JPS5842569B2 - Proximity switch - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a proximity switch, and more particularly to a circuit suitable for implementing a high frequency oscillation type proximity switch into an IC.

A conventional proximity switch is constructed as shown in FIG.

In Fig. 1, the proximity switch 1 includes an IC circuit 11, a detection coil 12 externally connected to the IC circuit 11, a capacitor 13 forming a parallel resonant circuit with the detection coil 12, an output transistor 15, a current detection resistor 16, and a protection circuit. 17, a timer circuit capacitor 18, and a bypass capacitor 19.

The resistance for adjusting sensitivity is omitted.

The IC circuit 11 includes a detection circuit 31, a constant voltage circuit 32, a timer circuit 33, an output circuit 33a1, and the like.

The detection circuit 31 is composed of an oscillation circuit 31a formed including the detection coil 12, and a signal processing circuit 31b that generates an output signal according to a change in the oscillation width of the oscillation circuit 31a.

The timer circuit 33 is configured to include an externally connected capacitor 18, and is configured to start charging this capacitor 18 when the power is turned on; and to time-up when the charging voltage of this capacitor 18 reaches a predetermined value. It is configured.

From when the power is turned on, the output circuit 33a is controlled to forcibly prohibit the output signal while the timer circuit 33 times out at 1.

This prevents an erroneous output signal from being output due to an unstable state when the power is turned on.

The bypass capacitor 19 is connected to the output terminal of the constant voltage circuit 32, and prevents the oscillation output from being superimposed on the constant voltage power supply line.

In FIG. 1, the transistor 15 is turned off when no object approaches the detection coil 12, and turned on when an object approaches.

Then, as shown by the dotted line between the output terminal and the negative power supply terminal,
If you connect a relay, it will turn on when an object approaches.

Assuming that the current of the output signal flowing through this relay is {circle around (1)} and the value of the resistor 16 is R, a voltage of IXR will be generated at both ends of the resistor 16.

Here, if the base-emitter voltage of the transistor 17 is VBE, then the transistor 17 is turned on when the current 2 increases to 1 which satisfies VnF,=IR.

Then, a portion of the base current of transistor 15 flows through this transistor, so that the current flowing to the collector of transistor 15 is limited.

In this way, the circuit consisting of the resistor 16 and the transistor 17 is
It operates in dangerous situations such as when a load such as a relay is short-circuited and an excessive current flows, and turns on the circuit such as the transistor 15.

By the way, assuming that the normal switching current of the transistor 15, that is, the switching current of the proximity switch 1, is 200 mA, the VBE of the transistor 17 varies from 0.5 to 0.5 depending on the temperature.
Since we want the voltage to vary between 0.7 V and at least 200 mA to flow, the value of the resistor 16 (0.5 R) must satisfy the relationship R<-=2.5 CΩ].

Therefore, if this resistance value R is 2.2 [Ω], then when VB = 0.5 [V] (7), the limiting current I = 227
[mA], ■BE=0.7[■], I=318[
mA].

Therefore, between the positive and negative power terminals of the proximity switch 1, 2
4 [V') is applied, the transistor 15
The maximum loss (when the load is short-circuited) is 7.63CW]'24
[V)X O,318(: becomes Al, transistor 1
5 requires an element with a large voltage that can withstand the above-mentioned loss, and therefore the shape becomes large and the heat generated by the proximity switch 1 also becomes large.

In view of the above, the present invention provides a large η as an output transistor.
To provide a proximity switch that does not require the use of a traditional switch, has the same protective function as the above while reducing heat generation during short circuit, and also solves problems peculiar to IC implementation.

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

In FIG. 2, the proximity switch 2 includes a detection circuit 31 including an oscillation circuit and a signal processing circuit, and a detection circuit 1. A constant voltage circuit 32, a timer circuit 33, and a transistor 34 constituting an output circuit.
35 and transistors 27 and 28 for short circuit protection are integrated into an IC.
It has a circuit formed as a circuit.

Then, the detection coil 12 is connected to this IC circuit in the same manner as described above.
A capacitor 13, a timer circuit capacitor 18, a bypass capacitor 19, a PNP type output transistor 25, and a current detection resistor 26 are each externally attached.

