AC relay protection circuit

Abstract

The utility model relates to an alternating-current relay protection circuit which comprises a power supply circuit, a driving circuit and an overcurrent protection circuit. The driving circuit is electrically connected with the power supply circuit and the alternating-current relay; the overcurrent protection circuit comprises a current sensor, a current sampling sub-circuit, a comparison sub-circuit and a switch sub-circuit which are sequentially connected in series, the input end of the current sensor is connected between the driving circuit and the alternating-current relay in parallel, and the switch sub-circuit is electrically connected with the driving circuit. Through cooperation of the current sensor, the current sampling sub-circuit, the comparison sub-circuit and the switch sub-circuit, the switch sub-circuit feeds back to the driving circuit, so that the alternating current relay can be disconnected, the purpose of over-current protection of the alternating current relay is achieved, and potential safety hazards are avoided.

Description

Technical Field

[0001] The present invention relates to the technical field of high-voltage switches, and in particular to an AC relay protection circuit. Background Art

[0002] AC relays are widely used in high-voltage, high-power applications, including power systems, control systems, automotive electronics, industrial automation, building automation, transportation, and medical equipment. AC relays play a vital role in electrical systems, controlling the on / off state of circuits and thereby controlling electrical equipment.

[0003] Currently, AC relays are not designed with specific protection circuits, resulting in the AC relay not being able to disconnect in time when an abnormality occurs. Over time, the AC relay will become damaged, not only rendering its control functions inoperable, but also posing a safety hazard. Summary of the Invention

[0004] Based on the above description, the present invention provides an AC relay protection circuit, which aims to solve the problem that the existing AC relay is easily damaged and cannot be disconnected in time, resulting in safety hazards.

[0005] The technical solution of the present invention to solve the above technical problems is as follows:

[0006] An AC relay protection circuit, characterized by comprising:

[0007] Power supply circuit;

[0008] a drive circuit, electrically connected to the power supply circuit and the AC relay;

[0009] The overcurrent protection circuit includes a current sensor, a current sampling subcircuit, a comparison subcircuit and a switch subcircuit connected in series in sequence. The input end of the current sensor is connected in parallel between the drive circuit and the AC relay, and the switch subcircuit is electrically connected to the drive circuit.

[0010] On the basis of the above technical solution, the present invention can also be improved as follows.

[0011] Furthermore, the power supply circuit includes a transformer, a first full-bridge rectifier circuit, a second full-bridge rectifier circuit and a linear regulator, the first full-bridge rectifier circuit is electrically connected to the negative pole of the transformer, the second full-bridge rectifier circuit is electrically connected to the positive pole of the transformer, the input end of the linear regulator is electrically connected to the second full-bridge rectifier circuit, and the ground end of the linear regulator is grounded.

[0012] Furthermore, the power supply circuit includes a first capacitor, which is connected in parallel between the output terminal of the linear regulator and the ground terminal.

[0013] Furthermore, the power supply circuit includes a first diode, which is connected in parallel between the input end and the output end of the linear regulator.

[0014] Furthermore, the driving circuit includes an optical fiber receiver, an optocoupler sub-circuit, a pull-up resistor, a PNP transistor and a plate-type relay connected in series in sequence, the optocoupler sub-circuit is electrically connected to the first full-bridge rectifier sub-circuit, the output end of the linear regulator and the switch sub-circuit, the input contact of the plate-type relay is electrically connected to the live wire contact of the transformer, the output contact of the plate-type relay is electrically connected to one end of the coil of the AC relay, and the other end of the coil of the AC relay is electrically connected to the neutral wire contact of the transformer.

[0015] Furthermore, the driving circuit includes a freewheeling diode, which is connected in parallel between two ends of the coil of the plate-type relay.

[0016] Furthermore, the driving circuit includes a first light emitting diode, which is connected in parallel between two ends of the coil of the plate-type relay.

[0017] Furthermore, the overcurrent protection circuit includes a rectifier and filter sub-circuit, which is electrically connected between the current sampling sub-circuit and the comparison sub-circuit. The rectifier and filter sub-circuit includes a rectifier diode, a filter resistor and a second capacitor. The rectifier diode and the filter resistor are connected in series in sequence, and one end of the second capacitor is connected in parallel between the end of the filter resistor away from the rectifier diode and the comparison sub-circuit.

