GB2193377A

GB2193377A - Optoelectronic coupler

Info

Publication number: GB2193377A
Application number: GB08711799A
Authority: GB
Inventors: Rheinhold Barlian; Otto Eberhardt; Rolf Nied
Original assignee: Individual
Current assignee: Individual
Priority date: 1986-05-21
Filing date: 1987-05-19
Publication date: 1988-02-03
Also published as: DE3617057A1; DE3617057C2; GB2193377B; GB8711799D0

Abstract

The coupler comprises a light-emitting diode (7) and a phototransistor (11) between which a light- conducting rod (12) is arranged to transmit the optical signal by means of total internal reflection. The rod (12) has an internal light-carrying core (14) and a sheath consisting of an optically thinner medium (15). The whole assembly is provided within a continuous tubular housing (16). <IMAGE>

Description

SPECIFICATION Optoelectronic coupler The invention relates to an optoelectronic coupler.

Optoelectronic couplers, for the most part also called briefly 'optocouplers' are used for the transmission of signals and the electrical separation of two circuits which are for the most part at different voltage potentials. The signal transmission takes place optically. The electrical signal is converted during transmission to an optical signal and radiated via a light-emitting diode (transmitter) to a phototransistor (receiver) and reconverted to an electrical signal. When using an optocoupler in a zone with an explosion hazard or in an environment with an explosion hazard, one of the most critical points in the separation between the so-called 'instrinsically safe' circuit in the sense of the "Explosion Protection Regulations", and the non-intrinsically-safe circuit (normal circuit line side), since voltage distortions from the line side must be prevented here.The basis for the specification of the insulation, air and creep distances of those components through which the separation takes place, is the series voltage, namely the sum of the voltages of the intrinsicaliy safe and the non-intrinsically safe circuits which can occur in operation or in the case of malfunction at the separating component. For the electrical separation between the circuits, air, light-transmitting casting resin or glass may be provided. The higher the series voltage, the greater the insulation distance must be (European Standard EN 50020).Since with the light-transmitting media used up to the present time, for the electrical separation especially, relatively high losses occur, a correspondingly high energy supply with a current flow on the transmitter side of about 20 mA is usuaily be provided for the light-emitting diode, as a result of which safety against explosion is considerably impaired. Irl order to prevent this, the optocoupler must be designed to an appropriate size.

The task of the invention consists of continuing to work towards an electronic coupler so that with simple resources in a small structural shape a low-loss light transmission is achieved with a low energy supply in an insulation distance of a size which is in accordance with the regulations, with high insulation resistance for the electrical separation.

According to the invention there is provided an optoelectronic coupler for signal transmission with electrical separation between two electrical circuits comprising a light-conducting rod disposed in an electrically insulating compound and located between an electrically energisable light emitting source and means responsive to the reception of light said light conducting rod comprising a core for light transmission and a sheath around the core, said sheath being of an optically thinner medium.

The term 'light' as used herein is intended to include radiation other than visible radiation for example infra red and ultra violet radiation.

Specific embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Fig. 1 shows an optocoupler according to the invention, enveloped in casting resin as seen from below; Fig. 2 shows the optocoupler according to Fig. 1 in sectional side view; Fig. 3 shows the transmitting section of the optocoupler similar to Fig.2 in a greatly enlarged, partly sectional side view; Fig. 4 shows a circuit diagram of the optocoupler according to the invention; Fig. 5 shows a further practical version of an optocoupler in accordance with the invention inside an explosion-protected bushing in sectional side view; Fig. 6 shows a-view towards the under side of the bushing according to Fig. 5; Fig. 7 shows an enlarged side view of a part of the optocoupler bushing in accordance with Fig.5;; Fig. 8 shows an exploded view of the lower section of the bushing according to Fig.7 with the lower insulator and located over it a circuit board with the optical transmission parts; and Fig. 9 shows a plan view of the upper circuit board of the bushing according to Fig.7.

The optocoupler 1 illustrated in the drawing is hermetically embedded, according to Figs. 1 and 2, in a rectangular housing formed from an electrically insulating setting compound 2.

At the lower side 3 of the encapsulation housing 2, four terminal pins 4, 4' project.

