JPH0260090B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an overcurrent detection circuit for an electronic switch, such as a proximity switch.

Conventionally, this type of overcurrent detection circuit has a resistor _R1 inserted between the emitter of the output transistor _Tr1 and a voltage _Vr1 across the emitter of the output transistor Tr1 as shown in Figures 1 and 2. ₂ , which is the simplest method to detect overcurrent, and is used in a wide variety of applications. Although this method is easy to implement, it has the disadvantage that the residual voltage at the output terminal when it is turned on becomes extremely large. In other words, the remaining voltage Von when transistor Tr ₁ is turned on depends on the output current value and the output current capacity of transistor Tr ₁ , but if Von is Von = 0.2 [V] and the output current It = 150
[mA] and 200 mA including the overcurrent detection circuit.
When a current of [mA] or more flows, this is considered an overcurrent. When the base-emitter voltage Vbe of the transistor Tr ₂ is detected to be 0.7 [V] or more, the value of the emitter resistance R ₁ becomes R ₁ =0.7 [V]/0.2 [A] = 3.5 [Ω]. Also, the voltage Vr ₁ across the emitter resistor R ₁ when the output current It = 150 [mA] is Vr ₁ = 3.5 × 0.15 ≒ 0.525 [V], and therefore the series circuit of the transistor Tr ₁ and the emitter resistor R ₁ The voltage Vx is Vx = Von + Vr = 0.2 + 0.525 = 0.725 [V]. By inserting the overcurrent detection circuit in this way, the remaining output voltage when turned on as an electronic switch is more than tripled, and as is clear from the calculation formula, the output power capacity of the output transistor Tr ₁ is increased and The drawback is that even if the ON output residual voltage Von is somewhat reduced, the ON output residual voltage including the overcurrent detection circuit hardly decreases.

Further, in the conventional overcurrent detection circuit, since the base-emitter voltage of the transistor _Tr1 was the overcurrent detection voltage, it had a large negative temperature coefficient.

The present invention aims to eliminate such conventional drawbacks, and the first object of the present invention is to reduce the detection voltage compared to the one that detects overcurrent using the voltage between the base and emitter of a transistor. This aims to reduce the overcurrent detection resistance of the electronic switch and reduce the remaining voltage when the switch is turned on.

An embodiment of the present invention will be described below with reference to the drawings.

FIG. 3 is a circuit diagram in which the overcurrent detection circuit according to the present invention is applied to a proximity switch. This proximity switch 10 consists of a proximity sensor IC circuit 1 and an output transistor to supplement the output current capacity of this IC circuit. It is composed of a current switching element 26. This IC circuit has an oscillation circuit 2, a comparator 3, an integration circuit 4, a comparator 5, 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. The output circuit 6, the power supply reset circuit 8, and the output controller 14 constitute an electronic switch controller 17. Further, a detection coil L ₁ , a resonance capacitor C ₁ , a variable resistance for sensitivity adjustment R ₂₂ , a bypass capacitor C ₂₃ , an integrating capacitor C ₂₄ , a power supply reset capacitor C ₂₅ and a load 9 are attached externally.

Therefore, when the metal body approaches or separates from the detection coil _L1 , the oscillation circuit 2 starts or stops oscillation, and the output circuit 6 outputs a logic L or H output.

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 the main part of the present invention, which basically includes four transistors, namely, a first transistor 22, a second transistor 23, a third transistor 24, and a fourth transistor 25.
An overcurrent detection circuit 16 is constituted by a block consisting of resistors 30, 31, and 32, a current source 20, and a transistor 21. Diode 27, resistor 3
5 constitutes an overvoltage protection circuit 18, which protects the overcurrent detection circuit 16 when a large voltage is applied to the node 41. This overvoltage protection circuit is for current sinking. 19 is a power source.
Note that the current switching element 26 may be any element as long as it controls the current flowing to the load 9, and may be constituted by a transistor, for example. Each transistor 23,
Connection terminals a and b are drawn out to the connection point between the transistors 25 and the respective resistors 31 and 32, and a signal output terminal G is provided at the connection point between the transistors 24 and 25, and this output terminal further controls the current switching element 26. It is connected to the input terminal of the controller 17, thereby controlling the output state of the controller 17. Further, in FIG. 4, the overvoltage protection circuit 18 is connected to terminal b. In FIG. 4, the overcurrent detection circuit 16 and the overvoltage protection circuit 18 are integrated into ICs.

Now, in Fig. 4, the current I ₀ from the current source 20 is
I ₀ = 5 [μA], resistance values of resistors 31 and 32 R ₃₁ , R ₃₂
When R ₃₁ = R ₃₂ = 10 [kΩ], the voltage V ₄₁ at node 41 becomes the overcurrent detection voltage, and its value is V ₄₁ = I ₀ ×
R ₃₁ = I ₀ × R ₃₂ = 5 × 10 = 50 [mV]. This is because when node 41 is open, transistors 24 and 25 are balanced, and output node 4
6 is in a critical state.