Note that the sensitivity adjustment resistor is omitted.

The timer circuit 33 includes transistors 333, 334, 335.
The charging voltage of the capacitor 18 is compared with a reference voltage provided by a resistive voltage dividing circuit made up of resistors 351 to 353.

When the charging voltage of the capacitor 18 is larger than the reference voltage, the transistor 333 is turned on, and when it is smaller, the transistor 333 is turned off.

Transistors 338-340 constitute a current mirror circuit, allowing a current approximately equal to the current flowing through the collector of transistor 333 to flow through the respective collectors of transistors 339 and 340.

Further, the transistors 335 to 337 constitute a current mirror circuit, and a current proportional to the collector current of the transistor 336 flowing through the resistor 354 flows through the collectors of the transistors 335 and 337, respectively.

Further, the collector current of the transistor 337 also flows as the collector current of the transistor 332.

The transistor 332 constitutes a current □ error circuit together with the transistors 331 and 343.
A current proportional to the collector current of transistor 33
Flow to the collector of 1°343.

Therefore, the current mirror circuit made up of transistors 331, 332, and 343 works as a constant current circuit, passing a constant current from the collector of transistor 331 to dynart 342.
is supplied to the capacitor 18 via the transistor 343.
A constant current is supplied to the collector of transistor 344 and the base of transistor 345.

To explain the operation of this timer circuit 33, the capacitor 18 is in a discharged state when the power is turned on, but it is charged by a constant current flowing through the diode 342, and its charging voltage gradually increases. come.

Therefore, when the charging voltage is lower than the reference voltage determined by the resistors 351, 352, and 353 (transistor 341 is off and transistor 344 is on), transistor 333 is off, so transistor 339 is off. No current flows through the collector, so a state in which no current is supplied to the output circuit made up of transistors 34 and 35, that is, a state identical to the state before the power is turned on is forcibly realized.

Then, the charging voltage of the capacitor 18 is
3, the transistor 333 turns on.

In this way, current begins to flow through the collector of the transistor 339, and the output circuit consisting of the transistors 34 and 35 is supplied with current and becomes operational.

Therefore, during the time 1 when the charging voltage of the capacitor 18 becomes the reference voltage, this timer circuit 33 is connected to the transistor 34.
．． By stopping the current supply to 35, the output signal is forcibly prohibited.

Once transistor 333 is turned on, transistor 3
Since current now flows through the collector of 40, transistor 341 is turned on and short-circuits resistor 353, causing the reference voltage to drop.

Therefore, even if the charging voltage of the capacitor 18 fluctuates around the initial reference voltage, stable operation is performed because a hysteresis is provided in this voltage detection, so to speak.

When an object approaches the detection coil 12, the detection current from the detection circuit 31 disappears, so the transistor 34 is turned off, and as a result, the transistor 25 is turned on.

Then, a load such as a relay connected between the negative power supply terminal and the output terminal is turned on.

At this time, if an excessive current flows due to a short-circuit accident in this load, the voltage across the resistor 26 increases and the transistors 28 and 27 are turned on.

Then, the transistor 344 is turned off because the base current is no longer supplied.

Therefore, the constant current supplied from transistor 343 now flows to the base of transistor 345, turning on transistor 345.

Therefore, since both ends of the capacitor 18 are short-circuited, the capacitor 18 is rapidly discharged.

At this time, since the transistor 2T is on, the reference voltage applied to the base of the transistor 334 is Ov.

Therefore, the transistor 333 remains on until the charging voltage of the capacitor 18 reaches OV, and the transistor 25 for this purpose also remains on, and as a result, the transistor 27 also remains on.

When the charging voltage of the capacitor 18 becomes OV, the transistor 333 is turned off, and therefore the collector current of the transistor 339 becomes 0, so that the transistor 35.25 is turned off.

Then, the transistors 28 and 27 are turned off, and the transistor 341 is also turned off, so that the state is the same as that described at the beginning immediately after the power is turned on, and the capacitor 18 is charged, and its charging voltage increases.

In this way, the transistor 339 is turned off for a certain period of time determined by the timer circuit 33.
5.25 will be forcibly turned off.

