Abstract:
An integrated solenoid system including a single housing containing a solenoid, a controller and one or more electrical connections. The controller includes temperature compensating means and/or voltage compensating means thereby providing predetermined, substantially constant currents to said solenoid. The housing includes an integral two-part end cover.

Description:
RELATED APPLICATION 
     This application claims the benefit of Provisional Patent Application Ser. No. 60/295,974, filed 5 Jun. 2001, now abandoned. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to an integrated system solenoid, and in particular to a single housing containing a solenoid, controller and one or more electrical connections. 
     BACKGROUND OF THE INVENTION 
     A solenoid is a common electrical device used to convert electrical energy into mechanical energy. Solenoids are well known in the art and are often utilized as a means of moving a component a predetermined distance at a predetermined time. In its most basic form, a solenoid is an electro-mechanical device that converts electrical energy into linear or rotary motion. Electrical current passes through a coil of insulated copper wire producing a magnetic field, which moves a ferro-magnetic plunger located within the core of the coil. Steel parts surround the coil to contain the flux path for maximum pull, push or rotational force. A solenoid can be used to open a valve, activate a switch, apply a brake or a number of other activities where mechanical movement is required and only an electrical energy source is available or practical. 
     A typical solenoid comprises a steel frame or shell that surrounds the coil of wire and directs the flux path. The coil assembly, when energized with an electrical voltage, creates the magnetic lines of force. A plunger, located within the coil assembly, reacts to the magnetic pull and moves to center itself within or along the coil toward and against a stop or pole piece. The pole piece provides a stop for plunger movement. 
     A solenoid system requires, at a minimum a solenoid and an electric control. In many applications, the electric control comprises an electronic control module. Most prior art solenoid systems require each individual solenoid and individual control module be connected to one another with a predetermined length of electrically conductive wire. While these prior art systems continue to be viable for many applications, the present invention advances the art by consolidating the multiple components into a single housing or enclosure. 
     While a controller located within a solenoid housing has been utilized in the past to control the movement of a solenoid, an integrated solenoid system having the structure and benefits, as set forth below, is believed to be novel. The inventor is aware of a product manufactured by the assignee of the present invention for Leslie Controls, Inc. of Tampa, Fla. known as a “Solicon” that incorporates a controller within the solenoid housing. However, the controller for the Solicon device is relatively complex, costly and requires considerable space. The inventor is also aware of a solenoid built by an unknown company in Poland that also places the controller within the housing. Again, temperature compensation is not provided in the control circuit and voltage compensation occurs only in a low power or “hold” mode of operation. Another major drawback of these designs resides in their overly large size and expensive circuitry. The inventor is not aware of any other prior art that teaches the unique combination of components and resulting benefits disclosed herein. 
     SUMMARY OF THE INVENTION 
     The present invention provides solenoid operational control by encasing the electronic control in the solenoid housing. By minimizing the control size, the invention provides more operational capacity without increasing the overall size of the solenoid housing. By reducing the number of separate components, the invention improves the cost effectiveness of a typical solenoid application. All of these improvements allow designers the maximum amount of flexibility in development. 
