Patent Publication Number: US-7915984-B2

Title: Starter solenoid switch with improved arrangement of resistor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from Japanese Patent Applications No. 2008-39233, filed on Feb. 20, 2008, and No. 2008-325261, filed on Dec. 22, 2008, the contents of which are hereby incorporated by reference into this application. 
     BACKGROUND OF THE INVENTION 
     1. Technical Field of the Invention 
     The present invention relates generally to solenoid switches (or electromagnetic switches) for controlling power supply to starter motors. More particularly, the invention relates to a solenoid switch which has an improved arrangement of a resistor that is used to limit electric current supplied to a starter motor. 
     2. Description of the Related Art 
     Japanese Patent No. 3767550, an English equivalent of which is U.S. Pat. No. 6,923,152 B2, discloses a starter for starting an internal combustion engine which includes a motor and a solenoid switch for driving the motor in two stages. 
     More specifically, the solenoid switch includes a pair of main contacts, a pair of auxiliary contacts, and a resistor. The main contacts are connected in parallel with the auxiliary contacts in an electric circuit of the starter for supplying electric power from a battery to the motor. The resistor is connected in series with the auxiliary contacts in the electric circuit. 
     During a starting operation, only the auxiliary contacts are closed in the first stage to supply limited current, which is limited by the resistor, to the motor. Consequently, the motor is energized to rotate at a low speed, facilitating establishment of an engagement between a pinion of the starter and a ring gear of the engine. As soon as the engagement between the pinion and the ring gear has been established, the main contacts are closed in the second stage to apply the full voltage of the battery to the motor, causing the motor to rotate at a high speed. 
     Moreover, in the solenoid switch, the resistor is arranged in a resin-made retainer so that it surrounds the radially outer periphery of a solenoid coil with an air gap formed between itself and the solenoid coil. 
     However, with the above arrangement of the resistor, the outer diameter of the solenoid switch is increased by an amount corresponding to the sum of the radial thicknesses of the air gap, resistor, and retainer. 
     Further, since the radially outer periphery of the solenoid coil is surrounded by the retainer via the resistor, it is difficult to dissipate heat generated by the solenoid coil in the radially outward direction. As a result, the temperature of the solenoid coil increases excessively, shortening the thermal withstand time of the solenoid coil. 
     To lower the temperature of the solenoid coil, one may consider enlarging the solenoid coil. However, this would increase the weight of the solenoid switch as well as make it difficult to minimize the solenoid switch. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above-mentioned problems. 
     According to the present invention, there is provided a solenoid switch which includes a solenoid coil, a fixed core, an annular magnetic plate, a movable core, a resin-made contact cover, first and second terminals, first and second fixed contacts, a movable contact, and a resistor. The solenoid coil has a longitudinal axis. The fixed core is surrounded by the solenoid coil. The annular magnetic plate is disposed on one side of the solenoid coil in an axial direction of the solenoid coil. The magnetic plate has a through-hole formed through a radial center thereof. The movable core is movable in the axial direction of the solenoid coil toward and away from the fixed core through the through-hole of the magnetic plate. The contact cover is arranged with the magnetic plate interposed between the contact cover and the solenoid coil in the axial direction of the solenoid coil. The first and second terminals are fixed to the contact cover and protrude outside of the contact cover so as to be connected to an electric circuit. The first and second fixed contacts are received in the contact cover and respectively electrically connected to the first and second terminals. The movable contact is received in the contact cover and configured to be moved along with the movable core to electrically connect and disconnect the first and second fixed contacts. The resistor is electrically connected between the first and second terminals to limit current flowing through the electric circuit when the first and second fixed contacts are electrically disconnected. The resistor is received in the contact cover and interposed between the magnetic plate and the first and second fixed contacts in the axial direction of the solenoid coil. 
     According to further implementations of the invention, the resistor has first and second ends that are respectively joined to the first and second terminals and located away from a radially inner surface of the contact cover by predetermined distances. 
     The resistor extends, on a plane perpendicular to the axial direction of the solenoid coil, between the first and second ends with at least two bends. 
     Thermal resistance of the resistor is so predetermined that when the resistor is continuously energized, the resistor melts before the contact cover reaches its softening temperature. 
     The electric circuit, to which the first and second terminals are to be connected, is an electric circuit for supplying electric power to a starter motor. 
