Patent Publication Number: US-2016226233-A1

Title: Signal transmission device

Description:
BACKGROUND OF THE DISCLOSURE 
     1. Field of the Disclosure 
     The present disclosure relates to a signal transmission device, and more particularly to a signal transmission device having an anti-surge mechanism. The signal transmission device has a small volume and low cost. 
     2. Brief Description of the Related Art 
     Surge may result from two reasons: one reason is because of lightning that causes lightning surge; the other reason is because a circuit is being powered on to cause power surge. Lightning surge is generated by nature. When employed in an area prone to lightning, an overload protection circuit is necessary to be provided. For example, in order for protection from a lightning surge, a lightning protection device, voltage dependent resistor or capacitor may be employed. A lightning protection tube may be mounted to protect circuits and release the energy of lightning or overload from a power system so as to protect electronic equipment from being damaged due to an overvoltage. The lightning protection tube may cut off the electric current so as to prevent a system from being shorted to the electrical ground. Basically, the lightning protection tube couples between a live wire and the electrical ground and in parallel with the circuits to be protected. When the overvoltage is over a threshold voltage, the lightning protection tube may be actuated to have the electric current pass therethrough and to limit a voltage amplitude and thereby the electronic equipment may be protected. When the overvoltage is gone, the lightning protection tube is promptly recovered to ensure regular power supply to the system. However, the lightning protection tube has a high cost and large volume. 
     SUMMARY OF THE DISCLOSURE 
     The present disclosure provides a signal transmission device with a metal plate sleeved around a signal terminal. An air radial gap exists between an annular surface of a hole in the metal plate and the signal terminal and acts as a surge protection structure. Comparing to the lightning protection tube or lightning protection element, the signal transmission device has a relatively low cost and small volume. 
     The present disclosure provides a signal transmission device. The signal transmission device includes a first metal plate; a first metal rod passing through a first hole in the first metal plate, wherein a first radial gap between the first metal rod and a first annular surface of the first hole is between 0.1 millimeters and 0.6 millimeters, wherein an electric current is configured to be discharged from the first metal rod to the first metal plate when a voltage difference between the first metal plate and the first metal rod is greater than or equal to 1 kV; and a circuit board connected to the first metal rod, wherein the circuit board comprises a first polymer layer, a patterned metal layer on the first polymer layer, and a second polymer layer on the first polymer layer and the patterned metal layer, wherein the patterned metal layer is connected to the first metal rod. 
     These, as well as other components, steps, features, benefits, and advantages of the present disclosure, will now become clear from a review of the following detailed description of illustrative embodiments, the accompanying drawings, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings disclose illustrative embodiments of the present disclosure. They do not set forth all embodiments. Other embodiments may be used in addition or instead. Details that may be apparent or unnecessary may be omitted to save space or for more effective illustration. Conversely, some embodiments may be practiced without all of the details that are disclosed. When the same reference number or reference indicator appears in different drawings, it may refer to the same or like components or steps. 
       Aspects of the disclosure may be more fully understood from the following description when read together with the accompanying drawings, which are to be regarded as illustrative in nature, and not as limiting. The drawings are not necessarily to scale, emphasis instead being placed on the principles of the disclosure. In the drawings: 
         FIG. 1  is an exploded perspective view illustrating a surge protection device in accordance with a first embodiment of the present invention; 
         FIG. 2  is an exploded cross-sectional view illustrating the surge protection device in accordance with the first embodiment of the present invention; 
         FIG. 3  is a cross-sectional view illustrating a circuit board of the surge protection device in accordance with the first embodiment of the present invention; 
         FIGS. 4 a , 4 b      5  and  6  are cross-sectional views illustrating an assembly for the surge protection device in accordance with the first embodiment of the present invention; 
         FIG. 7  is a cross-sectional view illustrating a surge protection device in accordance with a second embodiment of the present invention; 
         FIG. 8 a    is a cross-sectional view illustrating a first type of surge protection device in accordance with a third embodiment of the present invention; 
         FIG. 8 b    is a cross-sectional view illustrating a second type of surge protection device in accordance with the third embodiment of the present invention; 
         FIG. 8 c    is a cross-sectional view illustrating a third type of surge protection device in accordance with the third embodiment of the present invention; 
         FIG. 9 a    is a cross-sectional view illustrating a first type of surge protection device in accordance with a fourth embodiment of the present invention; 
         FIG. 9 b    is a cross-sectional view illustrating a second type of surge protection device in accordance with the fourth embodiment of the present invention; 
         FIG. 9 c    is a cross-sectional view illustrating a third type of surge protection device in accordance with the fourth embodiment of the present invention; 
         FIG. 10 a    is a cross-sectional view illustrating a first type of surge protection device in accordance with a fifth embodiment of the present invention; 
         FIG. 10 b    is a cross-sectional view illustrating a second type of surge protection device in accordance with the fifth embodiment of the present invention; 
         FIG. 11 a    is a cross-sectional view illustrating a first type of surge protection device in accordance with a sixth embodiment of the present invention; 
         FIG. 11 b    is a cross-sectional view illustrating a second type of surge protection device in accordance with the sixth embodiment of the present invention; 
         FIG. 12 a    is a cross-sectional view illustrating a first type of surge protection device in accordance with a seventh embodiment of the present invention; 
         FIG. 12 b    is a cross-sectional view illustrating a second type of surge protection device in accordance with the seventh embodiment of the present invention; 
         FIG. 13 a    is a cross-sectional view illustrating a first type of surge protection device in accordance with a eighth embodiment of the present invention; 
         FIG. 13 b    is a cross-sectional view illustrating a second type of surge protection device in accordance with the eighth embodiment of the present invention; 
         FIG. 14  is a cross-sectional view illustrating a surge protection device in accordance with a ninth embodiment of the present invention; and 
         FIG. 15  is a cross-sectional view illustrating a surge protection device in accordance with a tenth embodiment of the present invention. 
     
    
    
     While certain embodiments are depicted in the drawings, one skilled in the art will appreciate that the embodiments depicted are illustrative and that variations of those shown, as well as other embodiments described herein, may be envisioned and practiced within the scope of the present disclosure. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Illustrative embodiments are now described. Other embodiments may be used in addition or instead. Details that may be apparent or unnecessary may be omitted to save space or for a more effective presentation. Conversely, some embodiments may be practiced without all of the details that are disclosed. When the same reference number or reference indicator appears in different drawings, it may refer to the same or like components or steps. 
     The present disclosure provides a signal transmission device that may be installed on an electronic device, such as signal filter, signal receiver, signal transmitter, signal attenuator or any one that needs to be protected from surge. Multiple embodiments are introduced in the following paragraphs. 
     First Embodiment 
     In accordance with the first embodiment, a signal filter is illustrated as an example. Referring to  FIGS. 1 and 2 , an electronic device includes a cylindrical housing  100  and an inner electronic assembly  200  accommodated in the cylindrical housing  100 . The cylindrical housing  100  includes a nut portion  102  at a back end of the cylindrical housing  100 , an outer-thread portion  106  at a front end of the cylindrical housing  100  and a main body  104  between the nut portion  102  and outer-thread portion  106 . A through hole  108  passing through the cylindrical housing  100  may be divided into a first cylindrical space  1081  and a second cylindrical space  1082 . The first cylindrical space  1081  has an inner diameter greater than that of the second cylindrical space  1082 . The cylindrical housing  100  may be made of copper, iron, silver, nickel, tin, gold, copper-gold alloys, a copper-tin alloys, copper-nickel alloys, brass, brass alloys, phosphor bronze, beryllium copper, aluminum, aluminum alloys, zinc alloys, steel alloys or conductive polymers. The cylindrical housing  100  may be composed of the main body  104 , nut portion  102  and outer-thread portion  106   a  formed as a single integral part. 
