INPUT CIRCUIT HAVING A PHOTO-COUPLER

Abstract: Disclosed is an input circuit using a photo-coupler for circuit isolation which comprises a photo-coupler (9) and an indicator (11) for indicating the presence of an input signal both of which are adapted for an AC input signal as well as a DC input signal of either polarity. Typically, the input end of the photo-coupler (9) comprises a pair of light emitting diodes (9a, 9b) which are connected in parallel with each other directionally opposite and the indicator (11) likewise comprises a pair of light emitting diodes (11a, 11b) which are connected in a similar manner. Thereby, the input circuit can be adapted to input signals of all kinds and the indication of the presence of an input signal is accurate because the indicator (11) is directly driven by the input signal and is not affected by the condition of any external circuit.

Description: This invention relates to an input circuit using a photo-coupler for isolation between the input end of the input circuit and an internal circuit which is connected to the output end of the input circuit and in particular to an input circuit which is suitable for use as an input unit of a programmable controller. Typically, an input unit of a programmable controller comprises input circuits incorporating photo-couplers of the same number as its input points. The photo-couplers are used for isolating the internal circuit of the programmable controller from the influence of input signals. Figure 7 shows a typical input circuit using a photo-coupler 3 for isolation. This circuit comprises input terminals T0 and COM, a diode bridge 1 having input nodes which are connected to the input terminals T0 and COM, a light emitting diode 2 and a photo-coupler 3. The photo-coupler 3 comprises a light emitting diode 3a serving as the input end thereof and a photo-transistor 3b disposed adjacent to the light emitting diode 3a and serving as the output end thereof. The two light emitting diodes 2 and 3a are connected in series across the output nodes of the diode bridge 1. According to this circuit, the input signal supplied across terminals T0 and COM is rectified by the diode bridge 1 and the rectified output of the diode bridge 1 is supplied to an internal circuit by way of the uni-directional photo-coupler 3 for isolation while the presence of the input signal is displayed to the outside by the light emitting diode 2 which is connected in series with the other light emitting diode 3a serving as the input end of the photo-coupler 3. By use of the diode bridge 1, the input signal of this input circuit may be a DC signal of either polarity or even an AC signal, and not only the signal is transferred across the photo-coupler 3 but also the presence of the input signal is indicated by the light light emitting diode 2 irrespective of the nature of the input signal. However, according to this circuit, since the diode bridge 1 is relatively expensive and occupies a substantial area on the circuit board, the cost of the input circuit tends to be high and the size of the input circuit can not be reduced to the desired extent. Figure 8 is a circuit diagram of another example of an input circuit which is used in an input unit of a conventional programmable controller. In this circuit, the input signal applied across terminals T0 and COM is transferred to an internal circuit directly by way of a bi-directional input type photo-coupler 4 having a pair of light emitting diodes 4a and 4b which are connected in parallel to each other but directionally opposite and the presence of the input signal is indicated by a light emitting diode 5 which is connected in series with an output element of the photo-coupler 4 which is a photo transistor 4c in this case. According to this circuit, the cost and size reduction of the input unit is possible through elimination of a diode bridge, but since the light emitting diode 5 receives electric power from the internal circuit, the indication of the presence of an input signal may not be accurate since the function of the light emitting diode 5 is affected by the condition of the internal circuit. Thus, as shown in Figure 9, it is conceivable to improve the circuit of Figure 8 by connecting the light emitting diode 6 in series with the input side of the bi-directional input type photo-coupler 4, but if the input is a DC signal there is a restriction on the polarity [potential] of terminal T0 and a negative input is unacceptable. The circuit of Fig.9 forms the basis for the preamble of claim 1. From US-A-4 041 331 there is known an input circuit for accepting an external input signal by way of a photo-coupler for circuit isolation, comprising a photo-coupler having an input end, and an indicator which is connected in series with the input end of the photo-coupler. Both the photo-coupler and the indicator are of the unidirectional type. A photo-coupler of a bi-directional type is known per se from US-A-4 143 287. An indicator of a bi-directional type is known per se from US-A-3 987 401. In view of the problems of the prior art, an object of the present invention is to provide an input circuit using a photo-coupler for circuit isolation which can accept an AC input signal as well as a DC input signal of either polarity and indicate the presence of the input signal irrespective of the nature of the input signal. Another object of the present invention is to provide an input circuit using a photo-coupler which is both compact and economical to manufacture. In order to achieve these and other objects, the present invention is as claimed in the claim. According to the present invention, the indicator is of a bi-directional type comprising a plurality of pairs of individual light emitting diodes which are placed adjacent to each other and are connected in parallel to each other but directionally opposite, and the indicator comprises a circuit board carrying said plurality of light emitting diode pairs, a spacer plate (1214) having a plurality of windows corresponding to the positions of the light emitting diode pairs, and a light dispersing film which is placed over the spacer plate. In the drawings: Figure 1 is a circuit diagram showing the structure of an input circuit for one a plurality of input points of an input unit of a programmable controller to which the present invention is applied; Figures 2A and 2B are circuit diagrams similar to Figure 1 showing the action of an embodiment of the input circuit of the present invention; Figure 3 is a perspective view showing a light emitting element module incorporating the input circuits of the present invention and mounted on a circuit board of the input circuit; Figure 4 is a perspective view showing the reverse surface of the light emitting element module; Figure 5 is an exploded perspective view showing the internal structure of the display surface of