JPH0247898B2 - KINSETSUSUITSUCHI - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a proximity switch, and particularly to an improvement in a timer circuit for preventing malfunction when current is turned on.

Generally, this type of proximity switch 10, as shown in FIG. 1, includes a proximity sensor IC circuit 1 and an output transistor 2 for supplementing the output current capacity of this IC circuit.
0. The IC circuit 1 includes an oscillation circuit 2, a comparator 3, an integration circuit 4, a comparator 5, an output circuit 6, a constant voltage circuit 7, and a power supply reset circuit 8.
1, resonance capacitor 16, variable resistor for sensitivity adjustment 1
2, bypass capacitor 13, integrating capacitor 14,
A power supply reset capacitor 15 and the like are externally connected.

Therefore, when the metal body approaches or separates from the detection coil 11, the oscillation circuit 2 starts or stops oscillating, and the output circuit 6 produces an L or H output.

A conventional power supply reset circuit is shown in FIG. This is composed of constant current sources 30, 58, transistors 40, 51-54, resistors 55-57, and capacitor 41.

Now, when the power is turned on, the voltage of capacitor 41 is
V _CT becomes V _CT =Q/C=I _C ×t/C. Here, Q: load of the capacitor, C: capacitance of the capacitor, and I _C : charging current.

Therefore, when the transistor 40 is turned off and the reset is released, the voltage V _CT of the capacitor 41 starts to rise linearly from zero as shown in FIG. When the capacitor voltage V _CT becomes larger than the base potential V _TH of the transistor 51, the transistor 50 turns on, and at the same time, the transistors 51,
54 is turned off, and the base potential of the transistor 51 drops to _VTL . Therefore, unless the capacitor voltage V _CT becomes smaller than V _TL , transistor 51 is turned off and current flows through transistor 50.
I ₂ flows. At this time, the current flowing into the capacitor 41 is equal to the value obtained by subtracting the current flowing into the base of the transistor 50 from the current source _I1 , and the rising speed of the voltage V _CT increases at a slower rate than before, until it reaches a point close to the internal supply voltage V _S. Stabilize.

When the overcurrent detection circuit 60 detects an overcurrent in the output transistor 20 in this state, the output of the overcurrent detection circuit 60 turns on the transistor 40, discharges the capacitor of this timer circuit, and performs a reset. When the value of voltage V _CT falls below V _TL , transistors 50, 52, and 53 are turned off, and conversely, transistors 51 and 54 are turned on, and the base potential of transistor 51 rises to V _TH . As a result, the output is prohibited, so the output transistor 20 is turned off, the overcurrent detection circuit 60 is inverted, and the transistor 40 is turned off. Therefore, capacitor 41
begins charging until its charging voltage V _CT exceeds the voltage V _TH . Then, when the voltage V _CT exceeds the voltage V _TH , the output inhibition is canceled and the output transistor 2
Since overcurrent flows again to 0, transistor 4
0 is turned on and the voltage _VCT is discharged until it becomes lower than the voltage _VTL . By repeating this process, the state of the output transistor 20 becomes as shown in FIG. As can be seen from this figure, the initial width of the on-pulse is larger than the width of subsequent pulses. This is because the width of the output on-pulse depends on the discharging time of the capacitor 41, but initially it is the discharging time t ₁ from the internal supply voltage V _S to the voltage V _TL , and then from the voltage V _{TH to the voltage V TL} .
The discharge time to V _TL may be t ₂ , and (V _S − V _TL ) > (V _TH
−V _TL ), so t ₁ > t ₂ .

In this way, the output transistor 20 must withstand the overcurrent for a time _t1 , which is longer than the time _t2 .

It is generally known that the breakdown characteristics of a transistor depend on average power consumption and instantaneous power consumption, but in terms of instantaneous power consumption, the first on-pulse when transitioning from oscillation to non-oscillation is It is disadvantageous that the output transistor 20 is longer.
A material with good breaking properties is required.

The present invention aims to eliminate such conventional drawbacks, and one embodiment of the present invention will be described below with reference to the drawings.

That is, FIG. 5 shows a power supply reset circuit according to the present invention, in which 30 is a first constant current source, 31 is a second constant current source, 32, 33, 34, 35, 36, 3.
7, 38, 39, and 40 are transistors, 41 is a capacitor, and 49 is a diode. Note that 61 is an output prohibition circuit, 60 is an overcurrent detection circuit, and 20 is an output transistor. and transistor 36
and 37 form a Schmitt circuit.

In this figure, when the power is turned on and the transistor 40 is turned off to release the short-circuited state of the capacitor 41, charging of the capacitor 41 starts.
V _CT starts rising from zero. capacitor 41
When the voltage V _CT is close to zero, the transistor 36
is off, the transistor 37 is on, the current value I ₂ determined by the transistor 35 flows through the transistor 37, and the emitter voltage V _E of the transistors 36 and 37 is equal to the saturation voltage (V _SAT 0.1V) of the transistor 34, the diode The voltage V E ≈V _S −V _SAT −nV _D −V _D is obtained by subtracting the base-emitter voltage ( _{≈V D} ) of the transistor 37 from V _S by the voltage drop of 49 (=nV _D ).

Therefore, transistor 36 has a base voltage
It continues to turn off until V _CT becomes a value higher than V _E by V _D. The critical voltage V _TH of V _CT for transistor 36 to flip from off to on is V _TH = V _E + V _D = V _S − V _SAT (0.1V) − nV _D , and V _CT increases until it reaches V _TH Continue.
Then, when V _CT reaches V _TH , transistor 3
6 is turned on, but the current flowing into the base of transistor 37 flows into transistor 36, so transistor 37 is turned off. A current flows through the transistor 36 due to its current capacity with the transistors 34 and 35, but the current flowing to the emitter of the transistor 35 is the sum of the current I ₂ /N supplied from the transistor 34 and the current flowing from the base of the transistor 36. things flow.