     Referring to  FIGS. 1 and 2 , the inner electronic assembly  200  includes a first signal terminal  202 , a metal sleeve  204 , a first insulating annular plate  206 , a first water-proof insulating annular plate  208 , a first surge-protection metal annular plate  210 , a second insulating annular plate  211 , a circuit device  212 , a second signal terminal  214 , a second surge-protection metal annular plate  216 , a third insulating annular plate  217 , a second water-proof insulating annular plate  218 , a fourth insulating annular plate  220 , a fixing plate  221  and a fixing sleeve  223 . The circuit device  212  includes a circuit board  222 , multiple inductor coils  224 , two capacitors  226 , multiple resistors  228  and two metal sheets  230 , wherein the circuit board  222  may be a printed circuit board with a rectangular shape having two parallel longer edges and two parallel shorter edges. Referring to  FIG. 3 , the circuit board  222  may include a core substrate  2221  having multiple through holes  222   a  pass therethrough, multiple patterned metal layers  2222  and  2223 , such as copper or aluminum layers each having a thickness between 3 and 80 micrometers, and preferably between 3 and 50 micrometers, between 5 and 30 micrometers or between 10 and 80 micrometers, on an annular surface of each through holes  222   a , over a top surface of the core substrate  2221  and under a bottom surface of the core substrate  2221 , and multiple insulating polymer layers  2224  over the top surface of the core substrate  2221  and under the bottom surface of the core substrate  2221 . In this case, two of the patterned metal layers  2223  and three of the insulating polymer layers  2224  are formed over the top surface of the core substrate  2221 ; two of the patterned metal layers  2223  and three of the insulating polymer layers  2224  are formed under the bottom surface of the core substrate  2221 . The patterned metal layer  2222  in the through holes  222   a  may connect the patterned metal layers  2223  over the top surface of the core substrate  2221  and those under the bottom surface of the core substrate  2221 . The patterned metal layers  2223  may include multiple metal pads  2223   a  exposed by multiple openings  224   a  in the topmost and bottommost ones of the insulating polymer layers  2224 . A tin-containing solder may join the inductor coils  224 , capacitors  226 , resistors  228 , metal sheets  230 , first signal terminal  202  and second signal terminal  214  to the metal pads  2223   a.    
     Referring to  FIGS. 1, 2 and 3 , the first signal terminal  202  may be shaped like a metal wire or rod, having a diameter between 0.5 mm and 1.5 mm, and preferably between 0.5 mm and 1 mm or between 0.7 mm and 1.5 mm, bent with a horizontally-extending portion and a vertically-extending portion joining the horizontally-extending portion at a right angle. The vertically-extending portion of the first signal terminal  202  may be inserted into a through hole in the circuit board  222  and join the circuit board  222  by a tin-containing solder so as to connect with the patterned metal layers  2223 . The horizontally-extending portion of the first signal terminal  202  may pass across one of the shorter edges of the circuit board  222 . The second signal terminal  214  may include a metal wire or rod, having a diameter between 0.5 mm and 1.5 mm, and preferably between 0.5 mm and 1 mm or between 0.7 mm and 1.5 mm, passing across the other one of the shorter edges of the circuit board  222  to join one of the metal pads  2223   a  exposed by one of the openings  224   a  via a tin-containing solder, and a metal socket joining the metal wire or rod of the second signal terminal  214  for engaging with a metal wire or rod of a signal terminal, like the first signal terminal  202 , of another signal filter. The metal socket of the second signal terminal  214  may have an outer diameter, between 0.6 mm and 2.5 mm and preferably between 0.6 mm and 1.2 mm or between 0.8 and 2.5 mm, greater than a diameter of metal rod of first signal terminal  202 . The two metal sheets  230  may be mounted along the two respective longer edges of the circuit board  222 . Each of the metal sheets  230  may have a serrated portion  230   a  upwards extending from a corresponding one of the two longer edges of the circuit board  222  arranged in a horizontal level. Each of the metal sheets  230  may have a thickness between 0.02 mm and 2 mm, and preferably between 0.02 mm and 1 mm or between 0.5 mm and 2 mm. Each of the metal sheets  230  may be made of copper, iron, silver, nickel, tin, gold, copper-gold alloys, a copper-tin alloys, copper-nickel alloys, brass, brass alloys, phosphor bronze, beryllium copper, aluminum, aluminum alloys, zinc alloys or steel alloys. Each of the metal sheets  230  may be connected to the electrical ground of the circuit board  222 . The inductor coils  224  and the capacitors  226  are mounted to the metal pads  2223   a  at a top surface of the circuit board  222  via a tin-containing solder, wherein the inductor coils  224  are mounted between the capacitors  226  in a longitudinal direction and between the metal sheets  230  in a transverse direction. The resistors  228  are mounted to the metal pads  2223   a  at the bottom surface of the circuit board  222 . Two of the inductor coils  224 , capacitors  226  and resistors  228  may be connected to each other via a combination of the metal pads  2223 , patterned metal layers  2223  over and under the circuit board  222  and patterned metal layer  2222  in the through holes  222   a.    
     Referring to  FIG. 4 , the first insulating annular plate  206  may be inserted into a through hole  204   a  in the metal sleeve  204  until the first insulating annular plate  206  has a step abutting against a step  2041  of the metal sleeve  204 . The first insulating annular plate  206  may have an annular periphery radially abutting against an annular surface of the through hole  204   a  in the metal sleeve  204 . Next, the first water-proof insulating annular plate  208  may be inserted into the through hole  204   a  until the first water-proof insulating annular plate  208  abuts against the first insulating annular plate  206 . The first water-proof insulating annular plate  208  may have an annular periphery radially abutting against an annular surface of the through hole  204   a . Next, the first surge-protection metal annular plate  210  may be inserted into the through hole  204   a  until the first surge-protection metal annular plate  210  abuts against the first water-proof insulating annular plate  208 . The first surge-protection metal annular plate  210  may have an annular periphery radially abutting against an annular surface of the through hole  204   a . The second insulating annular plate  211  may be mounted to a step  2101  of the first surge-protection metal annular plate  210  before or after the first surge-protection metal annular plate  210  is mounted onto the first water-proof insulating annular plate  208  and into the through hole  204   a . The second insulating annular plate  211  may have an annular periphery radially abutting against an annular surface of the step  2101  of the first surge-protection metal annular plate  210 . The metal sleeve  204  may have an outer diameter substantially equal to an inner diameter of the through hole  108  in the first cylindrical space  1081  thereof. Each of the first and second insulating annular plates  206  and  211  may be made of a polymer, ceramic or glass material, such as plastic, polypropylene, polystyrene, polycarbonate, melamine resin or polytetrafluoroethene. The first water-proof insulating annular plate  208  may be made of a plastic, silicone, polymer elastomer or ceramic gasket. The first surge-protection metal annular plate  210  may be made of copper, iron, silver, nickel, tin, gold, copper-gold alloys, a copper-tin alloys, copper-nickel alloys, brass, brass alloys, phosphor bronze, beryllium copper, aluminum, aluminum alloys, zinc alloys or steel alloys. 
