JPH058608B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of the Invention] The present invention relates to a power supply reset circuit used in non-contact switches such as photoelectric switches and proximity switches, which stops output for a predetermined period of time to prevent malfunctions when the power is turned on. .

[Prior art and its problems]

Immediately after the power is turned on, the circuit voltage of a non-contact switch becomes unstable and its operation becomes unstable. In particular, in a high frequency oscillation type proximity switch etc., an object cannot be detected until after the oscillation circuit has started oscillation normally, so it is necessary to stop the output operation for a predetermined period of time after the power is turned on. Therefore, the non-contact switch is provided with a power reset circuit so that the non-contact switch outputs to the outside only after a predetermined period of time has passed after the power is turned on and the voltage reaches a predetermined voltage. Among such power supply reset circuits, there is known a reset circuit that charges an integrating capacitor with a minute current and delays the output depending on the time it takes for the voltage to reach a predetermined voltage. In this way, according to the reset circuit, the delay time is determined by the capacitance of the integrating capacitor.

However, in non-contact switches that require miniaturization, it is difficult to use capacitors that have a large shape, and for example, only capacitors with a capacity of several tens of pF or less can be used. Therefore, in order to lengthen the delay time, a Miller integration circuit using a Darlington connection was used to integrate the capacitor. FIGS. 4 and 5 are diagrams showing these conventional power supply reset circuits using Miller integration.
Two NPN transistors 2 and 3 are connected in Darlington, an integrating capacitor C is inserted between the collector and base of transistor 2, and the capacitance of capacitor C is equivalently multiplied by the current amplification factor (hfe) of the two transistors. The terminal voltage was detected by a comparator 4 to obtain the delay time. Similarly, in FIG. 5, a Miller integration circuit is constructed using a Darlington connection using an NPN type transistor 5 and a PNP type transistor 6.

According to such a conventional power supply reset circuit,
Since the capacitance of capacitor C is equivalently the product of hfe of two Darlington transistors, it is possible to lengthen the delay time. However, since the current amplification factor of the transistor changes depending on the temperature, if a Darlington connection is used, the temperature change rate will also be the product of this. Therefore, the delay time varies greatly depending on the temperature, resulting in a problem that the reset time when the power is turned on becomes unstable.

[Purpose of the invention]

The present invention was made in view of the problems of the power supply reset circuit of the conventional non-contact switch. It is an object of the present invention to provide a power supply reset circuit capable of performing a reset operation.

[Structure and effects of the invention]

The present invention is a power supply reset circuit that is used in a non-contact switch and stops output for a predetermined period of time after the power is turned on. A current amplifier is constructed using a current mirror circuit consisting of a source and a pair of transistors with different emitter areas, and a mirror integrating circuit that charges an integrating capacitor using the current amplifier, and compares the voltage of the integrating capacitor with a predetermined level. and a comparison means for stopping the reset when the charging voltage reaches a predetermined value.

According to the present invention having such characteristics, a current mirror circuit is used in the mirror integration circuit, and the emitter area is varied to amplify the current, so that changes in the amplification factor based on temperature changes can be minimized. It is possible to do so. Therefore, even if a capacitor of small capacity is used, it is possible to create a power supply reset circuit that always has a stable delay time when the power is turned on by using the Miller integration circuit.

[Explanation of Examples]

FIG. 1 is a circuit diagram showing one embodiment of a power supply reset circuit according to the present invention. In this figure, a voltage detection circuit 10 is provided between both ends of the power supply. The voltage detection circuit 10 is a detection circuit that detects whether the power supply voltage exceeds a predetermined value that allows operation, and provides an output to the constant current source 11 if the power supply voltage exceeds the predetermined value. The constant current source 11 supplies a constant current of, for example, several microamperes to the transistor 12 when an output is given from the voltage detection circuit 10. The transistor 12 and the transistor 13 form a current mirror circuit CM1, and an integrating capacitor C is connected between its collector and ground. The emitter area of transistors 12 and 13 is
nS, that is, it is assumed that the emitter of the transistor 13 is formed on the IC chip so as to be n times larger. The collector of the other transistor 13 of the current mirror circuit CM1 is connected to the base of the NPN transistor 14. The collector of the transistor 14 is commonly connected to the emitters of the two transistors 12 and 13 of the current mirror circuit CM1 and the constant current source 11, and one end of the transistor 14 is provided to the comparator 15. The comparator 15 provides an output that enables the non-contact switch to operate when the voltage of the power supply reset circuit reaches a predetermined value.

