Abstract:
A trolling motor having reverse battery protection such that an inadvertent reversal of the battery leads, as when connecting the trolling motor to a battery, will not cause unwanted operation of the trolling motor or damage to the trolling motor circuitry. The reverse battery protection includes a field effect transistor connected in series between one of the battery leads and the motor. The gate terminal of the transistor is in communication with the other battery lead such that, when the battery leads are properly connected to a battery, the transistor is driven to its conducting state and, when the battery leads are reversed, the transistor is driven to its non-conducting state.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to electrical trolling motors for a fishing boat More particularly, but not by way of limitation, the present invention relates to system to protect the electronic circuitry of a trolling motor from unintentional reverse battery connection. 
     2. Background of the Invention 
     Trolling motors are well known in the art and, most often, employ a permanent magnet DC motor. Generally speaking, a trolling motor is a relatively small electric motor coupled to a propeller for propelling a boat, or other water craft, at a relatively low speed. Typically, the electric motor and propeller are positioned at the bottom of a support column rotatably supported by a bracket which is attached to the boat. Traditionally, a control head located at the top of the support column houses electronic circuitry for controlling the motor. In addition, the control head may also house a steering motor and associated circuitry to provide rotation of the support column to steer of the boat when propelled by the trolling motor. 
     In a typical configuration, a trolling motor receives electrical power from one or more lead-acid batteries, preferably of the type designed for marine or deep cycle use. The trolling motor includes a cable terminating in a pair of color-coded clips for easy connection to the battery. Unfortunately, it is easy to inadvertently reverse the connection of such clips with respect to the polarity of the battery. Such reverse battery connections are a common occurrence. Unprotected, reverse battery connection can cause problems ranging from unanticipated activation of the trolling motor to catastrophic failure of the electronic circuitry controlling the motor and steering system. 
     To protect against a reverse battery condition, many trolling motors use either a power diode or a diode in conjunction with a relay or contactor. With regard to diode protection, a battery properly connected causes the diode to be forward biased and thus conductive while a reversed battery connection reverse biases the diode so that no current flows. Unfortunately, in the diode protection scheme, all of the trolling motor current must passes through the diode. Diodes which are rated for the maximum current drawn by a typical trolling motor are physically large. In addition, in light of the large currents flowing through the diode, as well as the voltage drop across the diode, significant amounts of heat are produced which must be dissipated in the environment and which reduce the efficiency of the trolling motor. 
     Alternatively, a contactor may be located in series with the power leads to the trolling motor such that the circuit to the motor and controller is open until the contactor is energized. A relatively small diode placed in series with the relay coil prevents current from flowing through the coil if the battery is reversed but allows current to flow when the battery is properly connected. While this eliminates virtually all of the heat loss of the diode scheme, the relay is somewhat large and costly, and energy is spent energizing the coil. 
     It is thus an object of the present invention to provide reverse battery protection for a trolling motor which does not induce a significant voltage drop and thus, does not produce significant amounts of heat 
     It is a further object of the present invention to provide reverse battery protection for a trolling motor which is relatively inexpensive. 
     It is still a further object of the present invention to provide reverse battery protection for a trolling motor which does not increase the overall number of electrical components which support operation of the motor. 
     SUMMARY OF THE INVENTION 
     The present invention provides a trolling motor having reverse battery protection which is relatively inexpensive and which results in relatively low losses. The reverse battery protection comprises a power metal oxide field effect transistor (“MOSFET”), connected in series with the primary power supplied to the trolling motor. In a preferred embodiment, MOSFET having an intrinsic diode is connected such that the intrinsic diode is forward biased when the battery is properly connected and reverse biased when the battery is reversed. The gate terminal is connected to the opposite rail of the power supply than that switched by the transistor. Thus, the MOSFET is driven to its on, or conductive, state when the battery is properly connected but is switched to its off state, or non-conductive, state when the battery is connected backwards. Since the device is switched off and the intrinsic diode is reverse biased when the battery is reverse connected, virtually no current will flow. In its conductive state, the MOSFET will typically exhibit very low impedance, on the order of a few milliohms, while in its non-conductive state, the MOSFET will exhibit very high impedance, in fact virtually infinite impedance. 