Now, when node 41 is connected to output current detection resistor _R1 , if the voltage generated across this resistor is less than 50 [mV], current flows from node 41 to resistor _R1 through resistor 35. Therefore, the current flowing through the resistor 32 decreases, the emitter voltage of the transistor 25 decreases, and the current flowing through the resistor 32 decreases.
The balance between nodes 4 and 25 is disrupted, and node 46 becomes a current sink. At this time, the output of the electronic switch controller 17 becomes, for example, H, and the current switching element 26 is in the on state.

Also, for some reason, the current switching element 26
When the current flowing through node 46 exceeds a predetermined value, the voltage generated across resistor _R1 exceeds 50 [mV], and node 46
becomes the current source. Controller 17 controls the current flowing through current switching element 26 in response to a change in the state of node 46.

Further, when the voltage of the resistor _R1 is 50 [mV], no current flows through the resistor 35, and this becomes the threshold voltage.

In this way, the voltage generated across the resistor R ₁ is 50
When it exceeds [mV], the overcurrent detection circuit 16 detects that point.

FIG. 5 shows an overvoltage protection circuit 18 connected to connection terminal a, and its operation is substantially the same as that shown in FIG. 4, except that current switching element 2
Node 46 becomes a current source when the current value flowing through node 6 is less than a predetermined value, and becomes a current sink when an overcurrent is detected, and the logic of node 46 is reversed compared to that shown in FIG.

Although the above description is based on the case where the point B is in the current sink output format, the same structure can be applied to the current source output format.

The circuit shown in FIG. 6 is a current source circuit opposite to that shown in FIG. 4, and the emitter of the first transistor 22 is connected to one terminal of the power supply 19 via a resistor 33. Further, the emitter of the third transistor 24 is connected to one terminal of the power supply via a resistor 34. Furthermore, an overvoltage protection circuit 18 is constituted by a diode 28 and a resistor 36, and this circuit is connected to a terminal c provided at a connection point between the emitter of the first transistor 22 and the resistor 33. Note that the overvoltage protection circuit may be connected to a terminal d provided at a connection point between the emitter of the third transistor 24 and the resistor 34. The logic level of node 46 in this case is reversed.

Note that FIG. 7 shows a modification of the current mirror circuit, and the modifications shown in FIG. 7b and c are well known compared to FIG. 7a.

In Figure 4, if the detection voltage is 50 [mV] and the overcurrent is 200 [mA], the resistance R ₁ is R ₁ = 50 mV/200 mA = 0.25 [Ω], and therefore the output current It = 150 [mA]. The remaining ON voltage Vx at the output terminal is Vx = Von + Vr ₁ = 0.2 + 0.25 x 0.15 = 0.2 + 0.0375 = 0.2375 [V], which is a significant improvement over the conventional one.

As is clear from this calculation example, in the conventional example, the resistance R ₁ is smaller than the remaining voltage Von when transistor Tr ₁ is ON.
However, according to the present invention, the voltage Vr ₁ across the resistor R ₁ is considerably smaller than the remaining voltage Von of the current switching element 26 when it is ON, and therefore when it is ON, the voltage Vr ₁ across the resistor R 1 is quite large. If a transistor with a large capacity is used to reduce the remaining voltage Von, it is possible to further reduce the remaining voltage Vx when the output terminal is ON.

[Brief explanation of drawings]

Figures 1 and 2 are circuit diagrams showing conventional overcurrent detection circuits, Figure 3 is a block diagram when the overcurrent detection circuit of the present invention is applied to a proximity switch, and Figure 4 is a circuit diagram of the overcurrent detection circuit of the present invention. A circuit diagram showing one embodiment of the detection circuit, FIG. 5 is a circuit diagram showing another embodiment of the invention, FIG. 6 is a circuit diagram showing still another embodiment of the invention, and FIG. 7 is a current mirror. FIG. 7 is a circuit diagram showing a modified example of the circuit. 1... IC circuit, 2... Oscillation circuit, 3... Comparator, 4... Integrating 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 resistor, C ₁ ... Resonant capacitor, C ₂₄ ... Integrating capacitor, C ₂₅ ... Capacitor, 16... Overcurrent detection circuit, 17 ...Electronic switch controller, 18...Overvoltage protection circuit, 19
...Power source, 20...Current source, 21...Transistor, 2
2 to 25...Transistor, 26...Current switching element, 27...Diode, 28...Diode,
29...Transistor, 30-32...Resistor, _R1 ...
Resistance, 41, 42...node, 46...output node.

Claims (1)

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