor R1 is _{R 1 = 50mV / 200mA = 0.25} [Ω] , and the thus remaining voltage Vx at ON at the output terminal in the output current It = 0.99 [mA] is _{Vx = Von + Vr 1 = 0.2} + 0 .25 x 0.15 = 0.2 + 0.0375 = 0.2375 [V], which is a great improvement over the conventional one.

As is clear from this calculation example, the transistor in the conventional example is
The voltage Vr ₁ across the overcurrent detection resistor R1 was considerably higher than the remaining voltage Von when the transistor Tr ₁ was on.However, according to the present invention, the voltage Vr ₁ across the overcurrent detection resistor R1 is the electronic switching element 2.
6 is considerably smaller than the ON-time remaining voltage Von. Therefore, if a transistor with a large capacity is used to reduce the ON-time remaining voltage Von, the remaining voltage Vx when the output terminal is ON
Can also be made smaller.

Furthermore, comparing the power consumption of the overcurrent detection resistor R1,
The voltage immediately before overcurrent detection required about 600 [mV] in the prior art, but in the present invention, as can be seen from the above calculation example.
50 [mV] and 1/12, which can be changed from 200 [mA] x 600 [mV] = 120 [mW] in the past to 200 [mA] x 50 [mV] = 10 [mW] . In order to reduce the size of the electronic switch, all the components are made small, but since the maximum power consumption of the small components is small according to the size, the present invention is also effective in this respect.

[Brief description of drawings]

1 and 2 are circuit diagrams showing a conventional overcurrent detection circuit, FIG. 3 is a block diagram when the overcurrent detection circuit in the present invention is applied to a proximity switch, and FIG. 4 is an overcurrent in the present invention. FIG. 5 is a circuit diagram showing one embodiment of a detection circuit, FIG. 5 is a circuit diagram showing another embodiment of the present invention, FIG. 6 is a circuit diagram showing still another embodiment of the present invention, and FIG. 7 is a current mirror. It is a circuit diagram which shows the modification of a circuit. 1 ... IC circuit, 2 ... oscillation circuit, 3 ... comparator,
4 ... integrator circuit, 5 ... comparator, 6 ... output circuit, 7 ... constant voltage circuit, 8 ... power supply reset circuit, 9 ...
… Load, 10… Switch, 14… Output controller, L ₁
…… Detection coil, R ₂₂ …… Variable resistance, C ₁ …… Resonance capacitor, C ₂₄ …… Integration capacitor, C ₂₅ …… Capacitor, 16
...... Overcurrent detection circuit, 17 ...... Electronic switch controller (controller), 18 ...... Overvoltage protection circuit, 19 ...... Power source, 20 ...... Current source, 21 to 25 ...... First to fifth transistors, 26 ... … Electronic switching elements, 27 …… Diodes,
28 …… Diode, 31,32,33,34,35 …… Resistance, R ₁ …… Overcurrent detection resistance, 41,42 …… Node, 46 …… Output node.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Shigeru Aoshima 1-12-2 Kawana, Fujisawa, Kanagawa Yamatake Honeywell Co., Ltd. Fujisawa Plant (72) Inventor Yoshikazu Kawashima 1-2-12, Kawana, Fujisawa, Kanagawa Yamatake Honeywell Co., Ltd. Fujisawa Factory (56) Reference JP-A-56-88513 (JP, A) JP-A-57-17226 (JP, A) JP-A-52-87649 (JP, A) JP-A-53- 23056 (JP, A) JP-A-58-144920 (JP, A)

Claims (1)

[Claims] 1. A load, a current switching element for controlling a current flowing through the load, and an overcurrent detection resistor are sequentially connected in series to a power supply, and one terminal of the power supply is connected to a first transistor of a first transistor. The emitter is connected, the collector of the first transistor is connected to the other terminal of the power source through the current source, and the emitter of the second transistor is connected to one terminal of the power source and the The emitter of the third transistor is connected to the other terminal through a resistor, the collectors of both transistors are connected to each other, and the emitter of the fourth transistor is connected to one terminal of the power supply. , The emitter of the fifth transistor is connected to the other terminal of the power source through a resistor, and the collectors of both transistors are connected to each other, The first transistor, the second transistor, and the fourth transistor are connected to each other, and the third transistor and the fifth transistor are connected to each other so as to form a current mirror circuit, and the third transistor and the fifth transistor are connected to each other. A connection terminal is drawn out from a connection point with each of the resistors, the terminal is selectively connected to a connection point between the current switching element and the overcurrent detection resistor, and the fourth transistor and the fifth transistor are connected. An overcurrent detection circuit for an electronic switch, which has a signal output terminal at a connection point thereof, connects the output terminal to an input terminal of a controller for controlling the current switching element, and controls an output state of the controller.