Very close to the left external side 5 of the setting-compound housing 2 there extends a connector 6 linking the connecting pin 4, to a low-current light-emitting diode 7 which is supplied with very low power and is fully capable of functioning with a current flow of only 2mA, so that a so-called "low-current light-emitting diode" may be used.

Very close to the right-hand external side 8 of the encapsulation housing 2, a circuit board 9 is provided, to the lower end section of which the connecting pin 4' is bonded. On the surface of the circuit board 9 directed towards the external side 8 electronic components are provided which are connected so that an electrical signal amplification is achieved. The electronic components 10 are preferably arranged in a space-saving flat design, the so-called "SMD technique" ("surface-mounted devices") on the circuit-board, so that a positive spacesaving is obtained.

On the surface of the circuit board 9 directed away from the right-hand external side 8, a phototransistor 11 is bonded, which re ceives the light sent by the luminescent diode or the low-current light-emitting diode 7. The phototransistor 11, which is a so-called electronic switch, becomes permeable to current in the case of the incidence of light.

A light-conducting rod 12 is arranged between the low-current light-emitting diode 7 and the phototransistor 11. In addition, it can also be seen from Figs. 2 and 3 that the lightconducting rod 12 has essentially the same diameter as the low-current light-emitting diode 7 and the photo-transistor 11.

The light-conducting rod 12 consists of two different glass layers or kinds of glass, that is, of the first internal kind of glass used as the light-conducting core 14 and the second kind of glass surrounding the core 14 as a sheath 15. The core 14 consists of an optically more dense medium than the sheath 15. By this means, total reflection of the light signal radiated by the low-current light-emitting diode 7 is achieved in the light-conducting rod 12. If the light falls at a certain angle onto the boundary surface-50, shown as a broken line, of the optically more dense core 14 and the optically thinner sheath 15, it is totally reflected and thus remains within the core 14, as the refractive index of the core 14 and the refractive index of the sheath 15 are different.

On the basis of this totally reflecting light conduction in the light-conducting rod 12, an exceptionally low-loss signal transmission is achieved, with losses from scatter and losses from decoupling especially being largely eliminated. As the light transmission in the lightconducting rod 12 takes place roughly without loss and practically all the light radiated is received by the photo-transistor 11, the lowcurrent light-emitting diode 7 may emit a correspondingly small light signal for which only a very small output is required, so that it is possible to work with a current of about 2 mA, which is about ten times smaller than with conventional optocouplers.Accordingly, practically no heating up worth mentioning occurs in the optocoupler 1, so that in applications in an environment with an explosion hazard, a high degree of safety against explosion is ensured, with the (ight-conducting rod 12, the diameter of which, for an optimal decoupling, is about two to five, preferably about three millimeters, fulfjlling several functions, namely the low-loss transmission of the light signal with electrical separation -of the two circuits, and safe separation, in the sense of explosion protection,-by observance of the precisely laid-down insulation distances.

Fig. 2 shows that at the end sections of the light-conducting rod 12 opposite to each other, in each case a sleeve shaped or hoseshaped tube 16, 16' is provided. The left-hand tube 16 overlaps a part of the low-current light-emitting diode 7 and a part of the lightconducting rod 12, whilst the right-hand tube 16' overlaps a part of the light-conducting rod 12 and a part of the phototransistor 11. The walls of the tubes 16, 16' may expediently be of an opaque type. Because of the two tubes 16, 16' the optocoupler 1 is arranged with the low-current light-emitting diode 7, the lightconducting rod 12 and the phototransistor 11 aligned exactly concentrically so that a perfect coaxial light transmission is ensured.

In the practical example in fig. 3, the tube 16 is of a continuous type and extends thus from the connection of the low-current lightemitting diode 7 as far as the connection of the phototransistor 11. It should be recognised here that the end sections 17 of the tube 16 are of a smaller diameter, so that the low-current light-emitting diode 7 and the phototransistor 11 are positively overlapped at the ends, by which means a permanently perfect fixing is ensured. The low-current lightemitting diode 7, the light-conducting rod 12 and the phototransistor 11 are thus are compactly combined as one constructional unit both in an axial direction and also in a radial direction and are supported in a protected manner so that no damage can occur during handling which occurs during assembly.

