Abstract:
A charger cradle used for charging a battery of a portable electrical device. An electromagnetic device is disposed within the charger cradle proximate a metal piece of the cordless handset when the cordless handset is placed in a receiving portion of the charger cradle. The electromagnetic device generates a magnetic field when charge contacts of the charger cradle and the cordless handset are in contact and a charging current is conducted from a charging circuit of the charger cradle. The magnetic field attracts the metal piece of the cordless handset and securely holds the cordless handset into the receiving portion while the charging circuit charges the battery of the cordless handset.

Description:
BACKGROUND 
       [0001]    I. Field of the Invention 
         [0002]    The invention relates generally to a charger for a cordless handset, and more particularly, to a charger that can securely hold the cordless handset while the cordless handset is in a charge position. 
         [0003]    II. Background of the Invention 
         [0004]    As known in the art, the electrical power used by cordless handsets is from chargeable batteries installed therein. As the amount of electricity provided by the chargeable batteries is limited, the chargeable batteries need be charged after being used for a period of time. Conventionally, while being charged, a cordless handset  10  is placed in a charger cradle  20 , as shown in  FIG. 1 . Typically, the cradle  20  includes two contacts  21  for electrically connecting with two charger contacts  11  of the cordless handset. It is important to ensure that the handset&#39;s charger contacts have a proper contact at all times with the cradle contacts, otherwise the battery will not be charged properly. Also, the handset should be secured in the charger cradle and arranged so that it does not drop out of the charger cradle easily. For example, the handset should be sufficiently secure such that vibrators used as a ringing indicator should not vibrate the handset out of the cradle. 
         [0005]    To meet the requirements mentioned above, the mechanical design of the handset is very important and cannot be underestimated. However, the design is limited by the industrial design of the handset and the charger cradle. With new slim designs for the handset and cordless telephones, designing a very stable handset and charger becomes a challenge and can limit the implementation of new designs. 
         [0006]    Another problem that is common in cordless telephones is that the charger cradle is designed in a way to securely hold the handset in place. Such design, however, reduces the ventilation of air around the battery area and, during charging, causes the battery and handset to warm up and, in some cases, became too hot. To solve the problem, in some designs, the charger cradle does not completely surround the handset or some mechanical solutions (such as clips) are used to secure the handset in the charger cradle. However, such methods are more complicated and have other limitations. 
         [0007]    Accordingly, it would be desirable to have a charger cradle that can secure the handset and prevent the handset from getting hot while charging without changing the industrial design. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    Certain embodiments of the invention provide a charger cradle for charging a battery of a cordless handset that securely holds the cordless handset while charging the cordless handset without changing the industrial design of the charger cradles. 
         [0009]    In some embodiments of the invention, a charger cradle includes a receiving portion for receiving and holding a cordless handset therein, the receiving portion having an electrical contact for contacting with a charge contact of the cordless handset, and a charging circuit for providing a charging current to the cordless handset through the contact of the electrical contact of the receiving portion and the charge contact of the cordless handset. The receiving portion includes a holding device that generates a magnetic field when the electrical contact of the receiving portion and the charge contact of the cordless handset contact with each other to attract the cordless handset securely into the receiving portion. 
         [0010]    Some embodiments of the invention provide a charger cradle for charging a battery of a cordless handset. The charger cradle includes a receiving portion for receiving the cordless handset, wherein the receiving portion includes an electrical contact, the electrical contact is located at a position corresponding to a charge contact of the cordless handset so that the electrical contact of the receiving portion contacts with the charge contact of the cordless handset, and a charging circuit for providing a charging current to charge the battery of the cordless handset when the electrical contact of the receiving portion is in contact with the charge contact of the cordless handset. The charger cradle further includes an electromagnetic device proximate to the receiving portion that generates a magnetic field when the electrical contact of the receiving portion and the charge contact of the cordless handset are in touch and the charging current is conducted for attracting a metal piece of the cordless handset into the receiving portion. 