the light emitting element module; Figure 6 shows the internal connection of the light emitting diode pairs; Figure 7 is a circuit diagram showing an example of conventional input circuit; Figure 8 is a circuit diagram showing another example of conventional input circuit; and Figure 9 is a circuit diagram showing a proposed hypothetical improvement of the conventional input circuits. Figure 1 is a circuit diagram of one of a number of input circuits of an input unit for a programmable controller to which the present invention is applied. Input terminals T0 and COM of this input circuit are connected to an AC power source 8a and an external contact 7 which may be a sensor consisting of a limit switch. Internally, the input terminals T0 and COM are connected to an input end of a photo-coupler 9 and a light emitting diode unit 11 which are connected in series across the input terminals T0 and COM. The input end of the photo-coupler 9 comprises a pair of light emitting diodes 9a and 9b which are connected in parallel to each other directionally opposite while the light emitting diode unit 11 likewise comprises a pair of light emitting diodes 11a and 11 which are also connected in parallel to each other but directionally opposite. The output end of the photo-coupler 9 comprises a transistor 9b which forms a part of an internal circuit or, in the present embodiment, a waveform shaping circuit 10. The waveform circuit 10 comprises a capacitor 101 for suppressing noises and an operational amplifier 102 for converting the output signal of the photo-coupler 9 into a binary signal. Typically, the binary signal is supplied, as required, to a system bus (not shown in the drawings) which is connected to a CPU by way of an input port consisting of a tri-state buffer. Normally, for each input unit, a plurality of such input circuits, for instance, eight sixteen or thirty-two input circuits are mounted on a single circuit board so as to correspond to the number (eight, sixteen or thirty-two points) of the input points. According to this circuit structure, since no diode bridge is required as opposed to conventional input circuits the input circuit can be made compact and economical to manufacture, and since the bi-directional input type light emitting diode unit 11 is directly driven by the input signal the presence of an input signal can be accurately detected without being affected by the state of the internal circuit. Furthermore, since the light emitting diodes 9a and 9b serving as the input end of the photo-coupler 9 can accept signal of either polarity there is no restriction on the polarity of the input signal source 8b or 8c even when it is a DC signal as shown in Figures 2A and 2B whereby the input circuit can be adapted to both positive and negative input signals. Figure 3 is a perspective view showing a light emitting element module 12 which incorporates therein eight bi-directional input type light emitting diode pairs 11 and is mounted on an circuit board 13 of the input circuit of a programmable controller having input circuits for eight input points. In this light emitting element module 12, bi-directional input type light emitting diode pairs 11 for eight input points are molded on synthetic resin and nine terminal pins 1201 to 1209 corresponding to nine input terminals (COM and T0 to T7) of the input circuit protrude from the rear face 12a thereof as shown in Figure 4. The light emitting element module 12 is mounted on the circuit board 13 of the input circuit with these terminal pins 1201 to 1209 passed through pin holes provided in the circuit board and soldered thereto. Figure 5 is an exploded perspective view showing the internal structure of the display surface of the light emitting element module 12. In the illustrated embodiment, the circuit board 1211 consists of a printed circuit board, a ceramic board or the like, and eight light emitting diode pairs 1212 are fixedly secured over this circuit board 1211. Each of these light emitting diode pairs 1212 comprises a pair of light emitting diode chips 1212a and 1212b and are electrically connected in parallel to each other in directionally opposite and forms an array with the rest of the diode pairs. One end of each light emitting diode pair is connected to the corresponding one of the terminals T0 to T7 while the other end thereof is commonly connected to a common terminal COM as shown in Figure 6. A spacer plate 1214 having a plurality of windows 1213 adjacent to the mounted positions of the light emitting diode pairs 1212 is layered over the circuit board 1211 and a light dispersing film 1215 is layered thereover. Therefore, each of the windows 1213 will be wholly lighted up as shown in Figure 3 when either one of the light emitting diode chips is lighted up. Thus, by mounting this light emitting element module 12 on the circuit board 13 of the input circuit and connecting the terminal pins 1201 to 1209 as shown in Figure 1, the presence of an input signal at each input point can be directly seen from the front face of the circuit board 13 of the input circuit. In the above described embodiment, the output element of the bi-directional input type photo-coupler consisted of a transistor, but if the internal circuit is driven by an AC power source the output element may then be a triac. Also, the bi-directional input type photo-coupler 11 is not limited by the present embodiment but may be other light emitting elements which are conductive in two directions. Although the present invention has been shown and described with reference to the preferred embodiment thereof, it should not be considered as limited thereby.

Claims: An input circuit for accepting an external input signal by way of a photo-coupler for circuit isolation, comprising: a photo-coupler (9) having an input end, the input end of the photo-coupler (9) being of a bi-directional type comprising a pair of light emitting diodes (9a, 9b) which are connected in parallel to each other but directionally opposite, and an indicator (11) which is connected in series with the input end of the photo-coupler (9), characterized by the indicator (11) being of a bi-directional type comprising a plurality of pairs of individual light emitting diodes (11a, 11b; 1212) which are placed adjacent to each other and are connected in parallel to each other but directionally opposite, and the indicator comprising a circuit board (1211) carrying said plurality of light emitting diode pairs (1212), a spacer plate (1214) having a plurality of windows (1213) corresponding to the positions of the light emitting diode pairs (1212), and a light dispersing film (1215) which is placed over the spacer plate (1214).

Inventors: SHIRAI, HIRONOBU C/O OMRON TATEISI ELECTRONICS CO.
Applicants: OMRON TATEISI ELECTRONICS CO.
Application year: 1987
Main classification: H03K17
IPC codes: H01H9, H03K17, G05B19