The circuit diagram of the optocoupler 1 according to the invention illustrated in Fig.4 shows that, between the two connections 4 of the low-current light-emitting diode 7, a further diode 18 is provided as a reverse voltage protection, whilst between the connections 4', a further transistor 19 and a resistance 20 are connected to the phototransistor 11 for signal amplification.

With reference to Fig. 2 it should also be mentioned that the setting compound 2 which hermetically encloses the components is made as short as possible in the axial direction of the optocoupler 1 and is only somewhat greater than the distance between the lefthand connecting section 6 of the low-current light-emitting diode 7 on the one hand and the electronic components 10 of the circuit board 9 on the other side opposite, so that, in all, a definitely small and compact device can be achieved.

In the case of the practical example illustrated in Figs. 5 to 9, the optocoupler 1 is supported in a tubular bushing 21 and is designed with the latter as an explosion-protected cable bushing. For this purpose, in the interior of the bushing 21, which has externally a thread 22, two circuit boards 23, 24 are provided, arranged at a distance from each other, the plane of which lies tranversely to the longitudinal axis 25 of the bushing. The circuit boards 23, 24 are essentially formed as circular discs and have a diameter which is somewhat smaller than the internal diameter of the bushing 21, so that alignment can be obtained in the interior.The connections 26 of the low-current light-emitting diode 7 and the phototransistor 11 of the two optocouplers 1 in this embodiment which azure aligned in paral lel are bonded to the inner sides of the circuit boards 23, 24 which are turned towards each other. Both the optocouplers 1 are essentially of the type of optocoupler 1 illustrated in Fig.

3 and thus have in each case a low-current light-emitting diode 7 and a phototransistor 11 in the continuous hose-shaped tube 16, between which the light-conducting rod 12 is situated. Both the optocouplers 1 of the present embodiment are of the type known as "two-channel optocouplers" which are connected in opposition. This means that the low-current light-emitting diode 7 of one optocoupler 1 is connected to the circuit board 23, whilst the low-current light-emitting diode 7 of the other optocoupler 1 aligned in opposition is bonded to the other circuit board 24 opposite. Instead of the two optocouplers 1, it is also possible to provide 3 or more optocouplers 1 in the same alignment or aligned in opposition.It should be noted here that the optocoupler 1 according to the invention can expediently be of a type both for DC and AC operation at the input and output, so that there is what is called a universal optocoupler 1, which can be operated irrespective of the particular kind of voltage.

The electronic components 10 for the protective circuit and amplification of the optocoupler 1 are also arranged on the sides of the two circuit boards, 23 24 which are directed towards each other. electrical conductors 27 are arranged on the external side of the circuit board 24, which are led out on the top end face of the bushing 21. On the lower side, as shown in Fig.5, an insulator 28 is arranged, which has conductor terminals 30 preferably arranged in the form of a star (Fig.6) between insulating bars 29, the terminals 30 are electrically connected to the circuit board 23.

Further input or output conductors may be connected to the conductor terminals 30. the insulator 28 is fitted -tightly into- the end face opening of the bushing 21 by a more or less circular disk-shaped end section 31. The conductors 27 connected to the circuit board 24 are led through an insulating spacer plate 32 and soldered to terminals 33 which, in the present embodiment, are arranged next to each other in a row on an insulating strip 34, with an insulating wall 35 in each case being situated between the terminals 33. The bushing 21 is completely packed with the setting compound 2, so that there is a tight seal on the insulator 28, and the optocouplers 1 with the circuit boards 23, 24, the electronic components 10, the conductors 27 and the spacer plate 32 which are completely embedded in the setting compound 2.It is also possible to provide instead of the spacer plate 32 which are completely embedded in the setting compound 2. It is also possible to provide instead of the insulator 28 and the terminals 33, on one or both end faces of the bushing 21, in each case individual single-core nonsheathed leads or multicore sheathed cables or terminal connections of another type.

Fig. 8 shows how the conductor terminals 30 of the contact ends 36 in the insulator 28 can be soldered to the soldering eyelets 37 of the circuit board 23. It can also be seen that the optocouplers 1 have a rectangular or square external contour in cross section. This has been achieved by further encapsulating the hose-shaped tube 16 in plastic, as a result of which a rectangular-shaped rod with high strength is obtained, so that during the further assembly and during encapsulation in the setting compound 2 no damage can occur to the optocoupler 1. It can be seen from Fig.9 how the individual leads 27 on the circuit board 24 can be bonded to their soldering eyelets 37.