After this certain period of time has elapsed, the transistor 339
Assuming that the state of the detection circuit 31 and the state of the load do not change at this time, the transistor 27 is turned on again and the above operation is repeated, so as explained above, the output current is When the voltage becomes excessive, the charging voltage of the timer circuit capacitor becomes OV (1), and the discharge is exhausted, after which the output transistor 25 is forcibly turned off for a period of time determined by the timer circuit. .

Therefore, the output transistor 25 repeats on and off unless the overcurrent condition is resolved, but since the off time is a relatively long time determined by the timer circuit 33, the average current flowing through the output transistor 25 is It will be extremely small.

Therefore, even in the event of an overcurrent, the heat generation (loss) of the output transistor 25 does not increase, making it possible to use an output transistor with a small wattage.

Further, according to the present invention, a parallel circuit consisting of a resistor 281 and a diode 284 is connected between the emitter of the PNP transistor 28 and the power supply Vcc line.

This is to solve problems that occur when this circuit is integrated into an IC.

That is, in a bipolar monolithic IC, it is inevitable that a parasitic capacitance CB will occur on the base side of the PNP transistor 28.

On the emitter side, this CB appears multiplied by hF'E, and this is expressed as C9.

Now, assuming that there is no resistor 281 and diode 284 and that the emitter is directly connected to the power supply Vcc line, we will discuss the problems that arise in this case.

When positive side noise enters the power supply Vcc line and ■cc is raised to the positive side, this positive side voltage causes current to flow from the emitter to the base through the PN junction of the transistor 28 and to the capacitor CB. It will be charged.

This charging current becomes the base current, so transistor 2
8 is turned on, and as a result, transistor 28 is also turned on.

In order to prevent malfunctions due to this noise, a resistor 281 is inserted as shown in the figure.

In this way, CE will be utilized and CB will be made by C8.
The charging current will be absorbed.

Since CE>CB, at least the values of the resistor 281 and the resistor 282 connected to the base are selected so that the rise time constant on the base side is faster than the rise time constant on the emitter side.

By doing this, the power supply Vc is reduced due to positive noise.
Even if c is raised to the positive side, the transistor 28 will not malfunction, and stable operation can be expected.

On the other hand, if negative noise enters the power supply Vcc line and lowers the voltage Vcc, the base side will discharge faster due to the time constant relationship described above.
The charge of co flows into CB through the PN junction between the emitter and base of transistor 28.

This current becomes a base current, causing the transistor 28 to malfunction.

Therefore, as shown in the figure, a diode 28 is installed for bypassing the emitter of the transistor 28 to the power supply Vcc line.
4 is inserted.

In this way, when the power supply voltage Vcc is lowered due to noise, 01i on the emitter side is discharged faster than CB on the base side to prevent malfunction.

As explained above, by connecting the resistor 281 and the diode 284, the problem of poor noise resistance that is inevitable when a bipolar monolithic IC is formed is solved.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing a conventional example, and FIG. 2 is a circuit diagram showing an embodiment of the present invention. 1.2... Proximity switch, 11... IC circuit, 12
...Detection coil, 15, 25, 25a... Output transistor, 16, 26, 26a... Current detection resistor,
17.27...Short circuit protection transistor, 18...
Capacitor for timer circuit, 19... bypass capacitor,
31... Detection circuit, 32... Constant voltage circuit, 33...
・Timer circuit.

Claims (1)

[Claims] 1. An oscillation circuit formed including an external detection coil, a signal processing circuit that generates an output signal according to the oscillation width of this oscillation circuit, an output circuit, and a capacitor that starts charging when the power is turned on. Compare the charging voltage and reference voltage,
A timer circuit is configured to perform a timer operation for a certain period of time from when the power is turned on, and when time-up occurs, the output circuit is disabled and the power supply is reset. In the proximity transistor, the IC circuit includes a PNP type first transistor that turns on when the current of the output signal flowing through the vibration transistor reaches a predetermined value or more; a second transistor that is turned on in response to the transistor being turned on to lower the reference voltage of the timer circuit; and a third transistor that is turned on in response to the second transistor being turned on and that discharges the capacitor of the timer circuit. A proximity switch comprising: a transistor; a resistor connected between the emitter of the first transistor and a power source; and a bypass diode connected in parallel with the resistor from the emitter to the power source.