     It is common knowledge in the art of solenoid design that the force produced by the solenoid has a direct relationship to the current flowing through the coil of the solenoid. The current flow through the coil of a direct current (DC) solenoid is directly related to resistance of the coil and the voltage applied across the coil. If the coil is wound from copper wire as is typical, the resistance of the coil will vary in response to changes in temperature of the copper wire. The resistance of copper changes by about 0.4% per degree Centigrade change in temperature. Such changes in temperature may be caused by external factors, i.e. the ambient temperature of the environment the solenoid exists in, heat radiated or conducted to or from the solenoid, etc. Coil temperature will also increase as a result of wattage dissipated by the coil due to current flow through the coil wire because of electrical resistance in the wire. The power dissipated (P) is a function of current (I) squared multiplied by the resistance (R) of the coil, P=I 2 R. 
     As a result, without any compensating means, the performance of a solenoid is affected by variations in applied voltage and by variations in coil temperature. Solenoids are commonly applied in mobile equipment applications. These can be particularly demanding because operating voltage levels can vary greatly. Requiring operation over voltage ranges of 16 volts maximum, down to 10 or even 6 volts minimum are realistic situations. Compounding the problem, operating temperatures of the coils can vary from −40 degrees Centigrade to over 100 degrees Centigrade. Under such varying conditions, a given coil may operate at currents that vary as much as 400% from minimum to maximum over the full range of extreme voltage and temperature combinations. The solenoid must be designed to generate adequate force under the minimum current conditions. Accordingly, it will have far more force than required and consume far more power than required when conditions are at maximum or even nominal values. Furthermore, any device performing electrical control of the solenoid must handle the excessive currents that will be generated under conditions of high voltage and low temperature. These factors all tend to lead to the selection of increased component size and increased costs. 
     It is also known to those skilled in the art of design and application of solenoids that one means to overcome these unwanted variations is to apply electrical power to the solenoid by some device that regulates current. Current regulating controllers typically involve more complexity and resultant circuitry size and cost than controllers that do not regulate current. A typical means to monitor or sense current involves a current sense resistor placed in series with the coil so it experiences the same current as the coil. These are often bulky, may be costly and generate additional heat in the controller circuit. Providing means to remove the added heat may be problematic. 
     It is an object of this invention to substantially compensate, by means of a novel electronic controller, for changes in supply voltage and changes in solenoid temperature so as to gain most or all of the benefits of conventional current regulated solenoid controllers while avoiding many of the factors that typically adversely affect size and cost of such controllers. This is achieved in part by placing the controller in the same housing as the solenoid such that a temperature sensitive resistor (thermistor) conveniently and conventionally mounted to the circuit board experiences temperatures that are substantially the same as the solenoid coil. 
     It is an object of the present invention to provide a solenoid assembly wherein the solenoid and controller are contained in one housing. 
     It is an object of the present invention to provide a single device containing a solenoid and controller thereby eliminating the need for two separate enclosures, electrical connections between the separate enclosures, additional installation space, and additional installation costs. 
     It is another object of the present invention to provide an integral electrical connector formed in the assembly housing. 
     It is another object of the present invention to provide a solenoid system having no lead wires in the system. Lead attachment involves costly labor and the lead attachment can prove to be one of the more mechanically fragile elements of the solenoid construction. 
     It is yet another object of the present invention to provide an efficient, yet robust circuit to approximately compensate for voltage and temperature fluctuation. 
     It is yet another object of the present invention to provide an integral solenoid system housing having a two-piece cover that provides tolerance for potting irregularities and deformities that commonly occur and have aesthetic impact. 
     It is yet another object of the present invention to provide a means of connection with a watertight electrical connector. 