     Referring to  FIG. 4 , an axial through hole  206   a  in the first insulating annular plate  206  may have the same inner diameter, between 0.4 mm and 1.2 mm, and preferably between 0.4 mm and 0.9 mm or between 0.7 mm and 1.2 mm, as that of an axial through hole  211   a  in the second insulating annular plate  211  and as that of an axial through hole  208   a  in the first water-proof insulating annular plate  208 . An axial through hole  210   a  in the first surge-protection metal annular plate  210  may have an inner diameter greater than that of the axial through hole  206   a , that of the axial through hole  211   a  and that of the axial through hole  208   a  by between 0.3 mm and 1 mm and preferably between 0.3 mm and 0.9 mm or between 0.5 mm and 1 mm. The first surge-protection metal annular plate  210  may have an axial thickness between 0.5 mm and 3 mm, and preferably between 0.5 mm and 1.5 mm, between 1 mm and 2 mm or between 1.5 mm and 3 mm. 
     Besides, the second surge-protection metal annular plate  216  may have the same material as the first surge-protection metal annular plate  210 . The third insulating annular plate  217  may have the same material as the second insulating annular plate  211 . The second water-proof insulating annular plate  218  may have the same material as the first water-proof insulating annular plate  208 . The fourth insulating annular plate  220  may have the same material as the first insulating annular plate  206 . The third insulating annular plate  217  may be mounted to a step  2161  of the second surge-protection metal annular plate  216  and may have an annular periphery radially abutting against an annular surface of the step  2161  of the second surge-protection metal annular plate  216 . Each of the second surge-protection metal annular plate  216 , second water-proof insulating annular plate  218  and fourth insulating annular plate  220  may have an outer diameter substantially equal to an inner diameter of the through hole  108  in the second cylindrical space  1082  thereof, to an outer diameter of the first insulating annular plate  206 , to an outer diameter of the first water-proof insulating annular plate  208  and to an outer diameter of the first surge-protection metal annular plate  210 . An axial through hole  217   a  in the third insulating annular plate  217  may have the same inner diameter, between 0.6 mm and 1.8 mm, and preferably between 0.6 mm and 1 mm or between 0.8 mm and 1.8 mm, as that of an axial through hole  220   a  in the fourth insulating annular plate  220  and as that of an axial through hole  218   a  in the second water-proof insulating annular plate  218 . Each of the axial through holes  217   a ,  220   a  and  218   a  may have an inner diameter greater than that of the axial through hole  206   a , that of the axial through hole  208   a  and that of the axial through hole  211   a  by between 0.3 mm and 1 mm, and preferably between 0.3 mm and 0.9 mm or between 0.5 mm and 1 mm. 
     The second surge-protection metal annular plate  216  may have an axial thickness between 0.5 mm and 3 mm, and preferably between 0.5 mm and 1.5 mm, between 1 mm and 2 mm or between 1.5 mm and 3 mm. 
     Referring to  FIGS. 4 a , 4 b    and  5 , the inner electronic assembly  200  is assembled as illustrated in the following paragraphs. The vertically-extending portion of the first signal terminal  202  may be first inserted into a through hole in the circuit board  222  and join the circuit board  222  by a tin-containing solder so as to connect with the patterned metal layers  2223  of the circuit board  222 . Next, the horizontally-extending portion of the first signal terminal  202  may be inserted sequentially into the axial through hole  211   a  in the second insulating annular plate  211 , the axial through hole  210   a  in the first surge-protection metal annular plate  210 , the axial through hole  208   a  in the first water-proof insulating annular plate  208 , and the axial through hole  206   a  in the first insulating annular plate  206  after the first insulating annular plate  206 , first water-proof insulating annular plate  208 , first surge-protection metal annular plate  210  and second insulating annular plate  211  are mounted into the through hole  204   a  in the metal sleeve  204 . Each of the axial through holes  206   a ,  211   a  and  208   a  may have substantially the same inner diameter as the diameter of the horizontally-extending portion of the first signal terminal  202 . The axial through holes  210   a  in the first surge-protection metal annular plate  210  may have an inner diameter greater than the diameter of the horizontally-extending portion of the first signal terminal  202  such that a first radial air gap  2102  may be formed between a cylindrical surface of the first signal terminal  202  and an annular surface of the axial through hole  210   a , wherein the first radial air gap  2102  may be between 0.05 mm and 0.8 mm, and preferably between 0.1 mm and 0.6 mm or between 0.15 mm and 0.5 mm. The first radial air gap  2102  may be formed as a first discharging structure. 
     Next, a second discharging structure may be formed as illustrated in the paragraph. The third insulating annular plate  217  is mounted to the step  2161  of the second surge-protection metal annular plate  216  at a left side thereof and has an annular periphery radially abutting against the annular surface of the step  2161  of the second surge-protection metal annular plate  216 . Next, the second water-proof insulating annular plate  218  is mounted onto a right side of the second surge-protection metal annular plate  216 . Next, The fourth insulating annular plate  220  is mounted onto a right side of the second water-proof insulating annular plate  218 . Next, the second signal terminal  214  may have the metal wire or rod to be inserted sequentially into the axial through hole  220   a  in the fourth insulating annular plate  220 , the axial through hole  218   a  in the second water-proof insulating annular plate  218 , the axial through hole  216   a  in the second surge-protection metal annular plate  216  and the axial through hole  217   a  in the third insulating annular plate  217 . Each of the axial through holes  217   a ,  218   a  and  220   a  may have substantially the same inner diameter as the diameter of the metal wire or rod of the second signal terminal  214 . The axial through holes  216   a  in the second surge-protection metal annular plate  216  may have an inner diameter greater than the diameter of the metal wire or rod of the second signal terminal  214  such that a second radial air gap  2162  may be formed between the metal wire or rod of the second signal terminal  214  and an annular surface of the axial through hole  216   a , wherein the second radial air gap  2162  may be between 0.05 mm and 0.8 mm, and preferably between 0.1 mm and 0.6 mm or between 0.15 mm and 0.5 mm. The second radial air gap  2162  may be formed as the second discharging structure. Next, a tin-containing solder may be formed to join the metal wire or rod of the second signal terminal  214  to the metal pads  2223   a  of the circuit board  222 , and thereby the second signal terminal  214  may be electrically connected to the patterned metal layers  2223  of the circuit board  222  via the tin-containing solder. Next, the second signal terminal  214  may have the metal socket to be inserted into a through hole in the fixing sleeve  223  from a front end thereof, wherein the fixing sleeve has a back end mounted to the fixing plate  221 , until the fourth insulating annular plate  220  abuts against the front end of the fixing sleeve  223  and the metal socket of the second signal terminal  214  is inserted into and engaged with the an axial through hole  221  a in the fixing plate  221 . Alternatively, the first and second water-proof insulating annular plates  208  and  218  and the second and third insulating annular plates  211  and  217  may be saved. 
