JP2008054042A - Insulated contact output circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To resolve the problem that a response time from turning-on to turning-off is made long by a long transition time from turning-on to turning-off of a photo coupler and an influence of electric charges stored in a parasitic capacity between a base and an emitter of an output transistor, in an insulated contact output circuit which insulates an input digital signal by the photo coupler and controls a base current of the output transistor by the photo coupler to control turning-on/off of the photo coupler. <P>SOLUTION: The input digital signal is inverted by an inverter, and an output of the inverter is inputted to a second photo coupler, and the base and the emitter of the output transistor are short-circuited by an output of the second photo coupler. Since the transition time from turning-off to turning-on of the photo coupler is much shorter than that from turning-on to turning-off, the base current of the output transistor is bypassed to the emitter by the second photo coupler, and electric charges stored in the parasitic capacity between the base and the emitter can be discharged, so that the response time from turning-on to turning-off of the output transistor can be shortened. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a high-speed contact output circuit insulated by a photocoupler.

  In a PLC (programmable logic controller), an open collector contact output circuit insulated by a photocoupler is used as a contact output. FIG. 2 shows the configuration of such an insulated open collector contact output circuit.

  In FIG. 2, the photocoupler 20 includes a light emitting diode 21 and a phototransistor 22 driven by the output light of the light emitting diode 21. The signal source 10 outputs a pulse signal having a high level of Vcc and a low level of 0 V, and its output is connected to the cathode of the light emitting diode 21 via a resistor 61. The anode of the light emitting diode 21 is connected to the power source Vcc.

  The emitter of the phototransistor 22 is connected to the base of the transistor 30 via a resistor 62, and the collector is connected to the positive side of the power supply 50. The collector of the transistor 30 is connected to one end of the load 40, and the emitter is connected to the negative side of the power supply 50. The other end of the load 40 is connected to the positive side of the power source 50, and the base of the transistor 30 is connected to the negative side of the power source 50 via a resistor 63.

  In such a configuration, when the output of the signal source 10 becomes low level, the light emitting diode 21 is turned on and the phototransistor 22 is turned on. Therefore, a base current flows through the transistor 30 and turns on, and a current flows through the load 40. When the output of the signal source 10 becomes high level, the light emitting diode 21 is turned off and the phototransistor 22 is turned off. Since no base current is supplied to the transistor 30, the transistor 30 is turned off and no current flows through the load 40.

  Thus, the current flowing through the load 40 can be controlled by the output of the signal source 10. Since the photocoupler 20 is inserted between the signal source 10 and the transistor 30 as an output contact, the input and output of this open collector contact output circuit are electrically insulated.

  Since the current transfer rate of the photocoupler 20 and the gain of the transistor 30 vary greatly, the photocoupler 20 and the transistor 30 are usually used in a saturation region in order to reliably turn on the output.

  As a semiconductor contact output circuit configured with a simple circuit and capable of high-speed response, for example, there is one described in Patent Document 1.

JP 2000-59197 A

  However, such an insulating open collector contact output circuit has a relatively high time of about several microseconds for the transistor 30 to transition from off to on. Since the recovery time of the transistor 30 is long, there is a problem that it is about 100 to 200 μsec.

  If the control period of the PLC is about several milliseconds, it can be used even when the transition time from on to off is about 100 μs. However, since the PLC control cycle has recently been increased to about 100 μsec, it has been difficult to use such an output circuit having a long transition time.

  Accordingly, an object of the present invention is to provide an insulated contact output circuit capable of increasing the speed with a simple configuration.

In order to achieve such a problem, the invention according to claim 1 of the present invention is:
A first photocoupler that receives a digital signal and is controlled to be turned on and off by the digital signal;
A transistor whose on / off is controlled by the first photocoupler;
An inverter that receives the digital signal and outputs a signal obtained by inverting the digital signal;
A second photocoupler whose on / off is controlled by the output of the inverter, the output of which is connected to the base and emitter of the transistor;
Is provided. The response time from on to off can be shortened.

The invention according to claim 2 is the invention according to claim 1,
An NPN transistor is used as the transistor. A current sink type contact can be configured.

The invention according to claim 3 is the invention according to claim 1,
A PNP transistor is used as the transistor. A current discharge type contact can be configured.

As is apparent from the above description, the present invention has the following effects.
According to the first, second, and third aspects of the present invention, an input digital signal is input to the first photocoupler, and the base current of the transistor is controlled by the output of the first photocoupler to control on / off of the transistor. An isolated contact output circuit, wherein an output of a second photocoupler operating in the opposite direction to the first photocoupler is connected to a base and an emitter of the transistor, and when the transistor is off, The emitter was short-circuited.

  Even if the transition time from on to off of the first photocoupler becomes long, the current supplied from the first photocoupler flows to the emitter by the second photocoupler and is not supplied to the base. Further, the electric charge accumulated in the parasitic capacitance between the base and the emitter is rapidly discharged. Therefore, there is an effect that the response time from contact ON to OFF can be shortened using a general-purpose photocoupler and transistor.