     In one embodiment the invention may be described as an integrated solenoid system comprising a solenoid, a control circuit having an output connected to said solenoid, a housing, the solenoid and control circuit each located within the housing, said control circuit including a temperature compensating means for monitoring system temperature and adjusting the control circuit output in consideration of system temperature, and said temperature compensating means being connected to said control circuit. 
     In another embodiment the invention may be described as an integrated solenoid system comprising a solenoid, a control circuit having an output connected to said solenoid, a housing, the solenoid and control circuit each located within the housing, and the control circuit including a voltage compensating means for monitoring supply voltage available at the solenoid system and adjusting the control circuit output. 
     In a third embodiment the invention may be described as an integrated solenoid system comprising a solenoid, a control circuit connected to said solenoid a housing, the solenoid and control circuit each located within the housing, said control circuit having control means to control a voltage output to the solenoid at a plurality of predetermined levels and having timing means to control the sequence and timing of said voltage output, said control circuit including a voltage compensating means for monitoring a supply voltage available at the solenoid system and adjusting the voltage output, and said control circuit including a temperature compensating means for monitoring a system temperature and adjusting the voltage output. 
     Another aspect of the invention includes a solenoid assembly comprising a housing, a solenoid, said solenoid being located within said housing, a primary cover, said primary cover being adapted to be received on one end of said housing, a secondary cover, said secondary cover being adapted to be received within an opening formed in said primary cover, and said primary cover being located on said housing end and said secondary cover being placed within said opening. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanied drawings in which: 
         FIG. 1  is a top plan view of the integrated system solenoid; 
         FIG. 2  is a rear elevation view; 
         FIG. 3  is a front elevation view; 
         FIG. 4  is a cross sectional view taken along line  4 — 4  of  FIG. 1 ; 
         FIG. 5  is a side elevation view; 
         FIG. 6  is an exploded perspective view; and 
         FIG. 7  is a schematic diagram of the control circuit. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention is depicted in  FIGS. 1 through 7 . Like parts illustrated and described herein are designated by like reference numerals. 
     Referring to the drawings, and partiuculary to  FIGS. 1 and 6  there is illustrated a solenoid  20 . The solenoid  20  is enclosed within a housing  30 . The housing may be formed from steel tubing or any other suitable material. In one embodiment and for purposes of example only, the housing is approximately 3.08 inches in length and has a diameter of 1.625 inches. A saddle type side mounting bracket  32  with two (2) holes  34  on 2.00 inch centers is attached to the housing  30  for mounting or mechanical interface. 
     Within the housing  30  is placed the bobbin  40  that is wound with magnetic wire  42 . The bobbin  40  is preferably fabricated from a nylon material and includes two passageways  44 ,  46  on one end. The first passageway  44  is formed where the tube portion  48  of the bobbin meets the side  50  and allows one end of the winding wire  42  to pass there through. The other passageway  46  is formed near the outer periphery of the bobbin side  50  and allows the second end of the winding wire  42  to also pass there through. In the illustrated embodiment, the coil contains 146 turns of 15.5 STAI size wire thereby forming the solenoid coil assembly  54 . The total resistance of the illustrated embodiment coil assembly is preferably between 0.142 and 0.160 ohms. 
     A brass tube  56  is positioned within the core  52  of the bobbin  40 . In the illustrated embodiment, the brass tube is 2.555 inches in length and 0.813 inches in diameter. The tube  56  has a 22 -gauge wall thickness. 
     The solenoid plunger  60  has a clearance fit within the inner diameter of the brass tube  56  as shown. The plunger is preferably formed from cold rolled steel. In the illustrated embodiment, the plunger  60  is 3.465 inches in length and 0.750 inches in diameter. The exposed end of the plunger may have one or more undercut grooves  62  formed therein and may have an opening  64  or threaded opening  66  formed therein also. 
