Patent Publication Number: US-6665415-B1

Title: Loudspeaker overcurrent protection

Description:
FIELD OF THE INVENTION 
     This invention relates to transducers. It is disclosed in the context of an electrodynamic loudspeaker, but is believed to have utility in other applications as well. 
     BACKGROUND OF THE INVENTION 
     Schemes for the protection of loudspeaker voice coils from excessive current are known. There are, for example, the schemes described in U.S. Pat. Nos. 3,544,720; 3,925,708; 3,959,736; 5,224,169; and, 5,847,610. There are also the loudspeakers disclosed in U.S. Pat. No. 5,838,809. This listing is not intended as a representation that a thorough search of the prior art has been conducted or that no more pertinent art than that listed above exists, and no such representation should be inferred. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may best be understood by referring to the following detailed description and accompanying single drawing FIGURE which illustrates the invention. The single FIGURE illustrates a fragmentary cross-section through a loudspeaker constructed according to the invention. 
    
    
     DISCLOSURE OF THE INVENTION 
     According to the invention, a transducer includes a motor assembly for providing a magnetic field across an air gap, a voice coil supported in the air gap, a diaphragm coupled to the voice coil for reciprocation with the voice coil, and conductors for coupling opposite ends of the voice coil to a source of alternating current to cause the voice coil to reciprocate in the air gap. At least some portion of the length of at least one of the conductors is fusible when exposed to a direct current having a magnitude greater than the maximum desired magnitude of direct current to which it is desired to expose the voice coil. 
     Illustratively according to the invention, the said portion of the length of the at least one conductor is formed from material having resistance per unit length which results in said portion of the length of the at least one conductor being fusible when exposed to direct current having magnitude greater than the maximum desired magnitude. 
     Illustratively according to the invention, the length of one of the conductors is fusible when exposed to direct current having magnitude greater than the maximum desired magnitude. 
     Illustratively according to this aspect of the invention, the length of one of the conductors is formed from material having resistance per unit length which results in said one conductor being fusible when exposed to direct current having magnitude greater than the maximum desired magnitude. 
     Additionally illustratively according to the invention, the lengths of both of the conductors are fusible when exposed to direct currents having magnitudes greater than the maximum desired magnitude. 
     Illustratively according to this aspect of the invention, the lengths of both of the conductors are formed from materials having resistances per unit length which results in the conductors being fusible when exposed to direct currents having magnitudes greater than the maximum desired magnitude. 
     Further illustratively according to the invention, at least some portion of the lengths of both of the conductors are fusible when exposed to direct current having magnitude greater than the maximum desired magnitude. 
     Illustratively according to this aspect of the invention, the said portions of the lengths of both of the conductors are formed from materials having resistances per unit length which result in said portions of the lengths of both of the conductors being fusible when exposed to direct currents having magnitudes greater than the maximum desired magnitude. 
     Detailed Descriptions of Illustrative Embodiments 
     Referring now to FIG. 1, a loudspeaker  9  includes a supporting frame  10  and a motor assembly. The illustrated motor assembly includes a backplate/center pole  12 , a permanent magnet  13 , and a front plate  14  providing a substantially uniform magnetic field across an air gap  15 . A voice coil former  16  supports a voice coil  17  in the magnetic field. Current related to the program material to be transduced by the loudspeaker  9  drives the voice coil  17 , causing it to reciprocate axially in the air gap  15  in a known manner. A cone  18  attached at its apex to an end of the coil former  16  lying outside the motor assembly is coupled by a surround  19  at its outer perimeter to the frame  10 . A spider  20  is coupled at its outer perimeter to the frame  10 . The spider  20  includes a central opening  22  to which the voice coil former  16  is attached. The suspension including the surround  19  and spider  20  constrains the voice coil  17  to reciprocate axially in the air gap  15 . 
     A typical, although by no means the only, mechanism for completing the electrical connection between the loudspeaker terminals  24 ,  25  and the voice coil wires  26 ,  27  is illustrated in the FIGURE. The voice coil wires  26 ,  27  are dressed against the side of the coil former  16 , and pass through central opening  22  and the intersection of the coil former  16  and the apex of the cone  18 . Wires  26 ,  27  are then dressed across the face  32  of the cone  18  to the points  28 ,  29  on the face of the cone  18  where they are connected to the flexible conductors  30 ,  31 . Connections  28 ,  29  are made by any of a number of available techniques. The coil wires  26 ,  27  illustratively are fixed to the face  32  of the cone  18  with (an) electrically non-conductive adhesive(s). 
     When the voice coil  17  is exposed to direct current, for example, accidentally during connection of the loudspeaker into the output circuit of an audio amplifier  40  or during a malfunction of such an audio amplifier  40 , voice coil  17  can be damaged sufficiently that a short circuit develops in voice coil  17 . A short circuited voice coil can become further overheated, with adverse consequences. Some audio amplifiers  40  are not capable of handling short circuited voice coils or other problems which develop as results of the short circuiting of voice coils. As noted above, in most loudspeakers, the ends  26 ,  27  of the voice coil  17  are normally coupled through connections  28 ,  29  to flexible conductors  30 ,  31 , and through these conductors  30 ,  31  to the audio amplifier  40  output circuitry. It is quite common to use so-called litzendraht, or litz, wire, a highly flexible, woven wire for conductors  30 ,  31 . 
     The present invention employs litz, or other suitable type wire, which performs normally in the presence of alternating current, such as normally appears across the output terminals of an audio amplifier  40 , but opens when exposed to a direct current having greater than a certain arbitrary magnitude. Under normal conditions, therefore, conductors  30 ,  31 , behave as any prior art conductors in this position in a loudspeaker. However, when exposed to a direct current having a magnitude greater than the established maximum, the conductors  30 ,  31  melt over, for example, five to ten seconds, opening the circuit to the voice coil  17  and reducing the likelihood of any adverse consequences of the exposure to direct current that the loudspeaker  9  or the audio amplifier  40  driving it might otherwise experience. This slow melting characteristic is achieved in the illustrative embodiment by choosing a material for conductors  30 ,  31  which has sufficiently high resistance per unit length that it is heated to its melting temperature by a direct current having a magnitude equal to or greater than the arbitrarily established maximum which is sustained over, in this example, five to ten seconds. This can be achieved in any of a number of ways, for example, by choosing (an) appropriate alloy(s) for some portion or all of the length(s) of one or both of conductors  30 ,  31 . 
     For example, in a loudspeaker  9  having a voice coil  17  with a nominal impedance of  4 Ω, it may be desirable to provide at least one conductor  30 ,  31  having at least a portion of its length fusible under sustained direct current greater than 3.8 amperes.