JPH0452009B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical field of interest] This invention relates to an overcurrent prevention circuit for a non-contact switch.

[Prior art]

The prior art will be explained using a proximity switch as an example. Generally, a proximity switch has a configuration as shown in FIG. In this figure, the proximity switch is
IC circuit 1, detection coil 15 connected to this IC circuit, capacitor 16 forming a resonance circuit with this detection coil, resistor 17 for operating distance adjustment, capacitor 18 for timer, output transistor 20, resistor 19 for overcurrent detection and a load 23 connected in series with a power supply 24 between terminals 21 and 22. The IC circuit 1 includes an oscillation circuit 3 connected to a detection coil 15 as a detection and signal processing system, a signal processing circuit 4 that outputs an output signal according to changes in the oscillation amplitude, and an external output circuit according to the output signal. An output circuit 11 that drives the transistor 20, a stabilization circuit 2, and an overcurrent detection circuit 10 for output protection.
It is composed of an overcurrent prevention circuit 5. The overcurrent prevention circuit 5 includes a charging circuit 6, a discharging circuit 7, a signal output circuit 8, and a voltage comparison circuit 9. The overcurrent prevention circuit 5 has two functions: one is to prevent output malfunction in an unstable state when the power is turned on, and the other is to prevent damage to the output transistor 20 when an overcurrent flows through it. It works to prevent. In the former, after the stabilized power supply 2 becomes active after the power is turned on, charging of the timer capacitor 18 is started by the charging circuit 6, and the output transistor 20 is forcibly turned off until the charging voltage reaches a predetermined value. It is. The latter is output transistor 2
0, a voltage is generated in the overcurrent detection resistor 19, this voltage is detected by the overcurrent detection circuit 10, and the discharge circuit 7 is detected by this signal.
is activated to promote discharge of the timer capacitor 18, and when the voltage falls below the lower limit threshold of the voltage comparison circuit 9, the output transistor 20 is turned off. When this output transistor is turned off, the resistor 1
9 becomes 0V, and the discharge of the timer capacitor 18 is stopped. Charging starts from this point, and the off state continues until the upper limit threshold. If the upper limit threshold is exceeded, an overcurrent will flow again, this will be detected, and the above operation will be repeated. At this time, since there is a considerable difference between the charging time and the discharging time of the timer capacitor 18, the on time of the output transistor 20 becomes pulse-like, which is sufficiently smaller than the off time. This suppresses the collector current of the output transistor 20, reduces heat generation, and prevents damage.

Conventionally, a circuit shown in FIG. 2 has been used to perform these functions.

Note that explanations of parts common to those in FIG. 3 will be omitted.

In this circuit, resistors 12, 13, 32, 3
8. The transistor 31 constitutes an overcurrent detection circuit that detects whether an overcurrent is applied to the resistor 19. The transistor 33 operates as a discharge circuit during overcurrent.
Further, the constant current source 40 and the transistors 35 and 36 each constitute a discharge circuit that operates when the power is turned on. The constant current source 47 and the transistor 48 form a buffer circuit, the transistor 49 forms a level shift circuit, and the constant current sources 37 and 41 and the transistor 4 form a buffer circuit.
A 2, 43, 44 Schmitt trigger circuit, ie, a voltage comparison circuit 9 is configured. transistor 4
Reference numerals 5 and 46 indicate current mirror circuits, which send an output prohibition signal to the output circuit 11.

When the proximity switch in FIGS. 3 and 2 is powered up, the regulated power supply is activated and discharged by transistor 35 until it is in the active region. Next, when the active region is reached, a signal is transmitted to the transistor 36, turning off the transistor 35, and the constant current source 39 starts charging the capacitor 18. At this time, transistor 48 is on, transistor 43 is off, and transistor 4
4, 45, and 46 are turned on, and a signal prohibiting output is issued. The threshold of the Schmitt trigger circuit is the upper threshold, and the collector-emitter voltage V _CE of the constant current source 37 acting as an active load,
Base-emitter voltage V _BE of transistor 43
I can do it. When the voltage Vreg of the stabilized power supply 2 is set to 2.4V, the emitter voltage of the transistor 44 is approximately 0V when the transistor 43 is on, and approximately 0.9V when it is off, and the upper limit threshold at the base of the transistor 43 is approximately 1.6. V, the lower threshold is approximately 0.7V. The potential of the capacitor is also approximately
The threshold values are 1.6V and approximately 0.7V, and the charging potential is approximately
When the voltage reaches 1.6V, transistor 43 turns on, transistors 44, 45, 46, and 48 turn off,
Cancel output prohibition. Thereafter, the potential of the timer capacitor 18 continues to increase its charging potential, rising to the voltage Vreg of the stabilized power supply 2 minus the voltage _VCE of the constant current source 37, and enters a normal operating state. During overcurrent detection, when the potential generated in the resistor 19 reaches approximately 0.7V, the transistors 31 and 33 turn on, and the timer capacitor 1
Discharge the charge of 8. When the potential of this capacitor drops from around the voltage Vreg of the stabilized power supply 2 to 0.7V, the output prohibition is lifted, but if further discharge occurs and the voltage drops below 0.7V, the transistors 44, 45,
46 and 48 turn on, generate an output signal, and inhibit the output of the output circuit 11. When output is prohibited,
The voltage drop across resistor 19 becomes 0V, transistors 31 and 33 are turned off, and charging begins. By setting a large duty ratio between the charging time and the discharging time and shortening the ON time of the output transistor 20, the time during which overcurrent flows is shortened, heat generation is reduced, and damage to the transistor is prevented.