         [0011]    In some embodiments of the invention, the electromagnetic/holding device includes a ferrite core and coil windings surrounding the ferrite core that generates the magnetic field when a current flows there through. 
         [0012]    In some embodiments of the invention, the electromagnetic/holding device includes a permanent magnet for attract the cordless handset into the receiving portion, and the permanent magnet is connected to the charging circuit by wires. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a schematic diagram showing a prior art charger cradle for holding a cordless telephone. 
           [0014]      FIG. 2  is a schematic diagram showing a charger cradle for holding a cordless telephone in accordance with the present invention. 
           [0015]      FIG. 3  illustrates another embodiment of the charger cradle in accordance with the present invention. 
           [0016]      FIG. 4  illustrates another embodiment of the charger cradle in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    Embodiments of the present invention provide a secure way for holding a cordless handset on a charger cradle while being charged. In accordance with the present invention, a magnetic field is generated by means of a charging current that attracts a metal part of the cordless handset to hold the cordless handset in place. 
         [0018]    An exemplary embodiment of a charger cradle for holding a wireless handset in accordance with the present invention is illustrated in  FIG. 2 . In this figure, like elements shown in  FIG. 1  are marked by similar reference numerals. As illustrated, charger cradle  20  includes a receiving part for receiving a cordless handset  10  that has charge contacts  21  therein for contacting with charge contacts  11  of cordless handset  10 . Charge cradle  20  contains an electromagnetic solenoid device including a ferrite core  22  surrounded with coil windings  23  and a charging circuit  24 . Cordless handset  10  is a conventional device that includes a battery  13  and a metal piece  12  on the bottom thereof proximate charge contacts  11 . 
         [0019]    According to a preferred embodiment of the invention, as soon as handset  10  makes contacts with charge cradle  20 , a current flows through the electromagnetic solenoid device that causes a magnetic field to attract metal piece  12  of cordless handset  10  in charge cradle  20 . The attraction of cordless handset  10  can securely holds cordless handset  10  in charge cradle  20  during a charging process. To remove cordless handset  10 , a user need only apply enough force to detach cordless handset  10 . As soon as one of the charge contacts is detached, the electromagnetic device loses its magnetic field so that cordless handset  10  can be removed easily. 
         [0020]    The mechanical and electrical design of the electromagnetic device is simple and the magnetic field generated by the electromagnetic solenoid device is adjustable depending on factors such as a charge current, number of windings around the ferrite core and the core material. Therefore, embodiments of the invention need not require extra current. The invention takes advantage of the current flow from charging circuit  24  to cordless handset  10  used for charging battery  13  of cordless handset  10  to generate the magnetic field to hold cordless handset  10  in place. When cordless handset  10  is not placed in charge cradle  20 , the magnetic field is automatically removed. 
         [0021]    The design of the invention also improves heat dissipation. As the invention uses the magnetic field to secure cordless handset  10 , it is not necessary that charge cradle  20  completely surround cordless handset  10  whereby heat generated during charging can be easily dissipated. 
         [0022]    The electromagnetic solenoid device used in charger cradle  20  includes an electric conductor  23  that is wound N times about a magnetic member, e.g., ferrite core  22 . In such device, a magnetic field is generated whenever a current is supplied to conductor  23  given that the current travels in a closed loop. Since a magnetic field is present, a mechanical force is induced and it can pull or push another magnetic material in a linear motion. 
         [0023]    There are two ways to calculate the force of the magnet: one of them depends on the magnetic field of the device, and the other one uses an energy balance method. To determine which method to use, it is necessary to know the path of the magnetic flux. Magnetic flux always travels in a closed path in a core or across air gaps; for example, the magnetic flux may go from north to south poles in a solenoid system that does not have a core. However, a core is usually preferred in most systems since it concentrates the magnetic flux and increases the magnetic force by reducing the air gap or air resistance in between the poles. In such a case, the magnetic force is determined by the strength of the magnetic field. In systems where the core of a more complicated shape and the air gap is small, the energy method should be used. 
         [0024]    The magnetic flux (B x ) generated by the electromagnetic solenoid device can be calculated as follows: 
         [0025]    For a rectangular shaped core, 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           B 
                           x 
                         