     The solenoid switch further includes a cup-shaped case that has first and second portions. The first portion includes a closed end of the case and has the solenoid coil received therein. The second portion includes an open end of the case and has an end portion of the contact cover fit thereinto. The first portion has a smaller outer diameter than the second portion. 
     The movable contact is located further from the magnetic plate than the first and second fixed contacts in the axial direction of the solenoid coil. 
     Each of the first and second terminals is shaped as a bolt. The first and second fixed contacts are formed respectively integral with the first and second terminals. 
     In a preferred embodiment of the invention, each of the first and second terminals is shaped as a bolt with a bore and two recesses. The bore opens on an axial end face of the bolt and has a predetermined depth. The two recesses are formed in a side surface of the bolt and opposed to each other in a radial direction of the bolt with the bore interposed therebetween. The resistor has first and second ends. The first end is inserted in the bore of the first terminal and joined to the first terminal by press-deforming bottoms of the recesses of the first terminal radially inward. The second end is inserted in the bore of the second terminal and joined to the second terminal by press-deforming bottoms of the recesses of the second terminal radially inward. 
     In another preferred embodiment of the invention, each of the first and second terminals is shaped as a bolt with a bore that opens on an axial end face of the bolt and has a predetermined depth. A brazing filler metal is provided in the bores of the first and second terminals. The resistor has first and second ends. The first end is inserted in the bore of the first terminal and joined to the first terminal by heating only part of the first terminal around the bore to melt the brazing filler metal in the bore. The second end is inserted in the bore of the second terminal and joined to the second terminal by heating only part of the second terminal around the bore to melt the brazing filler metal in the bore. 
     In yet another preferred embodiment of the invention, each of the first and second terminals is shaped as a bolt with a protrusion that protrudes from an axial end face of the bolt by a predetermined distance. The resistor has first and second ends that are respectively welded to the protrusions of the first and second terminals. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be understood more fully from the detailed description given hereinafter and from the accompanying drawings of preferred embodiments of the invention, which, however, should not be taken to limit the invention to the specific embodiments but are for the purpose of explanation and understanding only. 
       In the accompanying drawings: 
         FIG. 1  is a plan view of a starter which includes a solenoid switch according to the first embodiment of the invention; 
         FIG. 2  is a circuit diagram of the starter of  FIG. 1 ; 
         FIG. 3  is a partially cross-sectional view of the solenoid switch according to the first embodiment; 
         FIG. 4  is a plan view showing the inside of a contact cover provided in the solenoid switch of  FIG. 3  from an open end of the contact cover; 
         FIG. 5  is a time chart illustrating operation of the starter of  FIG. 1 ; 
         FIG. 6  is a partially cross-sectional view of a solenoid switch according to the second embodiment of the invention; 
         FIG. 7  is a partially cross-sectional view of a solenoid switch according to the third embodiment of the invention; and 
         FIG. 8  is a plan view showing the inside of a contact cover provided in the solenoid switch of  FIG. 7  from an open end of the contact cover. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be described hereinafter with reference to  FIGS. 1-8 . 
     It should be noted that, for the sake of clarity and understanding, identical components having identical functions in different embodiments of the invention have been marked, where possible, with the same reference numerals in each of the figures. 
     First Embodiment 
       FIG. 1  shows the overall structure of a starter  1  for starting an internal combustion engine of a motor vehicle, which includes a solenoid switch  10  according to the first embodiment of the invention.  FIG. 2  shows an electric circuit of the starter  1 . 
     The starter  1  includes: a housing  2  that is mounted to the engine (not shown); a motor  4  that is fixed to the housing  2  by means of a plurality of through-bolts  3 ; a pinion  6  (shown in  FIG. 2 ) that is configured to mesh with a ring gear  5  (shown in  FIG. 2 ) of the engine to transmit the torque generated by the motor  4  to the engine; a shift lever  7  (shown in  FIG. 2 ) that is configured to shift the pinion  6  in the axial direction of the starter  1  to bring the pinion  6  into and out of mesh with the ring gear  5 ; a solenoid switch  8  that serves as a main switch of starter  1 ; a resistor  9  for limiting electric current supplied from a battery  12  to the motor  4  during a starting operation; and the solenoid switch  10  according to the present embodiment which serves as an auxiliary switch of the starter  1 . Hereinafter, the solenoid switches  8  and  10  will be simply referred to as main switch  8  and auxiliary switch  10 , respectively. 
