JP3509323B2 - Photocoupler drive circuit - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photocoupler driving circuit.

[0002]

2. Description of the Related Art A conventional photocoupler driving circuit is shown in FIG. In this figure, the anode of the photocoupler 1 is connected to the power supply via a resistor R5 that limits the current, the cathode is connected to the collector of the transistor Q1, and the emitter of the transistor Q1 is grounded. This circuit drives the photocoupler by turning on / off the transistor Q1.

[0003]

In the case of the above-mentioned conventional circuit, the transistor Q1 is OFF, that is, the photocoupler is O.
In the case of FF, the circuit becomes high impedance and is weak against noise such as electromagnetic waves, and the noise sometimes causes the photocoupler to malfunction. As a countermeasure against this, conventionally, the circuit current has been increased or the wiring has been twisted, but this has not been sufficient. In order to solve such a problem, it is an object of the present invention to provide a photocoupler driving circuit that is resistant to noise.

[0004]

In order to solve the above problems, the present invention provides a photocoupler driving circuit in which a photocoupler is connected between an output terminal and an inverting input terminal of an operational amplifier. One end of the resistor is connected to one end of a second resistor, the other end of the first resistor is connected to a power source, the other end of the second resistor is grounded, and the non-inverting input terminal of the operational amplifier is connected. Is connected to one end of the third resistor, one end of the fourth resistor, and one end of the switching path in the switching mechanism, the other end of the third resistor is connected to a power source, and the other end of the fourth resistor is connected. And the other end of the switching path are grounded, and a diode is connected between the output terminal and the inverting input terminal of the operational amplifier in parallel with the photocoupler in the opposite direction to the photocoupler.

[0005]

The switching mechanism can be a simple mechanism by using a transistor. In this case, one end of the switching path is a collector and the other end of the switching path is an emitter.

[0007]

According to the present invention, with the switching mechanism OFF,
The photocoupler turns on and becomes a constant current drive. When the switching mechanism turns on, the photocoupler turns off and reverse bias is applied to the photocoupler, resulting in low impedance.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment according to the present invention will be described in detail with reference to FIG. In FIG. 1, the operational amplifier 2
To the inverting input terminal, one end of a resistor R1 as a first resistor and one end of a resistor R2 as a second resistor are connected, and the non-inverting input terminal is connected to one end of a resistor R3 as a third resistor and a fourth resistor. Is connected to one end of a resistor R4 as a resistor and a collector of an NPN transistor Q1 as a switching mechanism. The other ends of the resistors R1 and R3 are connected to a power source, the other ends of the resistors R2 and R4, and the NPN.
The emitter of the transistor Q1 is grounded. The resistance value of each of the resistors R1, R2, R3 and R4 is the inverting input of the non-inverting input (+ input) of the operational amplifier 2 when the NPN transistor Q1 whose collector is connected between the resistors R3 and R4 is OFF. For example, R1 = 4.7 kΩ, R2 = 470Ω, R3 so that the potential becomes higher than the (−input) potential.
= 10 kΩ and R4 = 12 kΩ are set. Further, the photocoupler 1 is connected between the output terminal and the inverting input terminal of the operational amplifier 2 so that the anode is on the output terminal side.

When driving the photocoupler 1, a resistor R1 is used.
Since R4 to R4 have the above-described relationship, by turning off the transistor Q1, the non-inverting input terminal voltage becomes higher in potential than the inverting input terminal voltage, and the operational amplifier 2 outputs.
At this time, the current If flowing in the photocoupler 1 rises and I
The inverting input terminal voltage represented by f × R2 also rises. When the voltage becomes higher than the non-inverting input terminal voltage, the output of the operational amplifier 2 decreases, the current If also decreases, and the inverting input terminal voltage also decreases accordingly. When the inverting input terminal voltage becomes lower than the non-inverting input terminal voltage, the output of the operational amplifier 2 rises again, and the inverting input terminal voltage rises in the same manner as described above. As described above, the inverting input terminal voltage of the operational amplifier finally becomes R3 and R
The current If flows in the photocoupler 1 in the direction equal to the non-inverting input terminal voltage determined by the ratio of 4, and the photocoupler is driven. On the contrary, when the transistor Q1 is turned on,
Since the voltage of the non-inverting input terminal becomes lower than the voltage of the inverting input terminal, the operational amplifier 2 does not output and becomes almost 0 volt, and the reverse bias of a constant voltage determined by the ratio of the resistors R1 and R2 is applied to the photocoupler. Become.

By doing so, the transistor Q1
When turned off, the photocoupler drives a constant current, and when transistor Q1 is turned on, a reverse bias is applied to the photocoupler, and it is possible to perform noise-resistant driving. Further, in this embodiment, the diode D1 is connected in parallel with the photocoupler 1 between the output terminal and the inverting input terminal of the operational amplifier 2 in the opposite direction to the photocoupler, so that the reverse bias current is generated when the photocoupler is OFF. Flows,
It is possible to make it stronger against noise.

The switching mechanism is not limited to the NPN transistor Q1 in this embodiment as long as the switching operation is possible, and may be a contactor, for example. The values and ratios of the resistors R1 to R4 can be arbitrarily set as long as the non-inverting input terminal voltage of the operational amplifier is higher than the inverting input terminal voltage when the switching mechanism is OFF.

[0012]

As described above, the photocoupler driving circuit according to the present invention has a simple circuit configuration, which drives a constant current when the photocoupler is ON and applies a reverse bias when the photocoupler is OFF. You can

[Brief description of drawings]

FIG. 1 is a circuit diagram of a photocoupler driving circuit according to the present invention.

FIG. 2 is a circuit diagram of a conventional photocoupler drive circuit.

[Explanation of symbols]

1 ... Photo coupler, 2 ... Operational amplifier, R1, R2
R3, R4 ... Resistance, Q1 ... Transistor, D1 ...
diode

Claims (2)

(57) [Claims] 1. A photocoupler driving circuit in which a photocoupler is connected between an output terminal and an inverting input terminal of an operational amplifier, wherein one end of a first resistor and one end of a second resistor are connected to the inverting input terminal of the operational amplifier. , The other end of the first resistor is connected to a power source, the other end of the second resistor is grounded, and one end of the third resistor and one end of the fourth resistor are connected to the non-inverting input terminal of the operational amplifier, And one end of a switching path in the switching mechanism, the other end of the third resistor is connected to a power supply, the other end of the fourth resistor and the other end of the switching path are grounded, and the operational amplifier is further connected. of
Between the output terminal and the inverting input terminal, in parallel with the photo coupler,
A photocoupler drive circuit in which a diode is connected in the opposite direction to this photocoupler . 2. The switching mechanism is a transistor, and one end of the switching path is a collector,
The other end of the switching path is an emitter.
The described photocoupler driving circuit.