[0018] Furthermore, it includes a switch state feedback circuit, which includes a first current limiting resistor, an optocoupler, a second current limiting resistor, a driver, an optical fiber transmitter, a second diode and a pull-down resistor. The first current limiting resistor, the optocoupler, the second current limiting resistor, the driver and the optical fiber transmitter are connected in series in sequence. One end of the first current limiting resistor is electrically connected to the second full-bridge rectifier circuit. The AC relay has a limit switch. The second input end of the optocoupler is electrically connected to the first normally open contact of the limit switch, and the second normally open contact of the limit switch is grounded. The first output end of the optocoupler is electrically connected to the output end of the linear regulator, and the input end of the driver is electrically connected to the switch sub-circuit. The negative electrode of the second diode is connected in parallel between the other end of the first current limiting resistor and the first input end of the optocoupler, the positive electrode of the second diode is electrically connected to the second input end of the optocoupler, and one end of the pull-down resistor is connected in parallel between the second output end of the optocoupler and the end of the second current limiting resistor away from the driver.

[0019] Furthermore, the switch state feedback circuit includes a second light-emitting diode and a third current-limiting resistor, the second light-emitting diode is connected in parallel between the other end of the first current-limiting resistor and the positive electrode of the second diode, and the third current-limiting resistor is connected in parallel between the second light-emitting diode and the third current-limiting resistor.

[0020] Compared with the prior art, the technical solution of this application has the following beneficial technical effects:

[0021] (1) This application uses a current sensor, a current sampling subcircuit, a comparison subcircuit and a switch subcircuit to cooperate with each other. The switch subcircuit provides feedback to the drive circuit, which can disconnect the AC relay, thereby achieving the purpose of overcurrent protection of the AC relay and avoiding safety hazards.

[0022] (2) In this application, when an optical signal is input to the optical fiber receiver, the optocoupler is turned on; the base voltage of the PNP transistor is pulled down, and the PNP transistor is turned on; the plate relay is closed to output 220V AC power, which can power the AC relay to close, thereby achieving the purpose of driving the AC relay to open and close by the optical signal.

[0023] (3) When the AC relay is closed, the limit switch is also closed. At this time, the optocoupler is turned on, and the voltage at the second output terminal of the optocoupler is charged to a high level. After isolation and conversion by the driver, the 5V output is sent to the optical fiber transmitter, thereby emitting an optical signal to light up the external indicator light to feedback that the AC relay has been closed. This can provide a reminder to the staff, and when the external indicator light goes out, it is convenient for the staff to handle it in time. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1A schematic structural diagram of an AC relay protection circuit provided in an embodiment of the present invention;

[0025] Figure 2 is a circuit diagram of a power supply circuit in an embodiment of the present invention;

[0026] Figure 3 is a circuit diagram of a driving circuit in an embodiment of the present invention;

[0027] Figure 4 is a circuit diagram of an overcurrent protection circuit in an embodiment of the present invention;

[0028] Figure 5 2 is a circuit diagram of a switch state feedback circuit in an embodiment of the present invention.

[0029] Description of reference numerals:

[0030] 1. AC relay;

[0031] 10. Power supply circuit; 11. Transformer; 12. First full-bridge rectifier circuit; 13. Second full-bridge rectifier circuit; 14. Linear regulator; 15. First capacitor; 16. First diode;

[0032] 20. Drive circuit; 21. Fiber optic receiver; 22. Optocoupler circuit; 23. Pull-up resistor; 24. PNP transistor; 25. Plate relay; 26. Freewheeling diode; 27. First light-emitting diode;

[0033] 30. Overcurrent protection circuit; 31. Current sensor; 32. Current sampling subcircuit; 33. Comparison subcircuit; 34. Switch subcircuit; 35. Rectification and filtering subcircuit; 351. Rectifier diode; 352. Filter resistor; 353. Second capacitor;

[0034] 40. Switch state feedback circuit; 41. First current-limiting resistor; 42. Optocoupler; 421. First input terminal; 422. Second input terminal; 423. First output terminal; 424. Second output terminal; 43. Second current-limiting resistor; 44. Driver; 45. Fiber optic transmitter; 46. Second diode; 47. Pull-down resistor; 48. Second light-emitting diode; 49. Third current-limiting resistor;

[0035] 50. Mains power grid;

[0036] 60. Optical transmitter;

[0037] 70. Indicator light. DETAILED DESCRIPTION

[0038] To facilitate understanding of the present application, the present application will be described more fully below with reference to the accompanying drawings. The accompanying drawings provide embodiments of the present application. However, the present application may be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to make the disclosure of the present application more thorough and comprehensive.