     The housing  2  has a flange portion  2   a , which is fixed to a surface (not shown) of the engine, and a switch-mounting portion  2   b  to which the main switch  8  is fixed. 
     The motor  4  is implemented by a commutator motor of a type well-known in the art. 
     More specifically, as shown in  FIG. 2 , the motor  4  includes an armature  4   a , a commutator  4   b  provided on an end portion (i.e., the left end portion in  FIG. 2 ) of the armature  4   a , and a pair of brushes  11  that are arranged around the radially outer periphery of the commutator  4   b  to make contacts with the commutator  4   b . In operation, upon closing a pair of main contacts (to be described later) of the electric circuit, current is supplied from the battery  12  to the armature  4   a  via the contacts between the brushes  11  and the commutator  4   b , causing the armature  4   a  to rotate. 
     The pinion  6  is provided together with a clutch  14  on an output shaft  13  which is driven by the motor  4 , so that rotation of the output shaft  13  is transmitted to the pinion  6  via the clutch  14 . 
     The main switch  8  is fixed, as shown in  FIG. 1 , to the switch-mounting portion  2   b  of the housing  2  by means of two through-bolts  17 . 
     The main switch  8  includes, as shown in  FIG. 2 , solenoid coils  15 , a plunger  16 , a pair of fixed contacts  18   a  and  19   a  that make up the main contacts of the electric circuit of the starter  1 , a pair of terminal bolts  18  and  19 , and a movable contact  20 . 
     The solenoid coils  15  create, when energized, a magnetic attraction for the plunger  16 . The magnetic attraction causes the plunger  16  to move to close the main contacts of the electric circuit. Further, when the solenoid coils  15  are deenergized, the magnetic attraction disappears. Then, the plunger  16  is returned, by the force of a return spring (not shown), to its initial position, thereby opening the main contacts of the electric circuit. 
     The fixed contact  18   a  is electrically connected to the high voltage-side (i.e., the side of the battery  12 ) via the terminal bolt  18 . On the other hand, the fixed contact  19   a  is electrically connected to the low voltage-side (i.e., the side of the motor  4 ) via the terminal bolt  19 . 
     The movable contact  20  is configured to move along with the plunger  16  to connect (or bridge) and disconnect (or separate) the pair of fixed contacts  18   a  and  19   a . More specifically, when the movable contact  20  makes contact with both the fixed contacts  18   a  and  19   a  to connect them, the main contacts of the electric circuit is closed. Moreover, when the movable contact  20  is detached from both the fixed contacts  18   a  and  19   a  to disconnect them, the main contacts are opened. 
     Both the terminal bolts  18  and  19  are fixed, as shown in  FIG. 1 , to a contact cover  21  of the main switch  8  which covers the fixed contacts  18   a  and  19   a  and the movable contact  20 . The terminal bolt  19  is electrically connected to the positive-side brush  11  of the motor  4  via a lead  22  (shown in  FIG. 1 ). The electrical connection of the terminal bolt  18  will be described later. 
     The solenoid coils  15  consist of a pull-in coil  15   a  and a hold-on coil  15   b . The pull-in coil  15   a  has one end electrically connected to an energization terminal  23  (shown in  FIG. 2 ), which is fixed to the contact cover  21  of the main switch  8 , and the other end electrically connected to the terminal bolt  19 . The hold-on coil  15   b  has one end electrically connected to the energization terminal  23  and the other end grounded via, for example, an iron core (not shown) of the main switch  8 . 
     The energization terminal  23  is, as shown in  FIG. 2 , electrically connected to the battery  12  via a starter relay  24 . In operation, when the starter relay  24  is turned on by an ECU  25 , electric current is supplied from the battery  12  to the energization terminal  23 , thereby energizing the solenoid coils  15 . Here, the ECU  25  is an ECU (Electronic Control Unit) for controlling operation of the engine. 