     Next, referring to  FIGS. 1, 2 and 6 , the inner electronic assembly  200  may be mounted into the through hole  108  in the cylindrical housing  100 . In this step, each of the serrated portions  230   a  of the metal sheets  230  mounted on the circuit board  222  may be inwardly bent in an arc between 0.1 π and 0.45 π, and preferably between 0.1 π and 0.25 π, between 0.15 π and 0.33 π or between 0.2 π and 0.45 π. Preferably, each of the serrated portions  230   a  of the metal sheets  230  may have substantially the same curvature radius as that of an annular surface of the through hole  108 . 
     Next, the inner electronic assembly  200  with its second signal terminal  214  is inserted into the through hole  108  in the cylindrical housing  100  in a direction from its nut portion  102  to its outer-thread portion  106 . Due to each of the second surge-protection metal annular plate  216 , second water-proof insulating annular plate  218  and fourth insulating annular plate  220  having an outer diameter less than an inner diameter of the through hole  108  in the first cylindrical space  1081 , the second surge-protection metal annular plate  216 , second water-proof insulating annular plate  218  and fourth insulating annular plate  220  may be moved in the through hole  108  from the first cylindrical space  1081  to the second cylindrical space  1082  and stop at the second cylindrical space  1082 . At this time, the metal sleeve  204  may be moved in the first cylindrical space  1081  and the serrated portions  230   a  of the two metal sheets  230  may surface-to-surface contact the annular surface of the through hole  108 . Next, the metal sleeve  204  may be tightly fitted with, riveted with or engaged with the first cylindrical space  1081  in the through hole  108 , and the second surge-protection metal annular plate  216  may be tightly fitted with, riveted with or engaged with the second cylindrical space  1082  in the through hole  108  such that the inner electronic assembly  200  may be fixed in the through hole  108  in the cylindrical housing  100 . 
     When the signal filter operates for signal processing, the first signal terminal  202  and the second signal terminal  214  may act as an input signal terminal and output signal terminal of the signal filter respectively or act as an output signal terminal and input signal terminal of the signal filter respectively. Taking an example of the first and second signal terminal  202  and  214  acting as an input signal terminal and output signal terminal of the signal filter respectively, when the signal filter operates for signal processing, lightning may occur to the signal filter such that a surge voltage between 1 kV and 8 kV or between 2 kV and 7 kV may be applied to the input signal terminal. At this time, a surge current may pass from the first signal terminal  202  to the first surge-protection metal annular plate  210  through the first radial air gap  2102  and then pass from the first surge-protection metal annular plate  210  to the electrical ground through the metal sleeve  204  and cylindrical housing  100 . Thereby, the signal filter may be protected from the surge current. Taking an example of the first and second signal terminal  202  and  214  acting as an output signal terminal and input signal terminal of the signal filter respectively, when lightning occurs to the signal filter, a surge current may pass from the second signal terminal  214  to the second surge-protection metal annular plate  216  through the second radial air gap  2162  and then pass from the second surge-protection metal annular plate  216  to the electrical ground through the cylindrical housing  100 . 
     When the surge current does not fully pass to the electrical ground through the first or second radial air gap  2102  or  2162 , the remaining surge current may be received by the capacitors  226  mounted on the circuit board  222  coupled to the metal sheets  230  via the patterned metal layers  2223 , wherein the metal sheets  230  surface-to-surface contact the annular surface of the through hole  108  in the cylindrical housing  100 . Thereby, the remaining surge current may pass from the capacitors  226  to the electrical ground through the patterned metal layers  2223 , metal sheets  230  and cylindrical housing  100 . Alternatively, the capacitors  226  may be saved. 
     The present invention provides a surge-protection metal annular plate at an input signal terminal with a radial air gap between an annular surface of an axial through hole in the surge-protection metal annular plate and a cylindrical surface of the input signal terminal being formed to protect a surge current. Comparing to the conventional lightning protection tube or lightning protection element, the signal transmission device in accordance with the present invention has a relatively low cost and small volume. 
     Second Embodiment 
     In the first embodiment, either of the first and second signal terminals  202  and  214  may act as an input signal terminal of the signal transmission device. For the purpose, the first discharging structure, i.e. the first radial air gap  2102 , and the second discharging structure, i.e. the second radial air gap  2162 , may be formed at the first and second signal terminals  202  and  214  respectively. However, in the second embodiment, one of the first and second signal terminals  202  and  214  may be regulated as an input signal terminal of the signal transmission device, and the other one of the first and second signal terminals  202  and  214  may be regulated as an output signal terminal of the signal transmission device. In this case, referring to  FIG. 7 , the first signal terminal  202  is regulated as an input signal terminal of the signal transmission device, and the second signal terminal  214  is regulated as an output signal terminal of the signal transmission device. For the purpose, the first discharging structure, i.e. the first radial air gap  2102 , may be formed at the first signal terminal  202  and the second discharging structure, i.e. the second radial air gap  2162 , may be saved. Alternatively, when first signal terminal  202  is regulated as an output signal terminal of the signal transmission device and the second signal terminal  214  is regulated as an input signal terminal of the signal transmission device, the second discharging structure, i.e. the second radial air gap  2162 , may be formed at the second signal terminal  214  and the first discharging structure, i.e. the first radial air gap  2102 , may be saved. The element, as illustrated in the second embodiment, indicated by the same reference number as that in the first embodiment may be referred to the illustration for that in the first embodiment. 
     Third Embodiment 
     In the first and second embodiments, each of the first and second discharging structures is one-stage discharging structure. Alternatively, each of the first and second discharging structures may be modified into a two-stage discharging structure as shown in  FIGS. 8 a , 8 b  and 8 c   . The element, as illustrated in the third embodiment, indicated by the same reference number as that in the first embodiment may be referred to the illustration for that in the first embodiment. The two-stage discharging structure modified from the first discharging structure includes the first surge-protection metal annular plate  210  and a third surge-protection metal annular plate  232  axially between the first surge-protection metal annular plate  210  and the first water-proof insulating annular plate  208 . The third surge-protection metal annular plate  232  may be made of materials as illustrated for composing the first surge-protection metal annular plate  210 . The third surge-protection metal annular plate  232  may have the same material as that of the first surge-protection metal annular plate  210 . Alternatively, the third surge-protection metal annular plate  232  may have different materials from that of the first surge-protection metal annular plate  210 . An axial through hole  232   a  in the third surge-protection metal annular plate  232  may have an inner diameter between 0.4 mm and 1.2 mm, and preferably between 0.4 mm and 0.9 mm or between 0.7 mm and 1.2 mm. 
     In a first case as illustrated in  FIG. 8 a   , the axial through hole  232   a  in the third surge-protection metal annular plate  232  may have the inner diameter substantially equal to that of the axial through hole  210   a  in the first surge-protection metal annular plate  210 . A radial air gap  2321  between an annular surface of the axial through hole  232   a  in the third surge-protection metal annular plate  232  and a cylindrical surface of the first signal terminal  202  may be substantially equal to the first radial air gap  2102 . 