     Abutting one end of the bobbin  40  is the pole piece  70 . The pole piece  70  is fabricated from cold rolled steel. The pole piece  70  provides a stop for the plunger  60  when it is in its fully retracted position. The pole piece  70  is assembled into the end of the housing assembly  30 . As shown in  FIG. 4 , the pole piece  70  is set into the housing a predetermined distance to create a cavity  72  for the control circuit  90  discussed below. 
     A nose piece  80  is assembled into the opposite end of the housing  30 . The nose piece  80  is also formed from cold rolled steel and includes a central opening  82  through which the plunger  60  protrudes. Again, a clearance fit is provided. 
     Within the cavity formed adjacent the pole piece  70  there is located the control circuit  90 . The control circuit  90  receives power and a control signal through three inputs J 1 - 1 , J 1 - 2  and J 1 - 3  and is connected to the coil assembly through two outputs J 2 - 1  and J 2 - 2 . 
     The control circuit  90  shown in  FIG. 7  is a Pulse Width Modulation (PWM) type controller used to drive the solenoid coil  54 . The controller  90  functions as a voltage over-energizer control. The basic benefit afforded by such a control is that solenoid  20  can be momentarily energized at a power (wattage) level well above its continuous duty capacity. The continuous duty capacity is a function of the solenoid&#39;s thermal constant (approximately B degree Centigrade temperature rise per watt of electrical energy being dissipated), the temperature rating of the components used to construct the solenoid  20 , and the ambient temperature of the environment where the solenoid operates. 
     When the solenoid  20  is operated at power levels that are a multiple of it&#39;s continuous duty capacity (i.e. over-energized), it is capable of doing significantly more work than when it is operated at a power level equal to its continuous duty capacity (normally energized). This is due to the fact that the magnetically derived force produced by solenoid  20  is directly related to the electrical energy being applied to the coil  54 . A more detailed description of operation and benefits of this type of controller can be found in U.S. Pat. No. 6,256,185 entitled Low Voltage Direct Control Universal Pulse Width Modulation (PWM) Module issued on 3, Jul. 2001, assigned to same assignee as the present invention and incorporated herein by reference. 
     Referring again specifically to  FIG. 7 , resistors R 7 , R 1 , and R 2  along with capacitors C 1  and C 2  and voltage regulator VR 1  compose a nominal 5-volt power supply. The power supply derives energy to operate from plus input J 1 - 3  and minus input J 1 - 1 . J 1 - 3 , the plus control serves as a power input source for signal level power and also in effect serves as the on/off command signal input for the controller. VR 1  is a shunt type voltage regulator, P/N TL1431ID as manufactured by ST Microelectronics of Geneva, Switzerland and others. This device acts as a dynamic current load that draws current to ground in greater or lesser amounts as required in attempting to maintain a constant voltage across its anode and cathode terminals. The regulated voltage is programmed via its reference terminal using the resistive divider comprised of resistors R 1  and R 2 . R 7  limits the current that VR 1  must control in order to regulate voltage down to 5 volts from the level applied at input J 1 - 3 . The J 1 - 3  input voltage will typically be in the range of 10 to 16 volts. The value of R 7  must be chosen low enough to allow adequate current to the controller circuit  90  under the lowest system voltage conditions for which the controller is specified to operate, 6 volts in the illustrated embodiment. Capacitors C 1  and C 2  store energy local to the circuit  90  so as to maintain a relatively stable 5-volt supply in the event of momentary disturbances in the voltage applied at J 1 - 3 . The combination of R 7 , C 1 , and C 2  furthermore compose a low pass filter circuit that smoothes any rapidly occurring disturbances in voltage that might occur at J 1 - 3 . Such voltage disturbances can be induced by other electrical apparatus connected to, or in close proximity to the electrical system in which the controller  90  operates. 
     Q 1  is a power MOSFET that functions as a very fast acting on/off switch connected between the coil  54  of the solenoid  20  and the power source (not shown), which in the case of mobile equipment, is usually a battery. The percent of time that Q 1  is on out of a given period of time is referred to as the output duty cycle (duty cycle):
 