     Referring now to  FIG. 3 , the auxiliary switch  10  includes: a cup-shaped case  26 ; a cylindrical solenoid coil  27  that has a longitudinal axis and is received in the case  26 ; a magnetic plate  28  that is disposed on the rear side of the solenoid coil  27 ; a fixed core  29  to be magnetized upon energization of the solenoid coil  27 ; a movable core  30  that is disposed on the rear side of the fixed core  29  to face it in the axial direction of the auxiliary switch  10  (i.e., the axial direction of the solenoid coil  27 ); a resin-made contact cover  31  that is disposed on the rear side of the magnetic plate  28  to close the open end of the case  26 ; a pair of terminal bolts  32  and  33  fixed to the contact cover  31 ; a pair of fixed contacts  45  and  46  that are formed respectively integral with the terminal bolts  32  and  33 ; and a movable contact  34  that is movable along with the movable core  30  to connect (or bridge) and disconnect (or separate) the fixed contacts  45  and  46 . It should be noted that in  FIGS. 1 and 3 , the forward and backward directions are introduced only for convenience of explanation. 
     The case  26  forms, together with the magnetic plate  28  and the fixed core  29 , a magnetic circuit (or a fixed magnetic path) of the auxiliary switch  10 . The case  26  has a small-diameter portion  26   a  and a large-diameter portion  26   b  that has a larger diameter than the small-diameter portion  26   a . The small-diameter portion  26   a  includes the closed end of the case  26  and has the solenoid coil  27  received therein. The large-diameter portion  26   b  includes the open end of the case  26  and has the magnetic plate  28  received therein. Moreover, between the small-diameter and large-diameter portions  26   a  and  26   b , there is formed a step portion  26   c.    
     The solenoid coil  27  is wound around a resin-made bobbin  35 . The solenoid coil  27  has one end electrically connected to an energization terminal  36  (shown in  FIG. 2 ) and the other end grounded. The energization terminal  36  is drawn from the inside to the outside of the contact cover  31  via a through-hole  31   a  which is formed, as shown in  FIG. 4 , through an end wall of the contact cover  31 . The energization terminal  36  is electrically connected to the ECU  25  as shown in  FIG. 2 . 
     The magnetic plate  28  is annular in shape and has a circular bore formed through the radial center thereof. The magnetic plate  28  is insert-molded in a resin member  37  that is formed integral with the bobbin  35 . The magnetic plate  28  abuts the inner surface of the step portion  26   c  of the case  26 , thereby being positioned in the axial direction of the auxiliary switch  10 . In addition, the solenoid coil  27  is mechanically fixed to the magnetic plate  28  via the resin member  37 . 
     The fixed core  29  is disposed on the radially inner periphery of the magnetic coil  27  with a rear end face thereof abutting the inner surface of the end wall of the case  26 . 
     The movable core  30  is movable in the axial direction of the auxiliary switch  10  through the circular bore of the magnetic plate  28 . The movable core  30  is urged backward by a return spring  38  that is interposed between a step portion of the fixed core  29  and a step portion of the movable core  30 . 
     The contact cover  31  has the shape of a cup with a circular open end. The contact cover  31  is assembled to the case  26  so that a front end portion of the contact cover  31  is fit into a back end portion of the case  26  and the front end face of the contact cover  31  abuts the rear end face of the magnetic plate  28 . Further, the contact cover  31  is fixed to the case  26  by crimping part or the whole of the circumference of the back end portion of the case  26  onto the front end portion of the contact cover  31 . 
     A seal member  39 , which is implemented by an O-ring, is provided between the contact cover  31  and the case  26  to prevent foreign matter, such as water, from entering the inside of both the contact cover  31  and the case  26 . 
     The terminal bolt  32  is electrically connected to the cathode of the battery  12  via a cable, as shown in  FIG. 2 . The terminal bolt  32  is fixed to the contact cover  31  by means of a washer  41  and a crimp washer  43 . On the other hand, the terminal bolt  33  is both electrically and mechanically connected to the terminal bolt  18  of the main switch  8  via a metal-made connecting member  40  (shown in  FIG. 1 ). The terminal bolt  33  is fixed to the contact cover  31  by means of a washer  42  and a crimp washer  44 . 
     The fixed contacts  45  and  46  are both received in the contact cover  31  and make up a pair of auxiliary contacts of the electric circuit of the starter  1 . 
     As described previously, in the present embodiment, the fixed contacts  45  and  46  are integrally formed respectively with the terminal bolts  32  and  33 . However, it should be appreciated that the fixed contacts  45  and  46  may also be separately formed respectively from the terminal bolts  32  and  33  and then joined respectively to the same by, for example, brazing. 