     In another aspect of the preferred embodiment the reverse battery protection MOSFET is used to sense trolling motor current to eliminate another component of the trolling motor controller. In a typical trolling motor controller, motor current is sensed to detect over current conditions, such as with a stalled motor. When an over current condition is indicated, the controller reacts by reducing the drive to the motor which in turn reduces the current to a safe level to prevent damage to components of the motor controller or to the motor itself. 
     In the prior art, motor current is either sensed through a power resistor placed in series with the trolling motor or through a trace on the circuit board having a known resistance. As previously mentioned, since the MOSFET exhibits a resistance of a few milliohms in its conductive state, motor current can be sensed across the reverse battery protection MOSFET in the same manner that it is sensed across any other type of sensing resistor. 
     Further objects, features, and advantages of the present invention will be apparent to those skilled in the art upon examining the accompanying drawings and upon reading the following description of the preferred embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a trolling motor having the inventive reverse battery protection in its general environment. 
         FIG. 2  provides an elevation view of the motor housing of a trolling motor having the inventive reverse battery protection. 
         FIG. 3  provides an exploded isometric view of a submerged motor controller having the inventive reverse battery protection. 
         FIG. 4  provides an exploded view of the controller section of the a motor housing 
         FIG. 5  provides an exploded isometric view of a submerged motor controller in a partially assembled state. 
         FIG. 6  provides an isometric view of the controller section of a motor housing. 
         FIG. 7  provides a schematic diagram of a preferred motor controller having the inventive reverse battery protection incorporated therein. 
         FIG. 8  provides a diagram of an n-channel MOSFET and its intrinsic diode. 
         FIG. 9  provides a diagram of a channel MOSFET and its intrinsic diode. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Before explaining the present invention in detail, it is important to understand that the invention is not limited in its application to the details of the construction illustrated and the steps described herein. The invention is capable of other embodiments and of being practiced or carried out in a variety of ways. It is to be understood that the phraseology and terminology employed herein is for the purpose of description and not of limitation. 
     Referring now to the drawings, wherein like reference numerals indicate the same parts throughout the several views, a trolling motor  10  having reverse battery protection is shown in its general environment in FIG.  1 . Typically trolling motor  10  is rotatably mounted to a fishing boat  12  by a mount  14 . Mount  14  allows the trolling motor to be placed in the water, as shown in  FIG. 1 , or to be laid on the deck of boat  12  when not in use. Preferably, trolling motor  10  includes: head  16  which typically houses a steering mechanism and associated control circuitry, if motor  10  is so equipped; a support column  18  extending from head  16  downward through the mount  14  and into the water to support motor assembly  20  in a submerged position. Propeller  22  connects to motor assembly  20  to propel the boat. Fin  28  improves the steering performance of trolling motor  10  and protects propeller  22  from submerged obstacles. The trolling motor speed and steering control may be provided by a foot pedal  30 , a hand control (not shown), an autopilot (not shown), or the like. 
     With further reference to  FIG. 2 , motor assembly  20  includes: housing  24 ; motor  48  ( FIG. 7 ) housed in housing  24 , which in turn rotates propeller  22 ; and controller assembly  26  for electrically driving the motor  48 . Preferably, electrical wires (not shown) originate in head  16  and are routed through support column  18  to supply electrical power to controller  26  and ultimately to the motor  48 . Preferably, controller  26  utilizes pulse width modulation (PWM) to control the voltage applied to the motor, and hence, to control the speed of the motor. 
     Generally speaking, PWM controllers for trolling motors are well known in the art. An example of a PWM controller  26  having the inventive reverse battery protection is shown in FIG.  7 . Preferably, controller  26  possesses a number of desirable traits including: current management to protect the motor and controller in the invent of propeller fouling or other stall condition; minimal parts count to allow packaging of the controller in a relatively small volume; both analog and digital inputs for speed control to allow easy interfacing with a foot pedal, hand control, autopilot, and the like; adaptability to motors of various voltage and current requirements; etc. The motor controller  26  as shown in  FIG. 7  possesses these traits which are further described in U.S. Pat. No. 6,507,164, entitled Current Based Power Management for a Trolling Motor, which is incorporated herein by reference. 
