JPS5838009B2 - Input/output isolated signal transmission circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an input/output isolated signal transmission circuit with improved noise resistance.

Conventionally, signal transmission circuits using photocouplers have been put into practical use, and the feature of such signal transmission circuits is that input and output can be electrically separated or insulated.

However, in conventional signal transmission circuits using photocouplers, when noise is added to the input side, the light emitting element driven by the input current emits light undesirably, resulting in an incorrect output being taken out from the light receiving element on the output side. It includes the problem. SUMMARY OF THE INVENTION An object of the present invention is to solve these problems and provide a novel signal transmission circuit with improved noise resistance.

One of the features of the present invention is that a normally closed semiconductor switch is connected in parallel to the light emitting element of the photocoupler to bypass the input current lower than the signal level, and to detect the input current equivalent to the signal level. The semiconductor switch is turned off by a signal.

In this way, almost all of the input current corresponding to the signal level to be transmitted is switched to the light emitting element, so that the noise resistance can be improved without impairing the signal transmission efficiency.

The invention will now be described in detail with reference to preferred embodiments shown in the drawings.

FIG. 1 is a circuit diagram of a digital input signal transmission circuit according to an embodiment of the present invention.

In FIG. 1, 1 and 2 are input terminals, and 3 and 4 are output terminals.

AC power supply 1 is connected to input terminals 1 and 2 via long-distance connection line A.
0, a signal source consisting of contacts 20 is connected.

Further, a full-wave rectifier circuit 30 is connected to the input terminals 1 and 2 via a current-limiting impedance composed of a capacitor 15 and a resistor 16.

A light emitting diode 51 on the input side of a photocoupler 50 is connected to the output side of the full-wave rectifier circuit 30 via a smoothing circuit consisting of a resistor 31 and a capacitor 32, and a current detection resistor 45.

Further, a source electrode 41 and a drain electrode 42 of a semiconductor switch 40 are connected to both ends of the light emitting diode 51, respectively, and a gate electrode of the semiconductor switch 40 is connected to a connection point between a smoothing circuit consisting of a resistor 31 and a capacitor 32 and a resistor 45. 43 is connected.

On the other hand, an output-side light-receiving transistor 52 of a photocoupler 50 is connected to the output terminals 3 and 4.

Furthermore, the collector of the light receiving transistor 52 (output terminal 3
) is connected to the DC power supply connection terminal 5 via a resistor 60.

Here, a depletion type junction field effect transistor is used as the semiconductor switch 40 having normally closed characteristics, and an example of its characteristics is shown in FIG.

In figure 2? If the gate-source voltage vGS is O
V, the drain-source resistance RDS is several +Ω or less, 4~
A field effect transistor having switching characteristics exhibiting a resistance value of several hundreds of kΩ or more can be used.Next, the operation of the circuit shown in FIG. 1 in the above configuration will be explained.

First, when the contact 20 is in the "open" state and the AC voltage of the AC power supply 10 is not applied to the input terminals 1 and 2 (when the signal source is in the open signal state), the gate voltage Vi of the semiconductor switch 40 is
(The voltage across the resistor 45) is approximately O, so the light emitting diode 51 is not driven (emit light).

In such a state, if inductive noise, etc. generated between the two lines of the input circuit on the signal source side is applied to the input terminals 1 and 2, this noise (AC) voltage will exceed the current limiting impedance consisting of the capacitor 15 and resistor 16. The signal is then rectified by a full-wave rectifier circuit 30 and smoothed by a resistor 31 and a capacitor 32.

This smoothed voltage causes an input current Ii to flow through the resistor 45, but if the gate voltage Vi, which is equal to the voltage drop generated across the resistor 45 due to this input current, is lower than the gate threshold voltage Vt of the semiconductor switch 40, then For example, both ends of the light emitting diode 51 are connected to the semiconductor switch 4.
The input current Ii is bypassed by the semiconductor switch 40 and therefore does not flow to the light emitting diode 51. Therefore, the photocoupler 50 malfunctions due to noise and fails. The desired noise transmission will no longer occur.

Next, when the contact 20 is in the "closed" state and AC voltage is applied to the input terminals 1 and 2 (when the signal source is in the closed signal state), the gate voltage Vi exceeds the threshold voltage Vt, so the semiconductor The source and drain of the switch 40 is in an off state (high resistance state), and the input current Ii flowing through the resistor 45 is switched from the semiconductor switch 40 to the light emitting diode 51 and flows.

FIG. 3 shows this current switching operation. Until the gate voltage Vi becomes equal to the threshold voltage Vt,
As shown by the broken line A, the input current Ii flows through the semiconductor switch 40, but when the gate voltage Vi exceeds Vt,
As shown by solid lines B and C, the input current Ii drives the light emitting diode 51, causing it to emit light.

Regarding the level at which the input current Ii reaches to switch the current from the semiconductor switch 40 to the light emitting element 51, the resistor 4 is selected in consideration of the noise level that is not desired to be transmitted.
It can be determined as appropriate by selecting various values of 5 and the threshold value of the switch 40.

As described above, according to the present invention, the level of the input current Ii required to turn off the semiconductor switch 40 can be appropriately set with respect to the noise induced between the two lines of the signal source side input circuit and the noise on the DC side. This makes it possible to block noise and efficiently transmit only useful signals.

The present invention is not limited to the embodiments described above, but can be implemented with various modifications.

For example, although a depletion mode P-channel junction field effect transistor has been exemplified as the semiconductor switch, other switch elements such as an insulated gate field effect transistor having the same effect as this may also be used.

Further, although an example is shown in which a resistor is used as a current detection element, a constant voltage element such as a Zener diode may also be used.

Furthermore, although a series circuit of a resistor and a capacitor is illustrated as the current limiting impedance, only a resistor may be used.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing a digital input signal transmission circuit according to an embodiment of the present invention, and FIG. 2 shows the control voltage (Gs) versus resistance (R) of the semiconductor switch used in the circuit of FIG.
．． FIG. 3 is a graph showing voltage (Vi) vs. current (Ii) characteristics for explaining the operation of the circuit shown in FIG. 1. Explanation of symbols, 1, 2... Input terminal, 3, 4...
...Output terminal, 10...AC power supply, 15,
16... Current limiting capacitor and resistor, 20.
...Contact, 30...Full-wave rectifier circuit, 31
, 32... Smoothing resistor and capacitor, 4
0...Semiconductor switch, 45...Resistor for current detection, 50...Photocoupler.

Claims (1)

[Claims] 1. In an input/output isolated signal transmission circuit using a photocoupler, a normally closed semiconductor switch is connected in parallel to the light emitting element of the photocoupler, and when the drive current of the light emitting element exceeds a predetermined level, the semiconductor switch is turned off. An input/output isolated signal transmission circuit characterized in that it is configured to have the following characteristics.