JPH02213218A - Dc 2-wire sensor - Google Patents

Abstract

PURPOSE:To attain ease of operation display by keeping a minimum light intensity when a load current is less and giving part of a sufficient load current to a light emitting element. CONSTITUTION:A detection signal of a sensor main circuit 1 is given to a switching element 4, and when the switching element 4 is turned on, a transistor(TR) Q8 of a current mirror circuit 11 is driven by a current I0 outputted from a constant current source 7, the current I0 flows to a TR Q7 and a light emitting diode 5 is lighted. The operation is displayed as a required minimum value through the lighting. When a collector current I2 is increased by the increase in the load current, the collector current I3 is increased and the drive current of the light emitting diode 5 is increased from the current I0 to the current I0+I3. Therefore, the light emitting diode 5 is lighted more brightly. Thus, the more the load current increases, the brighter the operating display becomes and then ease of observation is enhanced.

Description

DETAILED DESCRIPTION OF THE INVENTION Summary of the Invention In a DC two-wire sensor equipped with a light emitting element that displays the operating state, when the current flowing through the load is large, a part of the load current flowing through the output circuit of this sensor is transferred to the light emitting element. feedback to the drive circuit. This allows the drive current of the light emitting element to be increased by an extra current when the load is small while ensuring the minimum driving current for the light emitting element, and the operation display can be easily recognized.

Background of the invention Technical field The present invention relates to a DC two-wire sensor.

Prior art and its problems An example of a circuit diagram of a conventional two-wire DC sensor is shown in FIG. In a DC two-wire sensor, operating power for the sensor is supplied through a load. A detection signal from a sensor main circuit 1 including a proximity switch, a photoelectric switch, and other switches is applied to the base of a transistor Q of an output circuit 3. The output circuit 3 includes transistors Q 1 and Q 2 in addition to the transistor Ql, and the transistor Q 1 controls the transistor Q 2 , and the transistor Q 1 controls the output transistor Q 4 . Transistor Q1 is connected between Zener diode D as an output residual voltage circuit and output terminal OO via resistor R1, transistor Q is connected between output terminal 0.02 via resistor R, and transistor Q is connected directly to output terminal OO. 01102, respectively. Further, a Zener diode D2 is connected between the output terminals 010 and 010.

A load 20 and a DC power source 21 are connected in series between output terminals O and 02 of this two-wire DC sensor.

When transistor Ql is off, transistors Q2 and Q4
is off, the output voltage of the DC power supply 21 is applied to the constant voltage circuit 2 via the load 20, and a small amount of current flows, so that the operating voltage is applied from the constant voltage circuit 2 to the sensor main circuit 1.

Transistor Q is activated by the detection signal from sensor main circuit 1.
When transistors Q2 and Q2 are turned on, transistors Q2 and Q2 are also turned on, and load current flows to load 20 through transistor Q4. This load current is determined by the size of the load 20. At this time, the voltage required for the operation EI of the sensor main circuit 1 (output residual voltage) is determined.

By the way, it is a matter of course that an indicator for indicating the operating status of the sensor is required, and the light emitting diode 5 and its driving circuit 8 are provided for this purpose. This light emitting diode 5 is turned on by the detection signal of the sensor, that is, when the transistor Q4 is turned on.

When the output circuit 3 of this DC two-wire sensor is turned on and a load current flows, part of the load current is used to ensure the operation of the sensor as described above. When the load current of the load 20 is smaller than the operating current of the sensor, the sensor cannot operate stably. Conversely, this sensor cannot be used for loads where the load current is smaller than the current required for stable operation of the sensor. From the viewpoint of low power consumption, loads with small load currents are increasing. Therefore, it is preferable that the DC two-wire sensor has a small current consumption. However, in a DC two-wire sensor, the minimum required current is the current required to drive the sensor main circuit 1 and the constant voltage circuit 2, the current required to generate the output residual voltage, and the driving current necessary for displaying the display. If the drive current required for the display is lowered in order to reduce power consumption, the operation of the DC two-wire sensor will not be affected, but the operation display on the display will become very difficult to see.

Summary of the Invention Purpose of the Invention This invention maintains the minimum light emission intensity when the load current is low, but when a sufficient load current is flowing, a portion of it is passed through the light emitting element to make the operation display easier to see. The purpose of the present invention is to provide a DC two-wire sensor that can

Structure and Effects of the Invention The present invention has an output terminal connected to a power source through a load,
In a DC two-wire sensor where operating power is supplied through a load, an output circuit turns on and off between output terminals in response to a detection signal, and a drive that drives a light emitting element for operation indication with a predetermined current in response to a detection signal. A detection circuit that detects a load current flowing through the output circuit when the circuit is turned on, and a current addition circuit that adds a current corresponding to the magnitude of the detected load current to the above predetermined current and flows through the light emitting element. It is characterized by

Further, it is preferable to include a current limiting circuit that defines an upper limit value of the current applied to the light emitting element.