[0039] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which this application pertains. The terms used herein in the specification of this application are for the purpose of describing specific embodiments only and are not intended to limit this application.

[0040] It will be understood that spatial relational terms such as "under," "beneath," "below," "under," "above," "above," etc., may be used herein to describe the relationship of an element or feature shown in the figures to other elements or features. It will be understood that in addition to the orientations shown in the figures, spatial relational terms also include different orientations of the device in use and operation. For example, if the device in the drawings is turned over, the element or feature described as "under" or "beneath" or "beneath" the other elements will be oriented as "above" the other elements or features. Thus, the exemplary terms "under" and "under" may include both upper and lower orientations. In addition, the device may also include alternative orientations (e.g., rotated 90 degrees or other orientations), and the spatial descriptors used herein are interpreted accordingly.

[0041] When used herein, the singular forms "a", "an", and "the" may also include the plural forms, unless the context clearly indicates otherwise. It should also be understood that the terms "include / comprise" or "have" and the like specify the presence of stated features, integers, steps, operations, components, parts, or combinations thereof, but do not preclude the possibility of the presence or addition of one or more other features, integers, steps, operations, components, parts, or combinations thereof.

[0042] Refer to the attached Figures 1 to 4 As shown, the present invention provides a technical solution: an AC relay protection circuit, comprising a power supply circuit 10, a drive circuit 20 and an overcurrent protection circuit 30; the drive circuit 20 is electrically connected to the power supply circuit 10 and the AC relay 1; the overcurrent protection circuit 30 comprises a current sensor 31, a current sampling subcircuit 32, a comparison subcircuit 33 and a switch subcircuit 34 connected in series in sequence, the input end of the current sensor 31 is connected in parallel between the drive circuit 20 and the AC relay 1, and the switch subcircuit 34 is electrically connected to the drive circuit 20.

[0043] For example, the model of the current sensor 31 may be HLX1-5 (100) A / 2.5 mA, etc. The model of the operational amplifier chip of the current sampling subcircuit 32 and the comparison subcircuit 33 may be LM258, etc. The transistor of the switch subcircuit 34 is an NPN transistor.

[0044] According to this embodiment, the power supply circuit 10 is connected to the external mains power grid 50 for inputting mains power and supplying power to the drive circuit 20, the current sampling subcircuit 32, and the comparison subcircuit 33. The drive circuit 20 controls the AC relay 1 to close or open the AC relay 1. When the AC relay 1 is operating, the current sensor 31 detects the current of the AC relay 1. When an abnormality occurs in the AC relay 1, the current sampling subcircuit 32 collects the current signal of the AC relay 1, converts the current signal into a voltage signal, and performs computational amplification. When the amplified voltage signal is transmitted to the comparison subcircuit 33, the sampled current value is compared with the preset current protection value of the comparison subcircuit 33. When the sampled current value is greater than the preset current protection value, the switch subcircuit 34 provides feedback to the drive circuit 20, which can cause the AC relay 1 to open, thereby achieving the purpose of overcurrent protection for the AC relay 1.

[0045] Refer to the attached Figure 1 and 2 As shown, in some embodiments, the power supply circuit 10 includes a transformer 11, a first full-bridge rectifier circuit 12, a second full-bridge rectifier circuit 13 and a linear regulator 14, the first full-bridge rectifier circuit 12 is electrically connected to the negative pole of the transformer 11, the second full-bridge rectifier circuit 13 is electrically connected to the positive pole of the transformer 11, the input end of the linear regulator 14 is electrically connected to the second full-bridge rectifier circuit 13, and the ground end of the linear regulator 14 is grounded.

[0046] For example, the live contact and the neutral contact of the transformer 11 are connected to the external mains power grid 50. The model of the transformer 11 can be SCB9-2000 / 10, etc. The model of the linear regulator 14 can be LM317, etc.