                         = 
                         
                           
                             B 
                             π 
                           
                           [ 
                           
                             
                               tan 
                               
                                 - 
                                 1 
                               
                             
                             ( 
                             
                               
                                 
                                   dtl 
                                   tw 
                                 
                                  
                                 
                                   
                                     
                                       
                                         t 
                                         2 
                                       
                                       + 
                                       
                                         w 
                                         2 
                                       
                                       + 
                                       
                                         
                                           ( 
                                           
                                             d 
                                             + 
                                             l 
                                           
                                           ) 
                                         
                                         2 
                                       
                                     
                                     ) 
                                   
                                 
                               
                               - 
                               
                                 
                                   tan 
                                   
                                     - 
                                     1 
                                   
                                 
                                  
                                 
                                   ( 
                                   
                                     
                                       d 
                                       tw 
                                     
                                      
                                     
                                       
                                         
                                           t 
                                           2 
                                         
                                         + 
                                         
                                           w 
                                           2 
                                         
                                         + 
                                         
                                           d 
                                           2 
                                         
                                       
                                     
                                   
                                   ) 
                                 
                               
                             
                             ] 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0026]    where 
         [0027]    B x =Resultant flux between the solenoid electromagnetic device and steel [Tesla] 
         [0028]    B=Magnetic flux density of the solenoid electromagnetic device [Tesla] 
         [0029]    d=Distance between the solenoid electromagnetic device and the steel [m] 
         [0030]    2t=height or thickness of the core [m] 
         [0031]    2l=length of the core [m] 
         [0032]    2w=width of the core [m] 
         [0033]    in which 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           B 
                           = 
                           
                             k 
                              
                             
                                 
                             
                              
                             
                               μ 
                               0 
                             
                              
                             nI 
                           
                         
                       
                     
                   
                    
                   
                     
 
                   
                    
                   where 
                    
                   
                     
 
                   
                    
                   
                     
                       
                         
                             
                            
                           
                             
                               μ 
                               0 
                             
                             = 
                             
                               
                                 4 
                                  
                                 
                                     
                                 
                                  
                                 
                                   π 
                                    
                                   
                                       
                                   
                                   · 
                                   
                                     10 
                                     
                                       - 
                                       7 
                                     
                                   
                                 
                               
                               = 
                               
                                 Permeability 
                                  
                                 
                                     
                                 
                                  
                                 of 
                                  
                                 
                                     
                                 
                                  
                                 the 
                                  
                                 
                                     
                                 
                                  
                                 medium 
                               
                             
                           
                         
                       
                       
                         
                             
                            
                           
                             [ 
                             
                               H 
                                
                               
                                 / 
                               
                                
                               m 
                             
                             ] 
                           
                         
                       
                     
                     
                       
                         
                             
                            
                           
                             k 
                             = 
                             
                               Relative 
                                
                               
                                   
                               
                                
                               permeability 
                             
                           
                         
                       
                       
                         
                             
                            
                           
                             [ 
                             dimensionless 
                             ] 
                           
                         
                       
                     
                     
                       
                         
                             
                            
                           
                             
                               
                                 
                                   n 
                                   = 
                                   
                                     N 
                                     l 
                                   
                                 
                               
                             
                             
                               
                                 
                                   = 
                                   
                                     Turn 
                                      
                                     
                                         
                                     
                                      
                                     density 
                                   
                                 
                               
                             
                             
                               
                                 
                                   = 
                                   
                                     Number 
                                      
                                     
                                         
                                     
                                      
                                     of 
                                      
                                     
                                         
                                     
                                      
                                     turns 
                                      
                                     
                                         
                                     
                                      
                                     per 
                                      
                                     
                                         
                                     
                                      
                                     meter 
                                   
                                 
                               
                             
                           
                            
                           
                               
                           
                         
                       
                       
                         
                             
                            
                           
                             [ 
                             
                               turns 
                                
                               
                                 / 
                               
                                
                               m 
                             
                             ] 
                           
                         
                       
                     
                     
                       
                         
                             
                            
                           
                             I 
                             = 
                             Current 
                           
                         
                       
                       
                         
                             
                            
                           
                             [ 
                             Amp 
                             ] 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0034]    For a cylindrical core, the magnetic flux B x  can be expressed as 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           B 
                           x 
                         
                         = 
                         
                           
                             B 
                             2 
                           
                           [ 
                           
                             
                               
                                 d 
                                 + 
                                 l 
                               
                               
                                 
                                   
                                     
                                       ( 
                                       
                                         d 
                                         + 
                                         l 
                                       
                                       ) 
                                     
                                     2 
                                   
                                   + 
                                   
                                     r 
                                     2 
                                   
                                 
                               
                             
                             - 
                             
                               d 
                               
                                 
                                   
                                     d 
                                     2 
                                   
                                   + 
                                   
                                     r 
                                     2 
                                   
                                 
                               
                             
                           
                           ] 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0035]    B x =Resultant flux between the solenoid electromagnetic device and steel [Tesla] 
         [0036]    B=Magnetic flux density of the solenoid electromagnetic device, obtained from equation (2) [Tesla] 
         [0037]    d=Distance between the solenoid electromagnetic device and the steel [mm] 
         [0038]    I=Length of the core [mm] 
         [0039]    r=Radius of the core [mm] 
         [0040]    After B x  is determined, the mechanical force can be obtained from the following expression: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         F 
                         = 
                         