     The movable contact  34  is also received in the contact cover  31 . The movable contact  34  is located on the rear side of the fixed contacts  45  and  46  and coupled to the movable core  30  via a resin-made rod  47 . 
     The movable contact  34  is pressed on a contact-receiving surface  31   b  formed in the contact cover  31  by urging the movable core  30  backward with the force of the return spring  38 . Further, around the contact-receiving surface  31 , there is formed an annular recess within which a contact pressure spring  48  is disposed. The contact pressure spring  48  applies, when the movable contact  34  is brought into contacts with the fixed contacts  45  and  46 , pressure to the movable contact  34  for keeping the contacts between the movable contact  34  and the fixed contacts  45  and  46 . 
     The rod  47  has one end embedded in a radially-central portion of the movable core  30  and the other end that passes through the space between the fixed contacts  45  and  46  to abut the movable contact  34 . 
     The above-described auxiliary switch  10  is disposed, as shown in  FIG. 1 , close to the main switch  8  in the radial direction of the starter  1 . The auxiliary switch  10  is fixed to the housing  2  via a bracket  49 . 
     More specifically, the bracket  49  has a first end portion  49   a  and a second end portion  49   b . The first end portion  49   a  has a substantially discoid shape; it has a rear surface to which the auxiliary switch  10  is joined by, for example, welding. The second end portion  49   b  has two circular through-holes (not shown) formed therein. The second end portion  49   b  is fixed between the switch-mounting portion  2   b  of the housing  2  and the main switch  8  by means of the two bolts  17  which respectively pass through the two circular through-holes. 
     The resistor  9  is arranged in an axial space formed within the contact cover  31  of the auxiliary switch  10  between the magnetic plate  28  and the fixed contacts  45  and  46 . More specifically, as shown in  FIG. 3 , the resistor  9  is positioned in the axial direction of the auxiliary switch  10  at predetermined distances from the magnetic plate  28  and the fixed contacts  45  and  46 . The resistor  9  has a first end  9   a  electrically and mechanically connected to the bolt terminal  32  and a second end  9   b  electrically and mechanically connected to the terminal bolt  33 . 
     Further, as shown in  FIG. 4 , the first and second ends  9   a  and  9   b  of the resistor  9  are located away from the radially inner surface of the contact cover  31  by predetermined distances. Moreover, the resistor  9  is configured to extend, on a plane perpendicular to the axial direction of the auxiliary switch  10 , between the first and second ends  9   a  and  9   b  with at least two bends  9   c.    
     Furthermore, the thermal resistance of the resistor  9  is so predetermined that when the resistor  9  is continuously energized, the resistor  9  melts before the contact cover  31  is thermally damaged, more specially, before the contact cover  31  reaches its softening temperature (e.g., 260° C.). 
     After having described the overall structure of the starter  1  and the details of the auxiliary switch  10 , operation of the starter  1  will now be described with reference to  FIG. 5 . 
     First, at a timing T 1 , the ECU  25  energizes the solenoid coils  15  of the main switch  8 , causing a limited current A 1  to flow from the battery  12  to the motor  4 . Then, at a later timing T 2 , the ECU  25  further energizes the solenoid coil  27  of the auxiliary switch  10 , causing a full current A 2  to flow from the battery  12  to the motor  4 . 
     More specifically, at the timing t 1 , the ECU  25  turns on the starter relay  24 , causing electric current to flow from the battery  12  to the solenoid coils  15  of the main switch  8  to energize them. The solenoid coils  15  create, upon being energized, a magnetic attraction for the plunger  16 . The magnetic attraction attracts the plunger  16  to move in the leftward direction of  FIG. 2 , thereby causing the movable contact  20  to connect the fixed contacts  18   a  and  19   a  and the shift lever  7  to shift the pinion  6  rightward. 
     With the main contacts of the electric circuit (i.e., the fixed contacts  18   a  and  19   a ) closed, the limited current A 1 , which is limited by the resistor  9 , flows from the battery  12  to the motor  4 . As a result, the motor  4  rotates at a low speed, facilitating establishment of an engagement between the pinion  6  and the ring gear  5  of the engine. 