[0047] According to this embodiment, after transformer 11 receives 220V AC power, first full-bridge rectifier circuit 12 performs full-bridge rectification to produce -12V DC power, which is then rectified by second full-bridge rectifier circuit 13 to produce +12V DC power. Simultaneously, at second full-bridge rectifier circuit 13, linear regulator 14 converts the +12V DC power into +5V DC power, which in turn provides +5V power to various chips (e.g., optocoupler 42 and operational amplifier chip). Furthermore, transformer 11 also acts as an isolation device, shielding against interference from the AC power grid 50 and ensuring stable operation of the driver circuit 20.

[0048] Refer to the attached Figure 2As shown, in some embodiments, the power supply circuit 10 includes a first capacitor 15 , which is connected in parallel between the output terminal of the linear regulator 14 and the ground terminal.

[0049] According to this embodiment, when the linear regulator 14 outputs a voltage, the first capacitor 15 filters the output voltage of the linear regulator 14 , making the output voltage of the linear regulator 14 smoother, thereby improving the stability of the output voltage of the linear regulator 14 .

[0050] Refer to the attached Figure 2 As shown, in some embodiments, the power supply circuit 10 includes a first diode 16 , which is connected in parallel between the input and output terminals of the linear regulator 14 .

[0051] According to this embodiment, the first diode 16 ensures that the input voltage of the linear regulator 14 does not exceed the output voltage, thereby preventing the linear regulator 14 from being damaged due to an excessive voltage difference.

[0052] Refer to the attached Figures 2 and 3 As shown, in some embodiments, the driving circuit 20 includes a fiber optic receiver 21, an optocoupler sub-circuit 22, a pull-up resistor 23, a PNP transistor 24 and a plate relay 25 connected in series in sequence, the optocoupler sub-circuit 22 is electrically connected to the first full-bridge rectifier sub-circuit 12, the output end of the linear regulator 14 and the switch sub-circuit 34, the input contact of the plate relay 25 is electrically connected to the live wire contact of the transformer 11, the output contact of the plate relay 25 is electrically connected to one end of the coil of the AC relay 1, and the other end of the coil of the AC relay 1 is electrically connected to the neutral wire contact of the transformer 11.

[0053] For example, the optical fiber receiver 21 is connected to the external optical transmitter 60 via a wire. The model of the optical fiber receiver 21 may be HFBR2412TZ, etc. The model of the plate relay 25 may be NNC69KTL-2Z, etc.

[0054] According to this embodiment, when driving the AC relay 1 to close or open, when there is no optical signal input to the optical fiber receiver 21, the optocoupler 42 of the optocoupler sub-circuit 22 is not conductive; when there is an optical signal input to the optical fiber receiver 21, the optocoupler 42 is conductive; the base voltage of the PNP transistor is pulled down, and the PNP transistor is conductive; the plate relay 25 is closed to output 220V AC power, which can power the AC relay 1 to close, thereby achieving the purpose of controlling the opening and closing of the AC relay 1 by driving the optical signal.

[0055] Refer to the attached Figure 3 As shown, in some embodiments, the driving circuit 20 includes a freewheeling diode 26 , which is connected in parallel between the two ends of the coil of the plate relay 25 .

[0056] According to this embodiment, if the coil of plate-type relay 25 suddenly loses power, freewheeling diode 26 provides a loop for the coil of plate-type relay 25, allowing the current in the coil of plate-type relay 25 to gradually decrease to zero. This prevents damage to the coil of plate-type relay 25 and other components in the circuit, ensuring the safe and reliable operation of plate-type relay 25.

[0057] Refer to the attached Figure 3 As shown, in some embodiments, the driving circuit 20 includes a first light emitting diode 27 , which is connected in parallel between two ends of the coil of the plate relay 25 .

[0058] According to this embodiment, the first light emitting diode 27 is used to display the operating status of the plate relay 25, which can provide prompts to the staff.

[0059] Refer to the attached Figure 4 As shown, in some embodiments, the overcurrent protection circuit 30 includes a rectifier and filter sub-circuit 35, which is electrically connected between the current sampling sub-circuit 32 and the comparison sub-circuit 33. The rectifier and filter sub-circuit 35 includes a rectifier diode 351, a filter resistor 352 and a second capacitor 353. The rectifier diode 351 and the filter resistor 352 are connected in series in sequence, and one end of the second capacitor 353 is connected in parallel between the end of the filter resistor 352 away from the rectifier diode 351 and the comparison sub-circuit 33.