[Problems with conventional technology]

Upon receiving the overcurrent detection signal, the discharge circuit 7 is turned on, the Schmitt trigger circuit, that is, the transistor 43 of the voltage comparator circuit 9 is turned off, and the transistor 4
It takes a long time to turn on the output transistor 5 and turn off the output transistor 20. Therefore, the average current consumption when overcurrent is flowing is several tens of mA. Furthermore, the current instantaneously flowing through the output transistor 20 is several A.
Therefore, the loss of this transistor increases,
A large package is required.

[Purpose of the invention]

This invention shortens the time from overcurrent detection to turning off the output transistor, thereby reducing the power loss of the output transistor, downsizing the IC package, and downsizing the entire non-contact switch. The purpose is to

[Summary of the Invention] This invention provides a buffer circuit between a timer capacitor and a voltage detection circuit that detects the potential of this capacitor, and also provides a level shift transistor between the buffer circuit and the voltage detection circuit. A constant current circuit is connected to the input terminal of the voltage detection circuit, and the emitter of the level shift transistor is connected to the constant current circuit, thereby reducing the time from overcurrent detection until the output transistor turns off. It can be shortened.

〔Example〕

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

Note that the description of the same parts as shown in FIGS. 2 and 3 will be omitted.

That is, transistor 50 constitutes a level shift circuit, which is provided between the buffer circuit and the Schmitt trigger circuit. That is, the base and collector of transistor 50 are connected to transistor 4.
The emitter of the transistor 8 is connected to the base of the transistor 43. Furthermore, the emitter of transistor 50 is connected to the collector of transistor 51, and the emitter of this transistor is connected to the other terminal 22 of the power supply. Further, the base of the transistor 51 is connected to the base of the transistor 52, and the base and emitter are connected,
Thereby, transistors 51 and 52 constitute a current mirror circuit. The collector of transistor 52 is connected to constant current source 53 . That is, the emitter of the transistor 50 is substantially the current source 5.
Connected to 3.

〔motion〕

Since the operation of the circuit shown in FIG. 1 is basically the same as that of the circuit shown in FIGS. 2 and 3, the explanation thereof will be omitted, and the operation of the main part of the present invention will be explained.

That is, since the current value of the constant current source 47 is absorbed by the emitter side of the transistor 50 in an amount almost equal to that of the constant current source 53, a current value twice or more of the current value of the constant current source 53 is required. As a result, the charge accumulated between the base and emitter of the transistor 43 is constantly absorbed through the transistor 51.
When the potential of the timer capacitor 18 changes from high to 0V, the transistor 48 is turned on and no current flows through the transistor 50. Then, the charge corresponding to the base-emitter capacitance of transistor 43 is absorbed through transistor 51,
The response until the transistor 43 turns off becomes faster. When timer capacitor 18 goes from 0V to high, transistor 48 turns off and current flows through transistor 50. Approximately half of the current is absorbed through the transistor 51, but the remaining current allows the transistor 43 to operate stably.

〔effect〕

As mentioned above, this invention connects the emitter of the level shift transistor to the constant current circuit, so the time from overcurrent detection to the output transistor turning off is less than one-tenth that of the conventional one. Therefore, it is possible to select an output transistor with low power dissipation, and the IC package can also be made smaller.
Also, the base of the input transistor of the voltage comparison circuit
Since the charge in the emitter-to-emitter capacitance is quickly discharged, the output of the voltage comparator circuit is stabilized, which has the effect of increasing the reliability of the overcurrent prevention circuit.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of an overcurrent prevention circuit according to the present invention, and FIGS. 2 and 3 are circuit diagrams of conventional overcurrent prevention circuits. 1... IC circuit, 2... Stabilized power supply, 3... Oscillation circuit, 4... Signal processing circuit, 5... Overcurrent prevention circuit, 6... Photoelectric circuit, 7... Discharge circuit, 8...
Signal output circuit, 9... Voltage comparison circuit, 10... Overcurrent detection circuit, 11... Output circuit, 12... Resistor, 13... Resistor, 14... Transistor, 15
...Detection coil, 16...Capacitor, 17...
Resistor, 18...Timer capacitor, 19...Resistor, 20...Output transistor, 21...Terminal,
22...terminal, 23...load, 24...power supply, 3
1... Transistor, 32... Resistor, 33... Transistor, 35... Transistor, 36... Transistor, 37... Constant current source, 38... Resistor,
39... constant current source, 40... constant current source, 41...
Constant current source, 42...transistor, 43...transistor, 44...transistor, 45...transistor, 46...transistor, 47...constant current source, 48...transistor, 50...transistor, 51...transistor , 52...transistor, 53...constant current source.

Claims (1)

[Claims] 1 In a non-contact switch that drives a load using the output signal of the detector, an overcurrent detection circuit that detects the overcurrent flowing through the load, a timer capacitor connected to a constant current source, and the overcurrent detection circuit a discharging transistor that is driven by the output and discharges the capacitor; a voltage detection circuit that detects the potential of the capacitor; a buffer circuit provided between the voltage detection circuit and the capacitor; A level shift transistor provided between the voltage detection circuit and a constant current circuit connected to the input terminal of the voltage detection circuit and consisting of a constant current source and a transistor, the emitter of the level shift transistor being connected to the An overcurrent prevention circuit for a non-contact switch characterized by being connected to a constant current circuit.