  Hereinafter, the present invention will be described in detail with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of an insulated contact output circuit according to the present invention. In addition, the same code | symbol is attached | subjected to the same element as FIG. 2, and description is abbreviate | omitted.

  In FIG. 1, 71 is an inverter, which receives the output of the signal source 10 and inverts and outputs this signal. Reference numeral 73 denotes a photocoupler, which includes a light emitting diode and a phototransistor as in the photocoupler 20 of FIG. A resistor 72 is disposed between the output terminal of the inverter 71 and the cathode of the light emitting diode in the photocoupler 73. The anode of the light emitting diode in the photocoupler 73 is connected to the power supply Vcc. The collector of the phototransistor, which is the output of the photocoupler 73, is connected to the base of the transistor 30, and the emitter is connected to the emitter of the transistor 30.

  Next, the operation of this embodiment will be described. When the output of the signal source 10 is at a low level, a current flows through the light emitting diode in the photocoupler 20, the light emitting diode emits light, and the phototransistor in the photocoupler 20 becomes conductive. Therefore, a base current is supplied to the base of the transistor 30 and the transistor 30 is turned on.

  The output of the signal source 10 is inverted by the inverter 71 and supplied to the light emitting diode in the photocoupler 73. When the output of the signal source 10 is at a low level, the output of the inverter 71 is at a high level, so that no current flows through the light emitting diode in the photocoupler 73, and the photodiode in the same photocoupler is turned off. Therefore, the photocoupler 73 does not affect the operation of the transistor 30.

  When the output of the signal source 10 transitions from a low level to a high level, the output of the inverter 71 transitions to a low level, and the phototransistor in the photocoupler 73 is turned on. Therefore, the base and emitter of the transistor 30 are short-circuited and the transistor 30 is turned off. Since the response time when the photocoupler is normally turned on is several microseconds, the transistor 30 is turned off with a delay of several microseconds after the output of the signal source 10 transitions from the low level to the high level.

  Usually, the photocoupler requires a time of about several tens of microseconds from the on state to completely turning off. Therefore, even if the output of the signal source 10 transitions from a low level to a high level, the phototransistor in the photocoupler 20 is kept on for several tens of microseconds, and continues to supply the base current to the transistor 30 during this time.

  When the transistor 30 is on, the operating region is in the saturation region. Therefore, even if the base current is not supplied, it takes 100 μs or more to completely turn off due to the parasitic capacitance between the base and the emitter. Cost.

  However, since the output of the photocoupler 73 is turned on with a delay of several microseconds, the current flowing through the photocoupler 20 is bypassed, and the charge charged in the parasitic capacitance between the base and emitter of the transistor 30 is rapidly discharged. The Accordingly, the transistor 30 is turned off with a delay of several microseconds after the output of the signal source 10 transitions from the low level to the high level.

  In order to confirm the effect of the embodiment in FIG. 1, the measurement was performed by setting the output voltage of the power supply 50 to 24 V and passing a current of 10 mA through the load 40. The time for the transistor 30 to transition from on to off was about 150 μsec in the conventional example of FIG. 2, but was 20 μsec or less in the embodiment of FIG. 1, and the transition time was shortened to 13% or less.

  In this embodiment, an NPN transistor is used as the transistor 30 and a current sink type contact output is used. However, a current discharge type contact output can be configured using a PNP transistor. In this case, the emitter of the PNP transistor may be connected to the positive side of the power source 50, and the load 30 may be connected to the collector and the negative side of the power source 50 to draw the base current from the base.

  In the present invention, nothing is added to the portion connected to the load 40 which is a load connected to the outside as compared with the conventional contact output circuit. Therefore, there is no need to worry about compatibility with the conventional contact output, and only the response time can be increased.

  Further, the drive circuit for the photocouplers 20 and 73 and the drive circuit for the transistor 30 are not limited to the present embodiment, and various types can be used.

It is a block diagram which shows one Example of this invention. It is a configuration diagram of a conventional insulated open collector contact output circuit,

Explanation of symbols

10 Signal source 20, 73 Photocoupler 30 Transistor 40 Load 50 Power supply 61, 62, 63, 72 Resistance

Claims (3)

A first photocoupler that receives a digital signal and is controlled to be turned on and off by the digital signal;
A transistor whose on / off is controlled by the first photocoupler;
An inverter that receives the digital signal and outputs a signal obtained by inverting the digital signal;
A second photocoupler whose on / off is controlled by the output of the inverter, the output of which is connected to the base and emitter of the transistor;
An insulated contact output circuit comprising:   2. The isolated contact output circuit according to claim 1, wherein the transistor is an NPN transistor. 2. The isolated contact output circuit according to claim 1, wherein the transistor is a PNP transistor.

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