According to this invention, when the sensor outputs a detection signal, in response to this, the operation display light emitting element is driven with a predetermined current to the extent that the light emission from the operation display light emitting element can be visually recognized. This allows the minimum amount of current to flow through the light emitting element, thereby increasing the lower limit of the load current. When the load current is large and the current exceeds the current required for stable sensor operation, this excess current is fed back to the operation display light emitting element, increasing the drive current of the light emitting element. Therefore, the operation display becomes brighter and easier to see.

DESCRIPTION OF EMBODIMENTS FIG. 1 is a circuit diagram showing an embodiment of a DC two-wire sensor according to the present invention. In this figure, the same components as those shown in FIG. 4 are given the same reference numerals.

In the circuit shown in FIG. 1, the bases of transistors Q3 are commonly connected to the bases of transistors Q2. The emitter of transistor Q3 is connected to one output terminal 01 via resistor R8.

A load current detection circuit 6 is constituted by a transistor Q and a resistor R8. The collector of transistor Q3 is connected to current mirror circuit 10 via current limiting resistor R3. The current mirror circuit 10 is composed of a transistor Q5 and a transistor Q6, one of which is a transistor Q5.
5 is driven by current I3 flowing from transistor Q3.

The drive circuit 8 for the operation display light emitting diode 5 includes a current mirror circuit 11 composed of a transistor QT and a transistor Q8. One transistor Q7 constituting the current mirror circuit 11 has its collector connected to the light emitting diode 5, and the current mirror circuit I is connected to this connection point 12.
The collector of transistor Q6 of O is connected. Connection point 12 constitutes a current addition circuit. The other transistor Q8 constituting the current mirror circuit 11 has its collector and base connected in common, and is connected to the constant current source 7 via a switching element 4 that is controlled on and off according to the detection signal of the sensor main circuit 1. has been done.

The constant current source 7 supplies a predetermined current l that is more than the minimum value that causes the light emitting diode 5 to emit visible light. The fortune telling that outputs.

When the detection signal of the sensor main circuit 1 is applied to the base of the transistor Q1, the transistor Q4 is turned on as described above, and current flows to the load 20.

The detection signal of the sensor main circuit 1 is also given to the switching element 4, and when this switching element 4 is turned on, a current (2) is output from the constant current source 7. Current mirror circuit 1 by
Since the transistor Q8 of No. 1 is driven, the current lo also flows through the transistor Q, causing the light emitting diode 5 to emit light. This light emission provides the minimum necessary operation indication.

On the other hand, if the collector current of the transistor Q2 is 12 and the collector current of the transistor Q3 is 3, the following equation holds true.

V −V +RI ... (1
) BH38E4 8 3 where V is the base-to-E2 emitter voltage of the transistor Q2, and ■ is the base-to-emitter voltage of the transistor Q4.

The above equation (1) can be rewritten as the following equation.

(KT/Q) I n (I/I 82)
-(KT/Q)In(I/I)+R8I3
s4...(2) Here, K is Boltzmann's constant, T is absolute temperature, Q is the amount of electron charge, ■ is the reverse saturation current of transistor Q2,
(2), 4 is the reverse saturation current of the transistor Q4.

As can be seen from equation (2), the collector current! When 2 is small, collector current 3 hardly flows. When the collector current (2) increases due to an increase in the load current, the collector current (2) also increases, and this current (2) is applied to the transistor Q5 constituting the current mirror circuit IO via the resistor R3. When a current 3 flows through the transistor Q5, an equal current 3 flows through the transistor QB and is applied to the connection point I2. As a result, the driving current of the light emitting diode 5 is
It becomes lo+■3. Therefore, the light emitting diode 5
will emit more bright light. In this way, as the load current increases, the operating blue indicator becomes brighter and easier to see. The current 3 added to the drive current of the light emitting diode 5 is an extra current obtained by subtracting the current necessary to stably operate the sensor main circuit 1, constant voltage circuit 2, and output circuit 3 from the load current. , there will be no hindrance to the operation of these circuits.

By the way, the collector current (3) can be expressed by the following equation.

I ″(VR”8 ’3-VCE3 BE53
-■ )/R3...
(3) VR is the output residual voltage (voltage across the Zener diode D1), ■ is the voltage between the collector and emitter of the transistor Q3, and ■ is the voltage between the base and emitter of the transistor E5 Q5.

As shown in equation (3), the collector current 13 is limited by the resistor R3, and by making the resistor R3 variable, the maximum value of the drive current applied to the light emitting diode 5 can be adjusted.