FIG. 2 is a circuit diagram showing another embodiment of the power supply reset circuit according to the present invention. In this embodiment, the same parts as in FIG. 1 are indicated using the same reference numerals.
In this embodiment, NPN type transistors 16 and 17 are used in the current mirror circuit CM2, and a PNP type transistor 18 is used as a transistor for amplifying the mirror current. The emitter area ratio of the transistor 17 of the current mirror circuit CM2 is made n times as in the first embodiment, and the collector of the transistor 17 is connected to the base of the transistor 18, as in the embodiment of FIG. It is.

Next, the operation of the first embodiment described above will be explained. FIG. 3 is a graph showing changes in power supply voltage after power is turned on. In this figure, if the power is turned on at time to, then the voltage will rise as shown in Figure 3, but at time t1 it will reach a predetermined voltage level.
When the voltage reaches Vr, the voltage detection circuit 10 provides an output to the constant current source 11. Then, a constant current flows out from the constant current source 11 and is applied to the current mirror circuit CM1 and the transistor 14. Current mirror circuit CM1
Therefore, the collector current of the transistor 13 becomes n times the collector current of the transistor 12 according to the ratio of the emitter areas, and the NPN type transistor 14
join the base of Here, if the current amplification factor of the transistor 14 is hfe, it means that mirror integration is performed by an amplifier having an amplification factor of n.hfe, and the capacitance of the capacitor C becomes equivalently large. Then, at time t2, when the voltage at the collector terminal of this transistor 14 exceeds a predetermined value, the comparator 15 outputs an output and controls the non-contact switch to operate. Here, the current amplification factor of the transistor 14 changes depending on the temperature as in the conventional example, but
The emitter area of a current mirror circuit formed on an IC chip does not depend on temperature changes. Therefore, fluctuations in the amplification factor can be significantly reduced compared to the conventional example, and a stable reset time T can be obtained.

Similarly, in the embodiment shown in FIG. 2, when a current is supplied from the constant current source 11, the current flowing to the integrating capacitor C is amplified by the current mirror circuit CM2 and the transistor 18, and the current is amplified by mirror integration. As a result, the capacitance of capacitor C equivalently increases, and the terminal voltage of integrating capacitor C gradually increases. Therefore, the power supply reset time T can be obtained by the comparator 15.

In each of the embodiments described above, the current amplified by the current mirror circuit is further amplified by the transistor to equivalently increase the amplification factor of the mirror integral. The current may be further amplified by a current mirror circuit to equivalently increase the amplification factor of the mirror integral amplifier.

Further, in each of the embodiments described above, a current mirror circuit and a transistor are connected in cascade, but it is also possible to use two current mirror circuits to amplify the current.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing one embodiment of the power reset circuit of a non-contact switch according to the present invention, Fig. 2 is a circuit diagram showing another embodiment of the power supply reset circuit according to the present invention, and Fig. 3 is a circuit diagram showing the power supply reset circuit of the non-contact switch according to the present invention. Graphs showing the subsequent power supply voltage, and FIGS. 4 and 5 are circuit diagrams showing an example of a conventional power supply reset circuit. 1, 11...constant current source, 2-5, 12-14,
16-18...transistor, 10...voltage detection circuit, 15...comparator, C...integrating capacitor, CM1, CM2...current mirror circuit.

Claims (1)

[Claims] 1. A power supply reset circuit used in a non-contact switch that stops output for a predetermined period of time after power is turned on, comprising: an integrating capacitor; and charging of the integrating capacitor when the power supply voltage reaches a predetermined value. Construct a current amplifier using a current source to start and a current mirror circuit consisting of a pair of transistors with different emitter areas,
A Miller integrating circuit that charges the integrating capacitor using the current amplifier; and comparing means that compares the voltage of the integrating capacitor with a predetermined level and stops resetting when the charging voltage reaches the predetermined value. A power reset circuit for a non-contact switch characterized by: 2. The power supply reset circuit for a non-contact switch according to claim 1, wherein the mirror integration circuit is constructed by cascading a current mirror circuit and a current amplification transistor.