     After the engagement between the pinion  6  and the ring gear  5  has been established, at the timing t 2 , the ECU  25  energizes the solenoid coil  27  of the auxiliary switch  10 . Upon being energized, the solenoid coil  27  makes up an electromagnet together with the fixed core  29 . The electromagnet attracts the movable core  30  to move along with the movable contact  34  in the forward direction of  FIG. 3 , causing the movable contact  34  to connect the fixed contacts  45  and  46 . 
     With the auxiliary contacts of the electric circuit (i.e., the fixed contacts  45  and  46 ) closed, the resistor  9  is bypassed or short circuited, and consequently the full current A 2  flows from the battery  12  to the motor  4 . As a result, the motor  4  rotates at a high speed, and the torque generated by the motor  4  is transmitted to the engine via the engagement between the pinion  6  and the ring gear  5 , thereby starting the engine. 
     As soon as the engine has started, at a timing t 3 , the ECU  25  deenergizes the solenoid coils  15  of the main switch  8  as well as the solenoid coil  27  of the auxiliary switch  10 . Consequently, the plunger  16  of the main switch  8  is returned, by the force of the return spring (not shown), to its initial position, thereby causing the movable contact  20  to disconnect the fixed contacts  18   a  and  19   a  (i.e., open the main contacts of the electric circuit). At the same time, the movable contact  34  of the auxiliary switch  10  is returned, by the force of the return spring  38 , to its initial position, thereby causing the movable contact  34  to disconnect the fixed contacts  45  and  46  (i.e., open the auxiliary contacts of the electric circuit). As a result, the electric power supply from the battery  12  to the motor  4  is interrupted, causing the motor  4  to stop. 
     According to the present embodiment, the following advantages can be achieved. 
     In the present embodiment, during the initial time period t from the timing t 1  to the timing t 2  as shown in  FIG. 5 , only the limited current A 1  is supplied to the motor  4 . Consequently, the motor  4  is energized to rotate at a low speed, thereby reducing mechanical shocks that occur during establishment of the engagement between the pinion  6  and the ring gear  5  of the engine. As a result, wear of the pinion  6  and ring gear  5  is reduced, thereby improving the durability of the same. 
     Moreover, with the resistor  9 , the inrush current, which flows from the battery  12  to the motor  4  when the motor  4  starts to rotate, is reduced. As a result, the service lives of the fixed and movable contacts  18   a ,  19   a , and  20  of the main switch  8  as well as those of the brushes  11  of the motor  4  can be extended. 
     In the present embodiment, the resistor  9  is received in the contact cover  31  of the auxiliary switch  10  and interposed between the magnetic plate  28  and the fixed contacts  45  and  46  in the axial direction of the auxiliary switch  10 . 
     Since the resistor  9  is not arranged on the radially outer periphery of the solenoid coil  27 , the outer diameter of the auxiliary switch  10  is reduced in comparison with that of the solenoid switch disclosed in Japanese Patent No. 3767550. 
     Moreover, in the present embodiment, the case  26  of the auxiliary switch  10  is configured to have the small-diameter portion  26   a  and the large-diameter portion  26   b . The solenoid coil  27  is received in the small-diameter portion  26   a , while the resistor  9  is received in the contact cover  31  that is fit into the large-diameter portion  26   b.    
     With the above configuration, the outer diameter of the small-diameter portion  26   a  can be minimized, thereby making the auxiliary switch  10  compact. 
     Further, since the resistor  9  is received in the contact cover  31  and thus not exposed to the outside of the auxiliary switch  10 , it is possible to protect the resistor  9  from foreign matter, such as water, thereby improving the durability of the resistor  9 . In addition, since no flammable gas can reach the resistor  9 , it is possible to ensure the safety of the auxiliary switch  10  when the resistor  9  comes to glow after a long-time energization thereof. 
     Furthermore, since the resistor  9  is located away from the solenoid coil  27 , it does not influence dissipation of heat generated by the solenoid coil  27 . Moreover, with the magnetic plate  28  interposed between the solenoid coil  27  and the resistor  9 , it is possible to block heat generated by the resistor  9  from transferring to the solenoid coil  27 , thereby ensuring the thermal resistance and excitation performance of the solenoid coil  27 . 
     In the present embodiment, the movable contact  34  of the auxiliary switch  10  is located further from the magnetic plate  28  than the fixed contacts  45  and  46 . In other words, the movable contact  34  is not interposed between the magnetic plate  28  and the fixed contacts  45  and  46  in the axial direction of the auxiliary switch  10 . Consequently, there is no risk of the movable contact  34  making contact with the resistor  9 , thus improving the reliability of the auxiliary switch  10 . 