     Continuing with  FIG. 7 , preferably controller  26  comprises: microprocessor  32  having analog inputs  34  and  36 , a pulse width modulator output  40 , a digital output  42 , and a serial input  38  for receiving motor speed commands from, for example, a foot pedal  30  (FIG.  1 ); reversing relay  46  for changing the polarity of the power applied to motor  48 ; relay driver  50  for energizing reversing relay  46  at the direction of output  42 ; and solid-state switch  52  for energizing motor  48 ; and reverse battery protection MOSFET  54 . Analog input  36  provides an alternate means for providing a motor speed command to controller  26  from an analog signal, such as a potentiometer. 
     As previously mentioned, accidental reversal of the battery leads when the trolling motor is connected to a battery can result in unwanted results ranging from unanticipated activation of the motor to catastrophic failure of the electronic circuitry of the motor controller. With further reference to  FIG. 8 , as is will known in the art, power MOSFET devices, such as MOSFET  54 , contain an intrinsic diode  56 . Diode  56  is oriented such that it is reversed biased when the polarity across MOSFET  54  is in the direction which will result in normal current flow through the device, i.e., the cathode towards drain  58  and anode towards source  60  for an n-channel device. As seen in the circuit of  FIG. 7 , in the absence of reverse battery protection, if a battery is connected backwards such that +BATT is more negative than −BATT, the intrinsic diodes  62  and  64  of MOSFETs  50  and  52 , respectively, would be forward biased resulting in unwanted activation of the reversing relay  46  and motor  48 . It should be noted that reverse battery protection MOSFET  54  is installed with drain  58  and source  60  reversed so that intrinsic diode  56  is reverse biased, and hence no current will flow, upon an accidental reverse battery connection. 
     As is also well known in the art, typically the internal geometry of a MOSFET is somewhat symmetric in nature such that the MOSFET will switch in a normal fashion with drain  58  and source  60  reversed. Thus in the circuit of  FIG. 7 , the gate  66  of MOSFET  54  is connected to +BATT and the drain  58  is connected to −BATT. Thus, when the battery is properly connected, MOSFET  54  is switched to its conductive state. It should be noted that when the battery is properly connected, intrinsic diode  56  is forward biased and thus conducting, diode  56  would typically exhibit a voltage drop of a few hundred millivolts. However, since MOSFET  54  is switched to its conductive state, the voltage drop across device  54  will be defined by the on resistance of device  54 , typically a few milliohms, multiplied times the trolling motor current. Preferably a MOSFET  54  is selected having an on-resistance sufficiently low that the IR drop across device  54  is much lower than the turn-on voltage of intrinsic diode  56 . With such a device, the loss of energy and production of heat will likewise be smaller than when simply using a diode for reverse battery protection. 
     Another feature of the present invention lies in the fact that MOSFET  54  has a relatively fixed resistance when driven to its conductive state. In prior art trolling motor controllers, a current sense resistor is often employed to measure the electrical current flowing through the trolling motor. In the event the motor is stalled, or otherwise encounters heavier than expected loads, the output to the trolling motor can be reduced to prevent damage to the power transistor or the motor. In a controller having the inventive reverse battery protection, since the trolling motor current will flow through MOSFET  54 , the resistance of MOSFET  54  can be used in lieu of a separate current sensing resistor. Since the voltage across MOSFET  54  will be substantially proportional to the current flowing through the trolling motor, this voltage can be scaled by current sense amplifier  68  to produce a current sense voltage which is of an appropriate range for analog input  34  of microprocessor  32 . Microprocessor  32  can thus monitor the current flowing through motor  48 . If the current becomes excessive, microprocessor  32  simply reduces the duty cycle at its PWM output  40  to a level which allows monitoring of the overload condition, but which will protect transistor  52  and motor  48 . Typically the overload monitoring duty cycle will be in the range of ten percent duty cycle, or less. 
     As will be apparent to those skilled in the art, the inventive reverse battery protection scheme could alternatively be accomplished with a p-channel transistor connected in series with the positive battery lead, having its gate terminal connected to the negative lead. Turning to  FIG. 9 , it can be seen that in p-channel transistor  110 , intrinsic diode  112  is reversed, as compared to its n-channel counterpart, having the cathode of the intrinsic diode  112  connected to the source terminal  114 , and the anode connected to the drain terminal  116 . Thus, when used in a reverse battery protection circuit, MOSFET  110  will be inserted in a circuit in a reverse manner than normal such that intrinsic diode  112  will be forward biased when the battery is properly connected, and reversed biased when the battery leads are reversed. 