In addition, itcan be seen how the electronic components 10 may be positioned on the opposite side of the circuit board.

By means of the optocoupler 1 integrated in the bushing 21 it is possible simultaneously to achieve an electrical separation of the circuits inside a pressure-tight housing not illustrated here, and of the circuits outside the area with an explosion hazard.

The invention is not restricted to the above described embodiments and many variations and modifications can be made. For example in place of the type of connectors shown in the embodiment of Figs. 5 to 9 flying leads can be provided. The bushing 21 need not be externally threaded as at 22 but can be plane and secured in a housing or other fitment by known means. In fact the bushing 21 can, if desired, be omitted altogether and the setting compound 2 being moulded to the required shape.

In order to assist in obtaining axial alignment of the light emitting source, light conducting rod and light receiving means the ends of the rod may be hollowed out or counterbored so as to receive therein at one end the light emitting source and at the other end the light receiving means. It may be necessary to polish the hollowed or counterbored ends of the rod so as to maintain optical performance of the assembly.

The light emitting source, light conducting rod and light receiving means may all be of the same diameter but that is not essential. In the embodiments described with reference to the drawings the light emitting source is a light emitting diode and the light receiving means is a photo transistor. Other components which perform the same or a similar function can be used instead.

Claims

1. An optoelectric coupler for signal transmission with electrical separation between two electrical circuits, comprising a light-conducting rod disposed in an electrically insulating compound and located between an electrically energisable light emitting source and means responsive to the reception of light will generate an electrical signal, said light conducting rod comprising a core for light transmission and a sheath around the core, said sheath being of an optically thinner medium.

2. A coupler as claimed in claim 1, wherein the light-conducting rod, the light-emitting source and/or the light receiving means are arranged coaxially in the insulating compound.

3. A coupler as claimed in claim 1 or claim 2, wherein the light emitting source, the light receiving means and the light-conducting rod are enveloped at least at adjacent ends thereof by a tube which is preferably opaque.

4. A coupler as claimed in claim 3, wherein the tube is in one piece and extends around the ends of the light-emitting source and the light-receiving means.

5. A coupler as claimed in any preceding claim wherein a printed circuit board with electronic components for the amplification of the signal from the light-receiving means is also disposed in the insulating compound.

6. A coupler as claimed in claim 5, wherein light receiving means and at least one terminal pin extending from the insulating compound are provided on the one circuit board, the electronic components being arranged in a space-saving flat design (SMD technique or I.C.chip).

7. A coupler as claimed in claim 5 or claim 6, wherein a further terminal pin extends from the setting compound and connected directly to the light-emitting source the dimension of the insulating compound in the axial direction of the light-conducting rod being only slightly greater than the distance between the connector between the further terminal pin and the light-emitting source on the one hand and the circuit board on the other hand.

8. A coupler as claimed in any of claims 5 to 7, wherein the light-conducting rod, the light-emitting source, the light-receiving means and the circuit board- carrying the electronic components are supported by the insulating compound in the interior of a tubular bushing and, with the latter, are constructed as an explosion-protected cable bushing.

9. A coupler as claimed in claim 8, wherein the light-conducting rod is provided with the light-emitting source and the light-receiving means between two circuit boards arranged at a distance from each other, the planes of the circuit boards being aligned transversely to the longitudinal axis of the bushing, the diameters of the circuit boards being somewhat smaller than the inside diameter of the bushing and wherein electrical conductors for the light-emitting source the light-receiving means and the electronic components are led out from the sides of the circuit boards remote from the light-conducting rod.

10. A coupler as claimed in claim 9, wherein conductors to one circuit board are connected to conductor terminals arranged in a substantially star-shaped manner in at least one insulator sealing off the bushing at the end.

11. A coupler as claimed in claim 9 or claim 10, wherein conductors to the other circuit board are bonded to terminals which are arranged side by side and insulated in series.

12. An optoelectric coupler substantially as described herein with reference to Figs. 1 to 4 or figs. 5 to 9 of the accompanying drawings.