     Alternatively, in a second case as illustrated in  FIG. 8 b   , the axial through hole  232   a  in the third surge-protection metal annular plate  232  may have the inner diameter less than that of the axial through hole  210   a  in the first surge-protection metal annular plate  210 . The difference between the inner diameter of the axial through hole  232   a  and that of the axial through hole  210   a  may be between 0.1 mm and 0.9 mm, and preferably between 0.1 mm and 0.3 mm, between 0.2 mm and 0.6 mm or between 0.3 mm and 0.9 mm. A third radial air gap  2322  between an annular surface of the axial through hole  232   a  in the third surge-protection metal annular plate  232  and a cylindrical surface of the first signal terminal  202  may be less than the first radial air gap  2102 . The difference between the first and third air gaps  2102  and  2322  may be between 0.05 mm and 0.45 mm, and preferably between 0.05 mm and 0.15 mm, between 0.1 mm and 0.3 mm or between 0.15 and 0.45 mm. 
     Alternatively, in a third case as illustrated in  FIG. 8 c   , the axial through hole  232   a  in the third surge-protection metal annular plate  232  may have the inner diameter greater than that of the axial through hole  210   a  in the first surge-protection metal annular plate  210 . The difference between the inner diameter of the axial through hole  232   a  and that of the axial through hole  210   a  may be between 0.1 mm and 0.9 mm, and preferably between 0.1 mm and 0.3 mm, between 0.2 mm and 0.6 mm or between 0.3 mm and 0.9 mm. A fourth radial air gap  2324  between an annular surface of the axial through hole  232   a  in the third surge-protection metal annular plate  232  and a cylindrical surface of the first signal terminal  202  may be greater than the first radial air gap  2102 . The difference between the first and fourth air gaps  2102  and  2324  may be between 0.05 mm and 0.45 mm, and preferably between 0.05 mm and 0.15 mm, between 0.1 mm and 0.3 mm or between 0.15 and 0.45 mm. 
     Alternatively, with regards to the second discharging structure, the third surge-protection metal annular plate  232  may be further arranged axially between the second surge-protection metal annular plate  216  and the second water-proof insulating annular plate  218 . The defined radial air gaps  2321 ,  2322  and  2323  may be applied to a radial air gap between the annular surface of the axial through hole  232   a  in the third surge-protection metal annular plate  232  and the second signal terminal  214 , which may be substantially equal to the second radial air gap  2162 , or greater than or less than the second radial air gap  2162  with a difference between the annular surface of the axial through hole  232   a  and the second signal terminal  214  being between 0.05 mm and 0.45 mm, and preferably between 0.05 mm and 0.15 mm, between 0.1 mm and 0.3 mm or between 0.15 and 0.45 mm. 
     Fourth Embodiment 
     Referring to  FIGS. 9 a -9 c   , with regards to the first discharging structure, the difference between the third and fourth embodiments is that the cylindrical housing  100  in accordance with the fourth embodiment may be provided with a fourth surge-protection metal annular plate  234 , instead of the third surge-protection metal annular plate  232  illustrated in the third embodiment, and the first surge-protection metal annular plate  210  in accordance with the fourth embodiment has no step, like the step  2101  shown in the third embodiment, having the second insulating annular plate  211  mounted thereto, but the second insulating annular plate  211  is mounted to a step  2342  of the fourth surge-protection metal annular plate  234 . The fourth surge-protection metal annular plate  234  may be integral with the cylindrical housing  100  as a single part and protrude from the annular surface of the through hole  108  in the cylindrical housing  100 . The fourth surge-protection metal annular plate  234  may have the same material as that of the cylindrical housing  100 . An axial through hole  234   a  in the fourth surge-protection metal annular plate  234  may have an inner diameter between 0.4 mm and 1.2 mm, and preferably between 0.4 mm and 0.9 mm or between 0.7 mm and 1.2 mm. 
     Referring to  FIGS. 9 a -9 c   , the difference between the step of assembling the inner electronic assembly  200  and the cylindrical housing  100  in accordance with the fourth embodiment and that of assembling the inner electronic assembly  200  and the cylindrical housing  100  in accordance with the first embodiment is that the second insulating annular plate  211 , in the fourth embodiment, is mounted to the step  2342  of the fourth surge-protection metal annular plate  234 , followed by the first signal terminal  202  being moved into the through hole  108  in the cylindrical housing  100  in a direction from the outer-thread portion  106  to the nut portion  102  such that the horizontally-extending portion of the first signal terminal  202  may pass sequentially through the axial through hole  211   a  in the second insulating annular plate  211  and the axial through hole  234   a  in the fourth surge-protection metal annular plate  234 . Next, the metal sleeve  204  having the first insulating annular plate  206 , first water-proof insulating annular plate  208  and first surge-protection metal annular plate  210  mounted into the through hole  204   a  therein may be moved into the through hole  108  in the cylindrical housing  100  in a direction from the nut portion  102  to the outer-thread portion  106  until the first surge-protection metal annular plate  210  and a rear end of the metal sleeve  204  contact the fourth surge-protection metal annular plate  234  such that the horizontally-extending portion of the first signal terminal  202  may pass sequentially through the axial through hole  210   a  in the first surge-protection metal annular plate  210 , the axial through hole  208   a  in the first water-proof insulating annular plate  208  and the axial through hole  206   a  in the first insulating annular plate  206 . 
     In a first case as illustrated in  FIG. 9 a   , the axial through hole  234   a  in the fourth surge-protection metal annular plate  234  may have the inner diameter substantially equal to that of the axial through hole  210   a  in the first surge-protection metal annular plate  210 . A radial air gap  2341  between an annular surface of the axial through hole  234   a  in the fourth surge-protection metal annular plate  234  and a cylindrical surface of the first signal terminal  202  may be substantially equal to the first radial air gap  2102 . 
     Alternatively, in a second case as illustrated in  FIG. 9 b   , the axial through hole  234   a  in the fourth surge-protection metal annular plate  234  may have the inner diameter less than that of the axial through hole  210   a  in the first surge-protection metal annular plate  210 . The difference between the inner diameter of the axial through hole  234   a  and that of the axial through hole  210   a  may be between 0.1 mm and 0.9 mm, and preferably between 0.1 mm and 0.3 mm, between 0.2 mm and 0.6 mm or between 0.3 mm and 0.9 mm. A fifth radial air gap  2344  between an annular surface of the axial through hole  234   a  in the fourth surge-protection metal annular plate  234  and a cylindrical surface of the first signal terminal  202  may be less than the first radial air gap  2102 . The difference between the first and fifth air gaps  2102  and  2344  may be between 0.05 mm and 0.45 mm, and preferably between 0.05 mm and 0.15 mm, between 0.1 mm and 0.3 mm or between 0.15 and 0.45 mm. 
     Alternatively, in a third case as illustrated in  FIG. 9 c   , the axial through hole  234   a  in the fourth surge-protection metal annular plate  234  may have the inner diameter greater than that of the axial through hole  210   a  in the first surge-protection metal annular plate  210 . The difference between the inner diameter of the axial through hole  234   a  and that of the axial through hole  210   a  may be between 0.1 mm and 0.9 mm, and preferably between 0.1 mm and 0.3 mm, between 0.2 mm and 0.6 mm or between 0.3 mm and 0.9 mm. A sixth radial air gap  2346  between an annular surface of the axial through hole  234   a  in the fourth surge-protection metal annular plate  234  and a cylindrical surface of the first signal terminal  202  may be greater than the first radial air gap  2102 . The difference between the first and sixth air gaps  2102  and  2346  may be between 0.05 mm and 0.45 mm, and preferably between 0.05 mm and 0.15 mm, between 0.1 mm and 0.3 mm or between 0.15 and 0.45 mm. 