     In the present embodiment, the resistor  9  has the first end  9   a  electrically and mechanically connected to the bolt terminal  32  and the second end  9   b  electrically and mechanically connected to the terminal bolt  33 . Moreover, the terminal bolts  32  and  33  respectively have the fixed contacts  45  and  46  formed therein. Consequently, heat generated by the resistor  9  can be easily transmitted to the fixed contacts  45  and  46 . As a result, even when the temperature of the terminal bolts  32  and  33  are lowered by external cold air, it is still possible to prevent the electrical conductivity of the fixed contacts  45  and  46  from dropping due to, for example, dew formation and freezing. 
     In the present embodiment, the resistor  9  is located away from the magnetic plate  28  and the fixed contacts  45  and  46  by the predetermined distances. Further, as shown in  FIG. 4 , the first and second ends  9   a  and  9   b  of the resistor  9  are located away from the radially inner surface of the contact cover  31  by the predetermined distances. Consequently, it is difficult for the contact cover  31  to be damaged by heat generated by the resistor  9 . 
     Further, in the present embodiment, the thermal resistance of the resistor  9  is so predetermined that the resistor  9  melts before the resin-made contact cover  31  reaches its softening temperature. 
     When the movable contact  34  cannot normally connect the fixed contacts  45  and  46 , the resistor  9  will be continuously energized and thus come to glow. However, with the above configuration, the resistor  9  will melt before the contact cover  31  is thermally damaged. Consequently, it is possible to improve the reliability and safety of the auxiliary switch  10 . 
     In the present embodiment, the resistor  9  is configured to extend, on a plane perpendicular to the axial direction of the auxiliary switch  10 , between the first and second ends  9   a  and  9   b  with at least two bends  9   c.    
     With the above configuration, it is possible to set the resistance of the resistor  9  to a desired value by adjusting the length of the resistor  9 . In addition, during the process of joining the first and second ends  9   a  and  9   b  of the resistor  9  to the terminal bolts  32  and  33 , it is easy to bend the resistor  9  to bring the distance between the first and second ends  9   a  and  9   b  into agreement with a desired distance L as shown in  FIG. 4 . 
     Second Embodiment 
     This embodiment illustrates a method of joining the resistor  9  to the terminal bolts  32  and  33 . 
     Referring to  FIG. 6 , in the present embodiment, the terminal bolt  32  has a bore  32   a  that opens on the front end face of the terminal bolt  32  and has a predetermined depth. The terminal bolt  32  also has two recesses  32   b  that are formed in the side surface of the terminal bolt  32  and opposed to each other in the radial direction of the terminal bolt  32  with the bore  32   a  interposed therebetween. Similarly, the terminal bolt  33  has a bore  33   a  that opens on the front end face of the terminal bolt  33  and has a predetermined depth. The terminal bolt  33  also has two recesses  33   b  that are formed in the side surface of the terminal bolt  33  and opposed to each other in the radial direction of the terminal bolt  33  with the bore  33   a  interposed therebetween. It should be noted that in  FIG. 6 , the forward and backward directions are introduced only for convenience of explanation. 
     The first end  9   a  of the resistor  9  is inserted in the bore  32   a  of the terminal bolt  32 . Further, the terminal bolt  32  is crimped onto the first end  9   a  of the resistor  9  by press-deforming the bottoms of the recesses  32   b  radially inward. On the other hand, the second end  9   b  of the resistor  9  is inserted in the bore  33   b  of the terminal bolt  33 . Further, the terminal bolt  33  is crimped onto the second end  9   b  of the resistor  9  by press-deforming the bottoms of the recesses  33   b  radially inward. 
     With the above joining method according to the present embodiment, the first and second ends  9   a  and  9   b  of the resistor  9  can be securely joined to the terminal bolts  32  and  33  without heating the whole of the resistor  9  and terminal bolts  32  and  33  as in the case of applying furnace brazing. 
     Consequently, the strengths of the terminal bolts  32  and  33  can be prevented from being lowered during the joining process. As a result, it is possible to securely fasten cable terminals onto the terminal bolts  32  and  33  without damaging the terminal bolts  32  and  33 . 