                           0.577 
                            
                           
                             B 
                             x 
                             2 
                           
                            
                           A 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
         [0041]    where 
         [0042]    F=Force [lbs] 
         [0043]    B x =Resultant flux calculated using the equations above [Gauss](×10 −4  Tesla) 
         [0044]    A=Area of the poles [in 2 ] 
         [0045]    From the equations (1)-(4) above, the number of coils, the current applied, the length, material and geometry of the core, the distance between the core and the object are variables of interests in the electromagnetic device. A change in each of the variables may also lead to an increase or decrease in the magnetic force. The variables in the solenoid magnetic field and force equations are the design parameters of the system and the relationship between force and the variables will be determined to optimize the design. 
         [0046]    According to the equations (1)-(4), the relationships between the force and the variables can be expressed as follows: 
         [0000]      F∝I 2    
       The force is proportional to the square of the current, so by doubling the current, the force will be increase by four times. 
       [0047]      F∝N 2    
       The force is proportional to the square of the number of coils, which is similar to the relationship of force and current. 
       [0048]    
       
         
           
             F 
             ∝ 
             
               1 
               d 
             
           
         
       
     
         [0000]    The force is proportional to the inverse of the distance between the core and the ferrite object. Therefore one wants to double the force; the core must be moved closer to the ferrite object. 
         [0000]    
       
         
           
             F 
             ∝ 
             
               1 
               l 
             
           
         
       
     
         [0000]    The force is inversely proportional to the length of the core. If one wants to double the force, the length needs to be shortened by a half. 
         [0000]      F∝A 
         [0000]    The force behaves linearly with the surface area of the core. Doubling the force will require doubling the surface area. 
         [0049]    Using the above relationships, one can roughly approximate a minimum magnetic force that is required to hold a cordless handset in place while charging if a given set of parameters is known. 
         [0050]    According to the invention, it is estimated that 50 gram-force of attraction is sufficient to provide a tactile feeling when placing cordless handset  10  to charger cradle  20  and to reduce the chance of tipping over of cordless handset  10  when resting on cradle  20 . Using the following design parameters, the measured force is found to be 76 gram-force. Such result is effective to securely hold cordless handset  10  in charger cradle  20 . 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Number of Coils 
                 700 
               
               
                   
                 Current 
                 500 mAmps 
               
               
                   
                 Separation 
                 0.70 mm 
               
               
                   
                 Geometry of Core 
                 Diameter = 0.375 in 
               
               
                   
                   
                 Length = 1.5 in 
               
               
                   
                 Material of Core 
                 1018 low carbon steel 
               
               
                   
                 Relative 
                 2000 maximum 
               
               
                   
                 Permeability 
               
               
                   
                   
               
             
          
         
       
     
         [0051]      FIGS. 3 and 4  illustrate various embodiments of charge cradle  20  in accordance with the present invention. In  FIG. 3 , the device includes permanent magnets  25  each held by combination charge contact magnet holder  21  a that is connected with charging circuit  24  via a wire  26 . Permanent magnets may be used to augment the electromagnetic embodiment of  FIG. 2  (or  FIG. 4 ) or may be used separately from such embodiments.  FIG. 4  shows that a circuit for coil  27  is separated from charging circuit  24 . By applying an additional circuit, the coil current has more flexibility and is independent from the charging current that is usually in the range from 100 to 300 mA. Therefore, the current for coil  27  can go beyond the charging current, resulting in a significant increase in the mechanical force. 
         [0052]    The foregoing disclosure of the preferred embodiments of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many variations and modifications of the embodiments described herein will be apparent to one of ordinary skill in the art in light of the above disclosure. The scope of the invention is to be defined only by the claims appended hereto, and by their equivalents. 
         [0053]    Further, in describing representative embodiments of the present invention, the specification may have presented the method and/or process of the present invention as a particular sequence of steps. However, to the extent that the method or process does not rely on the particular order of steps set forth herein, the method or process should not be limited to the particular sequence of steps described. As one of ordinary skill in the art would appreciate, other sequences of steps may be possible. Therefore, the particular order of the steps set forth in the specification should not be construed as limitations on the claims. In addition, the claims directed to the method and/or process of the present invention should not be limited to the performance of their steps in the order written, and one skilled in the art can readily appreciate that the sequences may be varied and still remain within the spirit and scope of the present invention.