     Fifth Embodiment 
     Referring to  FIG. 10 a   , the difference between the fourth and fifth embodiments is that the fourth surge-protection metal annular plate  234  in accordance with the fifth embodiment has no step, like the step  2342  shown in the fourth embodiment, having the second insulating annular plate  211  mounted thereto, but the first surge-protection metal annular plate  210  in accordance with the fifth embodiment has a step, like the step  2101  shown in the first embodiment, having the second insulating annular plate  211  mounted thereto. With regards to the step of assembling the inner electronic assembly  200  and the cylindrical housing  100 , after the second insulating annular plate  211  mounted to the step  2101  of the first surge-protection metal annular plate  210 , the metal sleeve  204  having the first insulating annular plate  206 , first water-proof insulating annular plate  208  and first surge-protection metal annular plate  210  mounted into the through hole  204   a  therein may be moved into the through hole  108  in the cylindrical housing  100  in a direction from the nut portion  102  to the outer-thread portion  106  until the first surge-protection metal annular plate  210 , a rear end of the metal sleeve  204  and the second insulating annular plate  211  contact the fourth surge-protection metal annular plate  234  such that the horizontally-extending portion of the first signal terminal  202  may pass sequentially through the axial through hole  211   a  in the second insulating annular plate  211 , the axial through hole  210   a  in the first surge-protection metal annular plate  210 , the axial through hole  208   a  in the first water-proof insulating annular plate  208  and the axial through hole  206   a  in the first insulating annular plate  206 . Also, the axial through hole  234   a  in the fourth surge-protection metal annular plate  234  may have the inner diameter substantially equal to, less than or greater than that of the axial through hole  210   a  in the first surge-protection metal annular plate  210 . A radial air gap  2347  between an annular surface of the axial through hole  234   a  in the fourth surge-protection metal annular plate  234  and a cylindrical surface of the first signal terminal  202  may be substantially equal to the first radial air gap  2102 , or less than or greater than the first radial air gap  2102  with a difference between the radial air gap  2347  and the first radial air gap  2102  being between 0.05 mm and 0.45 mm, and preferably between 0.05 mm and 0.15 mm, between 0.1 mm and 0.3 mm or between 0.15 and 0.45 mm. 
     Alternatively, the fourth surge-protection metal annular plate  234  having the step  2342  having the second insulating annular plate  211  mounted thereto, as illustrated in the fourth embodiment, may be incorporated into the fifth embodiment as shown in  FIG. 10 b   . Thereby, the two second insulating annular plates  211  may be arranged to stably maintain the radial air gap  2347  and the first radial air gap  2102  and to prevent the first and fourth surge-protection metal annular plates  210  and  234  from contacting the first signal terminal  202  or being too close to the first signal terminal  202 . 
     Sixth Embodiment 
     Alternatively, each of the through holes  210   a ,  216   a ,  232   a  and  234   a  in the respective first, second, third and fourth surge-protection metal annular plates  210 ,  216 ,  232  and  234  may have an annular surface with one or more steps. Referring to  FIG. 11  a, taking the first surge-protection metal annular plate  210  as an example, the annular surface of the through hole  210   a  in the first surge-protection metal annular plate  210  may have a step  236  with a front annular surface  2361  and a back annular surface  2362 , wherein the front annular surface  2361  has an inner diameter greater than that of the back annular surface  2362  with a difference between the inner diameter of the front annular surface  2361  and the inner diameter of the back annular surface  2362  being between 0.05 mm and 0.45 mm, and preferably between 0.05 mm and 0.15 mm, between 0.1 mm and 0.3 mm or between 0.15 and 0.45 mm. A seventh radial air gap  2363  between the front annular surface  2361  and the first signal terminal  202  may be greater than an eighth radial air gap  2364  between the back annular surface  2362  and the first signal terminal  202  with a difference between the seventh and eighth radial air gaps  2363  and  2364  being between 0.05 mm and 0.45 mm, and preferably between 0.05 mm and 0.15 mm, between 0.1 mm and 0.3 mm or between 0.15 and 0.45 mm, wherein the eighth radial air gap  2364  may be between 0.05 mm and 0.8 mm, and preferably between 0.1 mm and 0.6 mm or between 0.15 mm and 0.5 mm. As mentioned above, each of the third and fourth surge-protection metal annular plates  232  and  234  may have the step  236  with the front annular surface  2361  and the back annular surface  2362  to form the defined seventh radial air gap  2363  between the front annular surface  2361  and the first signal terminal  202  and the defined eighth radial air gap  2364  between the back annular surface  2362  and the first signal terminal  202 . Each of the second and third surge-protection metal annular plates  216  and  232  may have the step  236  with the front annular surface  2361  and the back annular surface  2362  to form the defined seventh radial air gap  2363  between the front annular surface  2361  and the second signal terminal  214  and the defined eighth radial air gap  2364  between the front annular surface  2362  and the second signal terminal  214 . 
     Alternatively, referring to  FIG. 11 b   , taking the first surge-protection metal annular plate  210  as an example, the front annular surface  2361  has an inner diameter less than that of the back annular surface  2362  with a difference between the inner diameter of the front annular surface  2361  and the inner diameter of the back annular surface  2362  being between 0.05 mm and 0.45 mm, and preferably between 0.05 mm and 0.15 mm, between 0.1 mm and 0.3 mm or between 0.15 and 0.45 mm. A ninth radial air gap  2365  between the front annular surface  2361  and the first signal terminal  202  may be less than a tenth radial air gap  2366  between the back annular surface  2362  and the first signal terminal  202  with a difference between the ninth and tenth radial air gaps  2365  and  2366  being between 0.05 mm and 0.45 mm, and preferably between 0.05 mm and 0.15 mm, between 0.1 mm and 0.3 mm or between 0.15 and 0.45 mm, wherein the tenth radial air gap  2366  may be between 0.05 mm and 0.8 mm, and preferably between 0.1 mm and 0.6 mm or between 0.15 mm and 0.5 mm. As mentioned above, each of the third and fourth surge-protection metal annular plates  232  and  234  may have the step  236  with the front annular surface  2361  and the back annular surface  2362  to form the defined ninth radial air gap  2365  between the front annular surface  2361  and the first signal terminal  202  and the defined tenth radial air gap  2366  between the back annular surface  2362  and the first signal terminal  202 . Each of the second and third surge-protection metal annular plates  216  and  232  may have the step  236  with the front annular surface  2361  and the back annular surface  2362  to form the defined ninth radial air gap  2365  between the front annular surface  2361  and the second signal terminal  214  and the defined tenth radial air gap  2366  between the front annular surface  2362  and the second signal terminal  214 . 