[0060] According to this embodiment, when the amplified voltage signal is transmitted to the comparison sub-circuit 33, the amplified voltage signal is rectified by the rectifier diode 351, and the amplified voltage signal is filtered by the filter resistor 352 and the second capacitor 353, thereby ensuring the stability of the signal and making the comparison result of the comparison sub-circuit 33 more accurate.

[0061] Refer to the attached Figure 1 、 3As shown in Figure 5, in some embodiments, a switch state feedback circuit 40 is included, and the switch state feedback circuit 40 includes a first current limiting resistor 41, an optical coupler 42, a second current limiting resistor 43, a driver 44, an optical fiber transmitter 45, a second diode 46 and a pull-down resistor 47. The first current limiting resistor 41, the optical coupler 42, the second current limiting resistor 43, the driver 44 and the optical fiber transmitter 45 are connected in series in sequence, and one end of the first current limiting resistor 41 is electrically connected to the second full-bridge rectifier circuit 13. The AC relay 1 has a limit switch, and the limit switch The second normally open contact is grounded, the first output end 423 of the optocoupler 42 is electrically connected to the output end of the linear regulator 14, the input end of the driver 44 is electrically connected to the switch sub-circuit 34, the cathode of the second diode 46 is connected in parallel between the other end of the first current limiting resistor 41 and the first input end 421 of the optocoupler 42, the anode of the second diode 46 is electrically connected to the second input end 422 of the optocoupler 42, and one end of the pull-down resistor 47 is connected in parallel between the second output end 424 of the optocoupler 42 and the end of the second current limiting resistor 43 away from the driver 44.

[0062] For example, the model of the driver 44 may be SN75451BD, etc. The model of the optical fiber transmitter 45 may be HFBR1414TZ, etc. The optical fiber transmitter 45 is connected to the external indicator light 70 via a wire.

[0063] According to this embodiment, when AC relay 1 is closed, the limit switch is also closed. At this point, optocoupler 42 is turned on, and the voltage at second output terminal 424 of optocoupler 42 is increased to a high level. After isolation and conversion by driver 44, 5V is output to fiber optic transmitter 45, which then emits an optical signal, causing external indicator light 70 to illuminate, indicating that AC relay 1 is closed.

[0064] Refer to the attached Figure 5 As shown, in some embodiments, the switch state feedback circuit 40 includes a second light-emitting diode 48 and a third current-limiting resistor 49, the second light-emitting diode 48 is connected in parallel between the other end of the first current-limiting resistor 41 and the positive electrode of the second diode 46, and the third current-limiting resistor 49 is connected in parallel between the second light-emitting diode 48 and the third current-limiting resistor 49.

[0065] According to this embodiment, the second light emitting diode 48 is used to display the operating status of the optical coupler 42, which can provide a reminder to the staff. At the same time, the third current limiting resistor 49 plays a role in current limiting to prevent the second light emitting diode 48 from being damaged.

[0066] It should be noted that the first full-bridge rectifier sub-circuit 12, the second full-bridge rectifier sub-circuit 13, the optocoupler sub-circuit 22, the current sampling sub-circuit 32, the comparison sub-circuit 33 and the switch sub-circuit 34 are all existing technologies, so their specific structures and working principles will not be described in detail here.

[0067] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. An AC relay protection circuit, characterized in that: include: Power supply circuit (10); A drive circuit (20) electrically connected to the power supply circuit (10) and the AC relay (1); An overcurrent protection circuit (30) comprises a current sensor (31), a current sampling subcircuit (32), a comparison subcircuit (33), and a switch subcircuit (34) connected in series in sequence, wherein the input end of the current sensor (31) is connected in parallel between the drive circuit (20) and the AC relay (1), and the switch subcircuit (34) is electrically connected to the drive circuit (20).