 Duty cycle  (%)k=[( Q 1  on time )/( Q 1  on time+Q 1  off time )]×100%
 
     The circuit operates as follows: Q 1  is off (duty cycle =0%) when J 1 - 3  has no voltage applied. When voltage is applied to J 1 - 3  the controller takes a small amount of time (approximately 40 mS) to initialize itself. Once initialization is complete, Q 1  is turned on and off with a relatively high duty cycle that causes the solenoid to be over-energized. After approximately 0.25 seconds, the on/off duty cycle of Q 1  is reduced substantially such that the solenoid is being normally energized and therefore may be maintained energized for an indefinite period of time. Longer or shorter over-energized time periods may be used, as the specific application requires. The on/off switching of Q 1  is performed at a frequency of approximately 1K Hz. Higher or lower switching frequencies may be used. Higher frequencies tend to increase switching related losses and create more electrical magnetic interference (EMI). Lower frequencies result in greater fluctuation in coil current and hence greater fluctuation in magnetic force of the solenoid throughout the PWM cycle. 
     Typically the solenoid  20  moves the load attached to its plunger  60  while the coil  54  is over-energized. Once the work of moving the load is completed, the solenoid plunger  60  is in a position wherein the magnetic efficiency is relatively high and therefore only a small amount of electrical energy input is required to hold the load in position. 
     D 1  is a freewheeling diode of the Schottky variety. It is used to slow the decay of current flow and the associated magnetic field in the solenoid coil  54  during the periods when Q 1  is off. D 1  slows this decay to the point that the current and related force decay during the off portion of the PWM cycle is small enough to not significantly affect actuation and holding of the load. 
     Z 1  is a transient voltage suppression device of the Metal Oxide Varistor (MOV) variety. It serves to absorb abnormal high amplitude transient voltage spikes that sometimes occur in electrical systems. In so doing, it protects other components of the controller from being damaged. 
     U 1  is an 8-bit One Time Programmable (OTP), CMOS RISC micro-controller with self-contained oscillator. In the illustrated embodiment, it is a PIC12C508A variety as manufactured by Microchip Corp. It is capable of operating with no ancillary support components accept as required to create its required nominal 5-volt power supply (actual specified operating voltage range is 3.0 to 5.5). U 1  generates the PWM drive signal that controls Q 1  on and off via Q 1 &#39;s gate terminal. Applying nominal 5 volts between Q 1 &#39;s gate and source terminals causes Q 1  to turn on. Because Q 1 &#39;s source terminal is tied to ground, applying approximately 5 volts to Q 1 &#39;s gate turns Q 1  on. Conversely, reducing the voltage at Q 1 &#39;s gate to near 0 volts, or ground potential, causes Q 1  to turn off. 
     The code programmed into U 1  sets the PWM frequency and establishes the basic timing and over-energized and normally energized duty cycles. The PWM duty cycle in both over-energized and normally energized states are adjusted to compensate for variations in the main system supply voltage at J 1 - 2  and operating temperature of the solenoid. In so doing, the performance of the solenoid  20  is made far more stable and the stresses imposed on the solenoid  20  and the controller  90  are substantially reduced compared to what they are in an uncompensated system. 
     The combination of R 6 , R 5 , and C 3  under the control of U 1  terminals (pins)  2  and  3  comprise a simple and low cost dual slope analog to digital (A/D) converter that monitors the main system voltage as it presents itself at J 1 - 2 . Voltage determination is made by alternately allowing C 3  to charge from the unknown system voltage at J 1 - 2  via the series combination of R 6  and R 5  over a fixed period of time, and then discharging C 3  to the logic 0 threshold of U 1  pin  3 , via R 5  and U 1  pin  2 . U 1  pin  2  is allowed to float (set in tri-state mode) to facilitate charging C 3 . After the fixed integration time has elapsed, U 1  pin  2  is switched on (to ground) to facilitate discharge of C 3 . While discharging, Ul monitors the digital value on pin  3 , the voltage across C 3 , and keeps track of the amount of time that it takes to reach the logic 0 threshold. When the voltage falls to the lower threshold (logic 0) of U 1  pin  3 , U 1  pin  2  is switched back to a tri-state mode, allowing R 6  and R 5  to charge C 3  for the next integration period. The cycle thus repeats on a continuing basis. Therefore, the discharge time will be proportional to the applied battery voltage at J 1 - 2 . In the illustrated embodiment, the resistors are 1% initial tolerance with 100 ppM temperature coefficients. C 3  is a relatively low cost ceramic capacitor of the temperature stable (X7R dielectric) type. This type of capacitor is acceptable because variations in C 3  will only affect the peak voltage across C 3  during the integration time period. Its value is not a factor in the actual analog to digital conversion process. 