Here, if I ₁ + I ₂ /N<I ₂ , then I _X = I ₂ −
A capacitor discharge current of (I ₁ + I ₂ /N) occurs,
The base and emitter voltages of transistor 36 decrease. Eventually, when the saturation voltage V _SAT between the emitter and collector of the transistor 35 reaches a value close to zero, for example 0.1V, the current carrying capacity of the transistor 35 becomes lower than I ₂ and stabilizes when it reaches I ₁ + I ₂ /N. .

FIG. 7 shows V _CT in the above state. The value of V _CT at which transistor 36 is turned off, V _TL
becomes V _TL = V _D + V _SAT .

When the output transistor 20 is on and an overcurrent flows, the overcurrent detection circuit 60 raises the base potential of the transistor 40 so as to reset the timer circuit. Then this transistor turns on,
Although the capacitor 41 is discharged, since the voltage V _CT of the capacitor 41 is close to the voltage V _TL , the time for the voltage V _CT to reach a voltage lower than the voltage V _TL is as short as an elbow. When the voltage V _CT falls below V _TL , the transistor 36 is turned off, and the base current flows from the transistor 34 to the transistor 37, so that the transistor 3
7 turns on. The base potential and emitter potential of the transistor 37 are the same as when the power is turned on, and are not inverted until the base potential of the transistor 36 becomes equal to or higher than the voltage _VTH . This state is a state in which output is prohibited.
Since the current in output transistor 20 is gone, transistor 40 is turned off and charging of capacitor 41 begins, which continues until its charging voltage V _CT exceeds voltage V _TH . When the voltage V _CT exceeds the voltage V _TH , the output inhibition is canceled and overcurrent flows through the output transistor 20 again, so the transistor 40 is turned on and the voltage V _CT becomes the voltage V _TL.
is discharged until it drops below . By repeating this process, the state of the output transistor 20 becomes as shown in FIG. It can be seen from this figure that the initial width of the on-pulse is smaller than the width of subsequent pulses.

In addition, FIG. 9 compares the configurations of the conventional example shown in FIG. 2 and the embodiment of the present invention, that is, FIG. 5, for reference; a indicates the conventional example, and b indicates the embodiment of the present invention. ing.

As described above, this invention generates an output signal according to the oscillation amplitude of an oscillation circuit having a detection coil, and includes a timer circuit having a capacitor, and an output signal for forcibly holding the output signal in a predetermined state before the timer circuit times up. and a proximity switch having an overcurrent detection circuit that detects an overcurrent condition, which is an overload condition of the output circuit, and resets the timer circuit with a detection signal, the timer circuit includes a capacitor and a first circuit that charges the capacitor. A constant current source, a first transistor discharging the capacitor, and a second constant current source are used as collector loads,
a second transistor having the capacitor charging voltage connected to its base; and a collector of the second transistor;
In a Schmitt circuit having an emitter as a base, a third transistor connected to the emitter, and a third constant current source into which current from the emitters of the second and third transistors flows, the current of the first constant current source and Since the current of the third constant current source is set larger than the sum of the second constant current sources, the width of the first output pulse until the output transistor turns on and its output is inhibited in an overload state. is much narrower than the subsequent output pulse width. Therefore, since the output transistor only has to withstand the instantaneous power corresponding to the steady-state pulse width, there is an advantage that an output transistor having a correspondingly low breakdown resistance can be used.

[Brief explanation of drawings]

Fig. 1 is a block circuit diagram of a general proximity switch, Fig. 2 is a circuit diagram of a conventional power supply reset circuit, Fig. 3 is a characteristic diagram of a timer circuit, and Fig. 4 shows the relationship between capacitor discharge characteristics and output pulses. A characteristic diagram showing the relationship, FIG. 5 is a circuit diagram showing an embodiment of the power supply reset circuit in the proximity switch of the present invention, FIG. 6 is an equivalent circuit diagram of FIG. 5, and FIG. 7 is a characteristic diagram of the timer circuit. FIG. 8 is an output waveform diagram of the output transistor, and FIG. 9 is an equivalent circuit diagram showing a comparison between FIG. 2 and FIG. 5. 30...First constant current source, 31...Second constant current source, 32, 33, 34, 35, 36, 37, 3
8, 39, 40...transistor, 49...diode, 41...capacitor, 61...output prohibition circuit, 20...output transistor, 60...overcurrent detection circuit.

Claims (1)

[Claims] 1. A proximity switch that generates an output signal according to the oscillation amplitude of an oscillation circuit having a detection coil includes a timer circuit, and is controlled by the timer circuit, and forcibly outputs the output signal to a predetermined value before the timer circuit times up. and an overcurrent detection circuit that detects that the output current of the output circuit is an overcurrent, outputs the overcurrent detection signal, and resets the timer circuit. has a timer capacitor, a first current source for charging the capacitor, a discharge circuit that discharges the timer capacitor, and a Schmitt circuit that receives the capacitor charging voltage as input, and the Schmitt circuit has a second current source. is a load on the collector side, has a third current source on the emitter side, and has a base connected to one end of the timer capacitor, and a base connected to the collector of the first transistor, and a second transistor whose emitter is connected to the emitter of the first transistor, and the sum of the current capacity of the first current source and the current capacity of the second current source is the third current source. The Schmitt circuit described above is characterized in that the current capacity of the first current source is smaller than the current capacity of the second current source divided by the current amplification factor of the first transistor. Proximity switch in the timer circuit.