     Moreover, with the above joining method, only part of the terminal bolt  32  around the recesses  32   b  and only part of the terminal bolt  33  around the recesses  33   b  are press-deformed during the crimping. Consequently, the bending-deformations of the entire terminal bolts  32  and  33  can be reduced. 
     In addition, the resistor  9  may also be joined to the terminal bolts  32  and  33  by the following brazing method. 
     First, a filler metal paste is filled in the bores  32   a  and  33   a  of the terminal bolts  32  and  33 . Then, the first and second ends  9   a  and  9   b  of the resistor  9  are respectively inserted into the bores  32   a  and  33   a  of the terminal bolts  32  and  33 . Thereafter, only part of the terminal bolt  32  around the bore  32   a  and only part of the terminal bolt  33  around the bore  33   a  are heated to melt the filler metal paste, thereby joining the first and second ends  9   a  and  9   b  of the resistor  9  respectively to the terminal bolts  32  and  33 . 
     With the above brazing method, it is also possible to achieve the same advantages as with the joining method according to the present embodiment. 
     Third Embodiment 
     This embodiment illustrates anther method of joining the resistor  9  to the terminal bolts  32  and  33 . 
     Referring to  FIG. 7 , in the present embodiment, the terminal bolt  32  has a protrusion  32   c  that protrudes from the front end face of the terminal bolt  32  to have a predetermined protruding height from the front end face. Further, as shown in  FIG. 8 , the protrusion  32   c  has a rectangular bottom and tapers toward its top to have a trapezoidal cross section. Similarly, the terminal bolt  33  has a protrusion  33   c  that protrudes from the front end face of the terminal bolt  33  to have the predetermined protruding height from the front end face. Further, as shown in  FIG. 8 , the protrusion  33   c  has a rectangular bottom and tapers toward its top to have a trapezoidal cross section. 
     The first and second ends  9   a  and  9   b  of the resistor  9  are respectively disposed on the tops of the protrusions  32   c  and  33   c  of the terminal bolts  32  and  33 , and respectively joined to the tops of the protrusions  32   c  and  33   c  by projection welding. 
     Further, as shown in  FIG. 8 , the length of the protrusions  32   c  and  33   c  of the terminal bolts  32  and  33  is sufficiently larger (e.g., three times) than the diameter of the resistor  9 . Furthermore, as shown in  FIG. 7 , the protruding height of the protrusions  32   c  and  33   c  of the terminal bolts  32  and  33  is so predetermined as to locate the resistor  9  almost at the same distance from the magnetic plate  28  and the first and second fixed contacts  45  and  46  in the axial direction of the auxiliary switch  10 . 
     With the above joining method according to the present embodiment, the first and second ends  9   a  and  9   b  of the resistor  9  can be securely joined to the terminal bolts  32  and  33  without heating the whole of the resistor  9  and terminal bolts  32  and  33  as in the case of applying furnace brazing. 
     Consequently, the strengths of the terminal bolts  32  and  33  can be prevented from being lowered during the joining process. As a result, it is possible to securely fasten cable terminals onto the terminal bolts  32  and  33  without damaging the terminal bolts  32  and  33 . 
     Moreover, with the length of the protrusions  32   c  and  33   c  of the terminal bolts  32  and  33  sufficiently larger than the diameter of the resistor  9 , it is possible to reliably prevent the first and second ends  9   a  and  9   b  of the resistor  9  from being detached from the protrusions  32   c  and  33   c  during the projection welding. Further, it is also possible to accurately set the distance between the first and second ends  9   a  and  9   b  to the desired distance L. 
     While the above particular embodiments of the present invention have been shown and described, it will be understood by those skilled in the art that various modifications, changes, and improvements may be made without departing from the spirit of the invention. 
     For example, in the first embodiment, the auxiliary switch  10  is fixed to the housing  2  of the starter  1  via the bracket  49 . 
     However, when it is difficult to locate the auxiliary switch  10  along with the starter  1  in the engine compartment, it is possible to separately locate the auxiliary switch  10  from the starter  1  without being connected to the housing  2 . 
     Moreover, in the previous embodiments, the present invention is applied to the auxiliary switch  10  which is employed in the starter  1  for starting the internal combustion engine. 
     However, the present invention may also be applied to any other solenoid switch which is connected to an electric circuit to control current flowing through the electric circuit in two stages.