     The combination of R 9  and C 4  under the control of U 1  pins  5  and  7  comprises a very cost efficient temperature monitoring circuit. R 9  is a voltage dependant resistor (thermistor) that is soldered to the printed circuit board on the surface that is closest to the solenoid coil  54  when the controller printed circuit board  92  is installed in the assembly  10 . The temperature of R 9  is very representative of the temperature of the solenoid coil  54  due to proximity. Changes in coil temperature caused by ambient variation and/or by self (electrical) heating of the coil are tracked. Temperature determination is made in the following manner: C 4  is charged from voltage output at U 1  pin  7  through R 9  until the upper (logic 1) voltage threshold of U 1  pin  5  is reached. C 4  is then discharged through R 9  and U 1  pin  7  until the lower voltage (logic 0) threshold of U 1  pin  5  is reached. This charge/discharge cycle repeats itself with the circuit operating as a free running oscillator for which the operating frequency is a function of the values of R 9 , C 4 , the voltage output at U 1  pin  7 , and the voltage thresholds of U 1 - 5 . The voltage output at U 1  pin  7  is relatively stable due to the fact that U 1 &#39;s outputs are very efficient (low loss) at low current loads and due to the relatively tight regulation of U 1 &#39;s 5-volt power supply as managed by VR 1 . The logic thresholds of U 1  pin  5  are relatively stable because that pin of the 12C508A is a Schmidt trigger input, having more stringently defined voltage thresholds than the other input/output pins, and also due to good power supply voltage regulation. C 4  is a high quality film capacitor with 5% initial tolerance and low temperature drift. All these factors lead to changes in the frequency of oscillation being predominately dependant upon changes in the resistance of R 9 . R 9 &#39;s resistance is temperature dependant in a strictly defined manner. That allows the micro-controller to derive an approximate temperature by measuring the frequency of oscillation. 
     R 4  is provided to guarantee that the U 1  pin  6  and more significantly, the Gate of Q 1  is held low during the power-up initialization of U 1 . R 5  is provided to allow tempering the turn on and turn off speed of Q 1  if desired to reduce switching induced electrical noise or electro magnetic interference (EMI). 
     R 3  is a pull up resistor used to tie U 1  pin  4  to the 5-volt supply. This helps to maintain U 1  pin  4 , an unused input, at a stable logic 1. 
     The integrated solenoid system  10  is assembled as follows. Nose piece  80  is assembled to the housing  30  and crimped in place. The brass tube  56  is next inserted into the housing  30 . The coil assembly  54  is placed into the housing  30  with the brass tube  56  located inside the bobbin core  52 . The pole piece  70  is inserted into the opposite end of the housing  30  with the coil wires  42  extending through the notches  74  formed in the pole piece  70 . The pole piece  70  sits flush with the back side of the coil assembly  54 . 
     The cover  100  and circuit  90  are assembled. This includes pressing the three terminal pins  102  into the molded cover  100 . The circuit board assembly  92  is placed into the cover  100  with the pins  102  extending through corresponding holes  94  in the circuit board  92 . The circuit board  92  is soldered to the terminal pins  102  at each location forming cover and circuit assembly  120 . 
     The cover and circuit assembly  120  is next inserted into the housing  30  while guiding the coil magnet wires  42  through corresponding holes  96  in the circuit board assembly  92 . The housing assembly  30  is crimped at the cover end and the magnet wire ends are soldered to the circuit board  92 . The excess wire that extends above the surface of the circuit board is trimmed. After positioning the solenoid assembly  20  with the cover opening  104  facing upward, the coil and circuit board cavity  72  and coil cavity  108  are filled with potting material  106 . The secondary cover  110  is placed into the cover opening  104  while the potting material  106  is still in the liquid state. The secondary cover  110  is secured to the main cover  100  with snap tabs  112  and will also be anchored with anchor structure  114  in the potting material  106  once cured. Finally, the solenoid plunger  60  is inserted into the brass tube  56 . 
     Care has been taken in the physical layout of the control circuit  90  to place components that carry large amounts of current and potentially generate the most heat, are positioned for efficient routing of the copper foil pattern and for best thermal performance. 
     In the illustrated embodiment  10 , the operating voltage is in the range of 6.0-15.0 volts (V) continuous, 16V intermittent (30 minute), and 24V jump start for 5 minutes maximum. The pull current (Pulse) is less than 50 amperes average during pull-in. The hold current (sustained) may be less than 1.0 ampere. Maximum cycle rate of the solenoid system is designed to be approximately 10 cycles in one (1) minute, non-repetitive burst, and approximately four (4) cycles per minute sustained. The preferred electrical connection  130  is a 3-pin Delphi Packard Electric Metri-Pack Series 150 male style connector that is integral to solenoid cover  100 . 
     The electrical connections include first and second primary power input terminals. A third electrical terminal provides a 12-volt nominal, 20-milliamp nominal control signal. This feature allows the primary power wiring to be routed by most direct means to battery source. Light gage control wiring may be run to point of command. 
     The following is a list of exemplary components that may be used in the circuit illustrated in FIG.  7 . These components are merely exemplary and other components could be utilized or readily substituted without departing from the scope of the present invention. 
     