     Seventh Embodiment 
     Alternatively, each of the through holes  210   a ,  216   a ,  232   a  and  234   a  in the respective first, second, third and fourth surge-protection metal annular plates  210 ,  216 ,  232  and  234  may have a coned surface. Referring to  FIG. 12 a   , taking the first surge-protection metal annular plate  210  as an example, the through hole  210   a  in the first surge-protection metal annular plate  210  may have a coned surface  237  with a greatest inner radius R1 at a front end of the axial through hole  210   a  adjacent to the first water-proof insulating annular plate  208  and a smallest inner radius R2 at a rear end of the axial through hole  210   a  adjacent to the second insulating annular plate  211 . An axial distance H is defined between the greatest inner radius R1 and the smallest inner radius R2. An coned angle θ defined by tan −1  (R1−R2)/H may be between 2 and 45 degrees, and preferably between 2 and 15 degrees, between 5 and 30 degrees or between 8 and 45 degrees. A greatest radial air gap  2371 , i.e. eleventh radial air gap, between the coned surface  237  and the first signal terminal  202  is at a front end of the axial through hole  210   a  adjacent to the first water-proof insulating annular plate  208  and a smallest radial air gap  2372 , i.e. twelfth radial air gap, between the coned surface  237  and the first signal terminal  202  is at a rear end of the axial through hole  210   a  adjacent to the second insulating annular plate  211 . A difference between the eleventh and twelfth radial air gaps  2371  and  2372  being between 0.05 mm and 0.45 mm, and preferably between 0.05 mm and 0.15 mm, between 0.1 mm and 0.3 mm or between 0.15 and 0.45 mm. As mentioned above, each of the second, third and fourth surge-protection metal annular plates  216 ,  232  and  234  may have the coned surface  237  with the greatest inner radius R1 at a front end of the corresponding axial through hole  216   a ,  232   a  or  234   a  and the smallest inner radius R2 at a rear end of the corresponding axial through hole  216   a ,  232   a  or  234   a  so as to form the defined coned angle θ, the defined greatest radial air gap  2371 , i.e. eleventh radial air gap, between the coned surface  237  at the front end of the axial through hole  232   a  or  234   a  and the first signal terminal  202  or between the coned surface  237  at the front end of the axial through hole  216   a  or  232   a  and the second signal terminal  214 , the defined smallest radial air gap  2372 , i.e. twelfth radial air gap, between the coned surface  237  at the rear end of the axial through hole  232   a  or  234   a  and the first signal terminal  202  or between the coned surface  237  at the rear end of the axial through hole  216   a  or  232   a  and the second signal terminal  214 , and the defined difference between the defined eleventh and twelfth radial air gaps  2371  and  2372 . 
     Alternatively, referring to  FIG. 12 b   , each of the through holes  210   a ,  216   a ,  232   a  and  234   a  in the respective first, second, third and fourth surge-protection metal annular plates  210 ,  216 ,  232  and  234  may have a coned surface with the greatest inner radius R1 at the rear end of the corresponding axial through hole  210   a ,  216   a ,  232   a  or  234   a  and the smallest inner radius R2 at the front end of the corresponding axial through hole  210   a ,  216   a ,  232   a  or  234   a  so as to form the defined coned angle θ, the defined greatest radial air gap  2371 , i.e. eleventh radial air gap, between the coned surface  237  at the rear end of the axial through hole  210   a ,  232   a  or  234   a  and the first signal terminal  202  or between the coned surface  237  at the rear end of the axial through hole  216   a  or  232   a  and the second signal terminal  214 , the defined smallest radial air gap  2372 , i.e. twelfth radial air gap, between the coned surface  237  at the front end of the axial through hole  210   a ,  232   a  or  234   a  and the first signal terminal  202  or between the coned surface  237  at the front end of the axial through hole  216   a  or  232   a  and the second signal terminal  214 , and the defined difference between the defined eleventh and twelfth radial air gaps  2371  and  2372 . 
     Eighth Embodiment 
     Alternatively, each of the first, second, third and fourth surge-protection metal annular plates  210 ,  216 ,  232  and  234  may have one or more bumps protruding from an annular surface of the through holes  210   a ,  216   a ,  232   a  and  234   a  in the respective first, second, third and fourth surge-protection metal annular plates  210 ,  216 ,  232  and  234 . Referring to  FIG. 13 a   , taking the first surge-protection metal annular plate  210  as an example, the first surge-protection metal annular plate  210  may have an annular bump  2382  annularly protruding from an annular surface  2381  of the through hole  210   a  in the first surge-protection metal annular plate  210 . The annular bump  2382  has the smallest inner diameter less than an inner diameter of the annular surface  2381  of the through hole  210   a , wherein a difference between the smallest inner diameter of the annular bump  2382  and the inner diameter of the annular surface  2381  of the through hole  210   a  may be between 0.03 mm and 0.45 mm, and preferably between 0.03 mm and 0.1 mm, between 0.1 mm and 0.3 mm or between 0.15 and 0.45 mm. A thirteenth radial air gap  2391  between a tip of the annular bump  2382  and the first signal terminal  202  may be between 0.05 mm and 0.8 mm, and preferably between 0.1 mm and 0.6 mm or between 0.15 mm and 0.5 mm. The annular bump  2382  may have the surge current to be guided in focus such that the surge current may be efficiently guided. Alternatively, a plurality of the annular bump  2382  may be provided to annularly protrude in parallel from the annular surface  2381  of the through hole  210   a . In this case, the annular bump  2382  has a cross section shaped like a triangle, but may have another cross section shaped like a rectangle or a semi-circle. As mentioned above, each of the third and fourth surge-protection metal annular plates  232  and  234  may have the annular bump  2382 , or a plurality of the annular bump  2382 , annularly protruding from, or annularly protruding in parallel from, an annular surface of the corresponding through hole  232   a  or  234   a  so as to form the defined thirteenth radial air gap  2391  between the tip of the annular bump  2382  and the first signal terminal  202 . Each of the second and third surge-protection metal annular plates  216  and  232  may have the annular bump  2382 , or a plurality of the annular bump  2382 , annularly protruding from, or annularly protruding in parallel from, an annular surface of the corresponding through hole  216   a  or  232   a  so as to form the defined thirteenth radial air gap between the tip of the annular bump  2382  and the second signal terminal  214 . 
     Alternatively, referring to  FIG. 13 b   , taking the first surge-protection metal annular plate  210  as an example, the first surge-protection metal annular plate  210  may have multiple conical bumps  2383  protruding from an annular surface  238  of the through hole  210   a  in the first surge-protection metal annular plate  210 , wherein the conical bumps  2383  may be arranged in a ring around the horizontally-extending portion of the first signal terminal  202 . A distance s between tips of neighboring two of the conical bumps  2383  may be between 0.03 mm and 0.3 mm, and preferably between 0.03 mm and 0.1 mm, 0.05 and 0.15 mm or between 0.1 and 0.3 mm. A fourteenth radial air gap  2392  between a tip of one of the conical bumps  2383  and the first signal terminal  202  may be between 0.05 mm and 0.8 mm, and preferably between 0.1 mm and 0.6 mm or between 0.15 mm and 0.5 mm. Alternatively, the conical bumps  2383  may be arranged in multiple parallel rings around the horizontally-extending portion of the first signal terminal  202 . In this case, each of the conical bumps  2383  has a cross section shaped like a triangle, but may have another cross section shaped like a rectangle or a semi-circle. As mentioned above, each of the third and fourth surge-protection metal annular plates  232  and  234  may have the conical bumps  2383  protruding from an annular surface of the corresponding through hole  232   a  or  234   a  in a ring or multiple parallel rings around the first signal terminal  202  so as to form the defined fourteenth radial air gap  2392  between the tip of one of the conical bumps  2383  and the first signal terminal  202  and the defined distance s between tips of neighboring two of the conical bumps  2383 . Each of the second and third surge-protection metal annular plates  216  and  232  may have the conical bumps  2383  protruding from an annular surface of the corresponding through hole  216   a  or  232   a  in a ring or multiple parallel rings around the second signal terminal  214  so as to form the defined fourteenth radial air gap  2392  between the tip of one of the conical bumps  2383  and the second signal terminal  214  and the defined distance s between tips of neighboring two of the conical bumps  2383 . 