     Referring next to  FIG. 3 , in the preferred embodiment, the various components of controller  26  are located on three circuit boards  70 ,  72 , and  74 . Connectors  76 ,  78 , and  80  electrically interconnect the boards  70 ,  72 , and  74 . Spacers  84   a-f  separate boards  72  and  74  to provide clearance between the boards to accommodate the components mounted thereto. 
     As will be understood by those familiar with DC motor design, a trolling motor typically includes an armature (not shown) which includes a series of windings wound about an armature shaft. A commutator is mounted coaxial with the armature shaft and acts in concert with a pair of brushes to sequentially apply power to the windings of the armature as the motor rotates to continue urging rotation of the of the armature. By reversing the voltage to the brushes, and hence each winding, the direction of rotation of the armature may be reversed. With further reference to  FIG. 7 , reversing relay  46  performs the function of reversing the polarity of the power applied to the armature through the brushes. It should be noted that circuit board  74  includes brush housings  90   a  and  90   b  which both retain, and provide power to the, brushes (not shown). The brushes in turn interact with the commutator to power the windings of the armature. As a matter of design choice, two single-pole double-throw relays  92   a  and  92   b  are used to provide the double-pole double-throw function (as shown with relay  46  of  FIG. 7 ) for reversing. As will be apparent to those skilled in the art, the output of controller  26  is provided directly to the brushes of motor  48  through circuitry provided on board  74  without the need for additional wiring between controller  26  and motor  48 . This arrangement substantially reduces the opportunity for the emission of electromagnetic noise. 
     While the inventive reverse battery protection it suitable for use in any type of trolling motor controller, whether located in the control head or in the motor housing, the preferred embodiment is shown in a trolling motor having a submerged controller. In such an embodiment, preferably, the heat producing components, i.e., transistors  52  and  54 , of motor controller  26  are mounted on circuit board  70 . Preferably, circuit board  70  is of a heat-sinking type, such as direct copper bonded substrate or insulated metal substrate, and is positioned by board housing  86  to maintain contact for good thermal conduction from the heat producing components to the outer surface of controller assembly  26  (FIG.  2 ). 
     The remaining components of motor controller  26  are preferably located on board  72 . With further reference to  FIG. 4 ,  5 , and  6 , screws  88   a-c  are received in spacers  84   d-f , respectively to secure board  70  to board  72 . Boards  72  is then received on connectors  78  and  80  of board  70  to form motor controller  26  which is, in turn, received in controller housing  94 . Controller housing  94  includes a forward opening  96  for receiving controller assembly  26  and a rear bulkhead  98  which mates with heat sinking surface  100  of circuit board  70 . Board  70  includes aperture  102 , board  72  includes aperture  104 , board  74  includes aperture  106 , and bulkhead  98  includes an aperture (not shown) for passage of the armature shaft when the motor is assembled. When fully assembled, as shown in  FIG. 6 , the heat-sinking surface  100  (FIGS.  3  and  4 )of board  70  is held in physical contact with the rear bulkhead  98  of housing  94  to provide thermal conduction between the heat producing components and the housing  94 . 
     Preferably, controller housing  94  is formed of a material which is a good thermal conductor, most preferably, aluminum. 
     As will be apparent to those skilled in the art, in certain embodiments of the present invention, other heat producing electronic components may be included in the controller, i.e., voltage regulators, interface circuitry, etc. To the degree any of these components produce heat beyond that which can be readily dissipated in the free air surrounding the component, such components could likewise be located on circuit board  70  to provide thermal conduction to the environment 
     Finally it should be noted that, while the preferred embodiment of the inventive reverse battery protection circuit has been shown and described as incorporated into a trolling motor, the invention is not so limited. The reverse battery protected controller can be used with DC motors in other applications. 
     Thus, the present invention is well adapted to carry out the objects and attain the ends and advantages mentioned above as well as those inherent therein. While presently preferred embodiments have been described for purposes of this disclosure, numerous changes and modifications will be apparent to those skilled in the art. Such changes and modifications are encompassed within the spirit of this invention.