FIG. 2 is a circuit diagram showing another embodiment. In this figure as well, the same components as shown in FIG. 4 are given the same reference numerals.

Referring to FIG. 2, the collector of a transistor Q4 included in an output circuit I3 is connected to one output terminal 01 through a resistor R4.
is connected to. A resistor R is connected in parallel to this resistor R4.
5 and a resistor R6 are connected. The base of the transistor Q9 is connected to the connection point between the resistors R5 and R6, its emitter is connected to one output terminal 01, and its collector is connected to the operation display light emitting diode 5 via a current limiting resistor R7. A load current detection circuit 1B is configured by the resistor R, RSR, and the 5B transistor Q9.

The drive circuit 18 for the operation display light emitting diode 5 is composed of a constant current source 7 and a switching reference transistor Q1o. Constant current source 7 is connected between constant voltage circuit 2 and connection point 14 . Connection point 14 constitutes a current addition circuit. The switching transistor Q10 is controlled to be turned on or off in response to a detection signal from the sensor main circuit 1.

When the detection signal of sensor main circuit 1 is applied to the base of transistor Q1, transistor Q4 is turned on and load 2
Current flows through 0. The detection signal of the sensor main circuit 1 is a switching transistor Ql.

is also given to this switching transistor Q1o
When turned on, the light emitting diode 5 emits light by the current 1o output from the constant current source 7.

On the other hand, the current flowing through resistor R is 11 the current flowing through resistor R8! , the following equation holds true if I7 is the current flowing through transistor Q9.

I 4R4 - I BRB+VBE9 - (4) Rewriting the above m (4) equation results in equation (5).

(KT/Q) I n (17/ I s9) ■l
4R4-16R6 (5) vBE9 is the base-emitter voltage of transistor Q9, and Is9 is the reverse saturation current of transistor Q9. Also, although it is I-V /R5, the 8th
8E9 Equation (5) can be further rewritten as the following equation.

In (Iy / Isg) - (IR-(R/R)V) / 44858E9 (KT/Q)...(6) As can be seen from equation (6), the detection signal is output from the sensor main circuit 1. When this happens, current 14 flows and current I7 is applied to connection point 14. As a result, the drive current changes from 1 to IO+I7. Therefore, the light emitting diode 5 emits brighter light. In this way, even in the circuit shown in FIG. 2, when the load current increases, the operation display becomes brighter and easier to see. Also, the driving current I of the light emitting diode 5. The current I7 applied to the sensor main circuit 1, constant voltage circuit 2, and output circuit 13 is an extra current obtained by subtracting from the load current the current necessary to stably operate the sensor main circuit 1, constant voltage circuit 2, and output circuit 13, so there is no problem with the operation of these circuits. It doesn't happen at all.

FIG. 3 is a graph showing the relationship between the drive current of the light emitting diode 5 and the load current in the circuit shown in FIG. Current is added to the light emitting diode 5 when the load current exceeds 1°. This load current 110 can be expressed by the following equation.

1 - (R+R) ・v / (R2R1o)10
5 6 8E9 (7) Further, the maximum value of the driving current of the light emitting diode 5 is limited by the resistor R7. This limited current 111 is expressed by the following equation.

■-(v-v-v-V)/R11
RC205CEIO3 ... (8) ■ is the voltage generated in the light emitting diode, V is the voltage generated in the light emitting diode,
This is the collector-emitter voltage of the CEIQ transistor lO. From equation (8), by making the resistor R3 variable, the light emitting diode 5
It can be seen that the maximum value of the drive current can be adjusted.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of the invention, FIG. 2 is a circuit diagram showing another embodiment, and FIG. 3 is a graph showing the relationship between the drive current of a light emitting diode and the load current. FIG. 4 is a circuit diagram showing a conventional example. 1...Sensor main circuit, 3.13...Output circuit, 8.18...Drive circuit, 6.16...Detection circuit, 12.14...Connection point (current addition circuit), RSR7・
...Resistance for current limiting. that's all

Claims (2)

[Claims] (1) In a DC two-wire sensor where the output terminal is connected to the power supply through the load and operating power is supplied through the load, there is an output circuit that turns on and off between the output terminals in response to the detection signal, and an output circuit that turns on and off between the output terminals in response to the detection signal. a drive circuit that drives the light-emitting element for operation indication with a predetermined current; a detection circuit that detects the load current flowing through the output circuit when it is on; and a current that is added to the above predetermined current according to the magnitude of the detected load current. A two-wire direct current sensor, comprising: a current adding circuit that causes the light to flow through the light emitting element. (2) The DC two-wire sensor according to claim (1), further comprising a current limiting circuit that defines an upper limit value of the current applied to the light emitting element.