     Ninth Embodiment 
     Alternatively, the second and third insulating annular plates  211  and  217  mounted respectively to the steps  2101  and  2161  of the first and second surge-protection metal annular plates  210  and  216  may be replaced with first and second insulating tubes  311  and  317  respectively as shown in  FIG. 14 . The first and second insulating tubes  311  and  317  may be made of a material composing the second and third insulating annular plates  211  and  217 . The order of assembling the first and second insulating tubes  311  and  317  for the inner electronic assembly  200  may be different from that of assembling the second and third insulating annular plates  211  and  217  for the inner electronic assembly  200 . Referring to  FIG. 14 , taking the first surge-protection metal annular plate  210  as an example, with regard to the first discharging structure, the first insulating tube  311  may be sleeved in position on the horizontally-extending portion of the first signal terminal  202 , and then the horizontally-extending portion of the first signal terminal  202  may be inserted sequentially into the axial through hole  210   a  in the first surge-protection metal annular plate  210 , the axial through hole  208   a  in the first water-proof insulating annular plate  208 , and the axial through hole  206   a  in the first insulating annular plate  206  after the first insulating annular plate  206 , first water-proof insulating annular plate  208  and first surge-protection metal annular plate  210  are mounted into the through hole  204   a  in the metal sleeve  204  until the first surge-protection metal annular plate  210  has the step  2101  contacting the first insulating tube  311 . Thereby, the first radial air gap  2101  may be tightly sealed by the first insulating tube  311  and first water-proof insulating annular plate  208 . With regard to the second discharging structure, after the second signal terminal  214  has the metal wire or rod to be inserted sequentially into the axial through hole  220   a  in the fourth insulating annular plate  220 , the axial through hole  218   a  in the second water-proof insulating annular plate  218  and the axial through hole  216   a  in the second surge-protection metal annular plate  216  in position, the second insulating tube  317  is moved to be sleeved on the second signal terminal  214  until the second insulating tube  317  contacts the step  2161  of the second surge-protection metal annular plate  216 . Thereby, the second radial air gap  2162  may be tightly sealed by the second insulating tube  317  and second water-proof insulating annular plate  218 . 
     For the third embodiment as shown in  FIGS. 8 a -8 c   , with regard to the first discharging structure, the second insulating annular plate  211  may be replaced with the first insulating tube  311  to be sleeved on the horizontally-extending portion of the first signal terminal  202  and contact the step  2101  of the first surge-protection metal annular plate  210  such that the adjacent radial air gaps  2102  and  2321  as illustrated in  FIG. 8 a   , the adjacent radial air gaps  2102  and  2322  as illustrated in  FIG. 8 b    and the adjacent radial air gaps  2102  and  2324  as illustrated in  FIG. 8 c    may be tightly sealed by the first insulating tube  311  and first water-proof insulating annular plate  208 . With regard to the second discharging structure, the third insulating annular plate  217  may be replaced with the second insulating tube  317  to be sleeved on the second signal terminal  214  and contact the step  2161  of the second surge-protection metal annular plate  216  such that the adjacent radial air gaps  2162  and  2321 , the adjacent radial air gaps  2162  and  2322  and the adjacent radial air gaps  2162  and  2324  as illustrated in  FIG. 8 c    may be tightly sealed by the second insulating tube  317  and second water-proof insulating annular plate  218 . 
     For the fourth embodiment as shown in  FIGS. 9 a -9 c   , with regard to the first discharging structure, the second insulating annular plate  211  may be replaced with the first insulating tube  311  to be sleeved on the horizontally-extending portion of the first signal terminal  202  and contact the step  2342  of the fourth surge-protection metal annular plate  234  such that the adjacent radial air gaps  2102  and  2341  as illustrated in  FIG. 9 a   , the adjacent radial air gaps  2102  and  2344  as illustrated in  FIG. 9 b    and the adjacent radial air gaps  2102  and  2346  as illustrated in  FIG. 9 c    may be tightly sealed by the first insulating tube  311  and first water-proof insulating annular plate  208 . 
     For the fifth embodiment as shown in  FIG. 10 a   , with regard to the first discharging structure, the second insulating annular plate  211  may be replaced with the first insulating tube  311  to be sleeved on the horizontally-extending portion of the first signal terminal  202  and contact the step  2101  of the first surge-protection metal annular plate  210  such that the radial air gap  2102  may be tightly sealed by the first insulating tube  311  and first water-proof insulating annular plate  208 . Referring to  FIG. 10 b   , each of the second insulating annular plates  211  may be replaced with the first insulating tube  311  to be sleeved on the horizontally-extending portion of the first signal terminal  202 . The front one of the first insulating tubes  311  may contact the step  2101  of the first surge-protection metal annular plate  210  such that the radial air gap  2102  may be tightly sealed by the front one of the first insulating tubes  311  and first water-proof insulating annular plate  208 . The front one of the first insulating tubes  311  may contact a front side of the fourth surge-protection metal annular plate  234  to seal a front end of the radial air gap  2347 . The rear one of the first insulating tubes  311  may contact the step  2342  of the fourth surge-protection metal annular plate  234  such that the radial air gap  2347  may be tightly sealed by the front and back ones of the first insulating tubes  311 . 
     Alternatively, for the above embodiments that the second or third insulating annular plate  211  or  217  is replaced with the first insulating tube  311  or  317 , each of the through holes  210   a ,  216   a ,  232   a  and  234   a  in the respective first, second, third and fourth surge-protection metal annular plates  210 ,  216 ,  232  and  234  may have an annular surface with the step  236  as illustrated in  FIGS. 11 a  and 11 b    in the sixth embodiment, with the coned surface  237  as illustrated in  FIGS. 12 a  and 12 b    in the seventh embodiment or with one or more bumps  2382  or  2383  as illustrated in  FIGS. 13 a  and 13 b    in the eighth embodiment. 
     Tenth Embodiment 
     Alternatively, referring to  FIG. 15 , an annular grove  105  may be formed from an annular surface of the through hole  108  and adjacent to the nut portion  102  of the cylindrical housing  100 . The annular grove  105  may accommodate a water-proof rubber ring  107  such that the electronic device may have enhanced water proof. 
     The scope of protection is limited solely by the claims, and such scope is intended and should be interpreted to be as broad as is consistent with the ordinary meaning of the language that is used in the claims when interpreted in light of this specification and the prosecution history that follows, and to encompass all structural and functional equivalents thereof.