2. The AC relay protection circuit according to claim 1, characterized in that: The power supply circuit (10) comprises a transformer (11), a first full-bridge rectifier circuit (12), a second full-bridge rectifier circuit (13) and a linear voltage regulator (14), wherein the first full-bridge rectifier circuit (12) is electrically connected to the negative pole of the transformer (11), the second full-bridge rectifier circuit (13) is electrically connected to the positive pole of the transformer (11), the input end of the linear voltage regulator (14) is electrically connected to the second full-bridge rectifier circuit (13), and the ground end of the linear voltage regulator (14) is grounded.

3. The AC relay protection circuit according to claim 2, characterized in that: The power supply circuit (10) comprises a first capacitor (15), and the first capacitor (15) is connected in parallel between the output end of the linear regulator (14) and the ground end.

4. The AC relay protection circuit according to claim 3, characterized in that: The power supply circuit (10) comprises a first diode (16), and the first diode (16) is connected in parallel between the input end and the output end of the linear regulator (14).

5. The AC relay protection circuit according to claim 2, characterized in that: The driving circuit (20) comprises an optical fiber receiver (21), an optocoupler circuit (22), a pull-up resistor (23), a PNP transistor (24) and a plate-type relay (25) connected in series in sequence, wherein the optocoupler circuit (22) is electrically connected to the first full-bridge rectifier circuit (12), the output end of the linear voltage regulator (14) and the switch circuit (34), the input contact of the plate-type relay (25) is electrically connected to the live wire contact of the transformer (11), the output contact of the plate-type relay (25) is electrically connected to one end of the coil of the AC relay (1), and the other end of the coil of the AC relay (1) is electrically connected to the neutral wire contact of the transformer (11).

6. The AC relay protection circuit according to claim 5, characterized in that: The drive circuit (20) includes a freewheeling diode (26), and the freewheeling diode (26) is connected in parallel between two ends of the coil of the plate-type relay (25).

7. The AC relay protection circuit according to claim 6, characterized in that: The driving circuit (20) includes a first light-emitting diode (27), and the first light-emitting diode (27) is connected in parallel between two ends of the coil of the plate-type relay (25).

8. The AC relay protection circuit according to claim 1, characterized in that: The overcurrent protection circuit (30) includes a rectifier filter subcircuit (35), the rectifier filter subcircuit (35) is electrically connected between the current sampling subcircuit (32) and the comparison subcircuit (33), the rectifier filter subcircuit (35) includes a rectifier diode (351), a filter resistor (352) and a second capacitor (353), the rectifier diode (351) and the filter resistor (352) are sequentially connected in series, and one end of the second capacitor (353) is connected in parallel between an end of the filter resistor (352) away from the rectifier diode (351) and the comparison subcircuit (33).

9. The AC relay protection circuit according to any one of claims 2 to 7, characterized in that: The invention comprises a switch state feedback circuit (40), wherein the switch state feedback circuit (40) comprises a first current limiting resistor (41), an optical coupler (42), a second current limiting resistor (43), a driver (44), an optical fiber transmitter (45), a second diode (46) and a pull-down resistor (47), wherein the first current limiting resistor (41), the optical coupler (42), the second current limiting resistor (43), the driver (44) and the optical fiber transmitter (45) are sequentially connected in series, one end of the first current limiting resistor (41) is electrically connected to the second full-bridge rectifier circuit (13), the AC relay (1) has a travel switch, and the second input end (422) of the optical coupler (42) is electrically connected to the first normally open contact of the travel switch. The first output terminal (423) of the optical coupler (42) is electrically connected to the output terminal of the linear regulator (14), the input terminal of the driver (44) is electrically connected to the switch sub-circuit (34), the cathode of the second diode (46) is connected in parallel between the other end of the first current limiting resistor (41) and the first input terminal (421) of the optical coupler (42), the anode of the second diode (46) is electrically connected to the second input terminal (422) of the optical coupler (42), and one end of the pull-down resistor (47) is connected in parallel between the second output terminal (424) of the optical coupler (42) and the end of the second current limiting resistor (43) away from the driver (44).

10. The AC relay protection circuit according to claim 9, characterized in that: The switch state feedback circuit (40) comprises a second light-emitting diode (48) and a third current-limiting resistor (49), wherein the second light-emitting diode (48) is connected in parallel between the other end of the first current-limiting resistor (41) and the positive electrode of the second diode (46), and the third current-limiting resistor (49) is connected in parallel between the second light-emitting diode (48) and the third current-limiting resistor (49).

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