       
         
               
             
               
               
             
           
               
                   
               
               
                 Exemplary Components 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Resistors 
                   
               
               
                 R1 
                  10 KOhms, 0.1 watt 
               
               
                 R2 
                  10 KOhms, 0.1 watt 
               
               
                 R3 
                  10 KOhms, 0.1 watt 
               
               
                 R4 
                  10 KOhms, 0.1 watt 
               
               
                 R5 
                  15 KOhms, 0.1 watt 
               
               
                 R6 
                  1.0 MOhms, 0.1 watt 
               
               
                 R7 
                 390 Ohms, 1.0 watt 
               
               
                 R8 
                 200 Ohms, 0.1 watt 
               
               
                 Thermistors 
               
               
                 R9 
                 150 kOhms at 25 C. 
               
               
                 Capacitors 
               
               
                 C1 
                  1.0 micro F, 16 volt 
               
               
                 C2 
                  1.0 micro F, 16 volt 
               
               
                 C3 
                  4700 pico F, 50 volt 
               
               
                 C4 
                 0.018 micro F, 16 volt 
               
               
                 Metal Oxide Varistor 
               
               
                 Z1 
                 54 volts at 5 amps; 30 volts for 5 minutes 
               
               
                 Transistors 
               
               
                 Q1 
                 Power MOSFET, 60 volts, 71 amps 
               
               
                 Rectifier Diodes 
               
               
                 D1 
                 Schottky rectifier, 7 amp, 60 volt 
               
               
                 Voltage Regulators 
               
               
                 VR1 
                 Precision shunt regulator 
               
               
                 Integrated Circuits 
               
               
                 U1 
                 Microchip PIC Microprocessor, 8 pin SOIC 
               
               
                   
               
             
          
         
       
     
     While the invention has been described in conjunction with a specific embodiment, it is to be understood that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. For example instead of a 3-pin input (J 1 - 1 , J 1 - 2 , J 1 - 3 ) for battery negative, battery positive and control signal, the same function could be performed with a 2-pin configuration. The battery positive and control signal could be tied together allowing the control signal to be obtained directly from the battery. This could be done with the above-noted 3-pin connector or with a 2-pin connector integral to the solenoid cover. The circuit function would remain the same. This eliminates the need for a separate low power signal to command the solenoid on and off. 
     Other examples include replacing the MOV with a transient voltage suppression device of a different type such as a transorb. To accomplish the digital to analog conversion, a PIC micro-controller with a D to A converter built in could be utilized. Alternatively, a different micro-controller could be utilized in place of the one time programmable micro-controller. The shunt regulator could be replaced with a series regulator to achieve the voltage regulation required by the micro-controller. 
     The foregoing is considered as illustrative only of the principles of the invention. Furthermore, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described. While the preferred embodiment has been described, the details may be changed without departing from the invention, which is defined by the claims.