Abstract:
An improved circuit supplies electrical power to a vehicle accessory load, such as a load that is powered through a vehicle accessory load connector or cigarette lighter plug. The circuit utilizes battery voltage comparator and timer switching circuitry to interrupt the supply of power to the accessory load if the engine-driven alternator fails to charge the storage battery for at least a predefined time interval (i.e., when the vehicle engine is not running), and to automatically re-establish electrical power supply to the load when battery charging resumes (i.e., when engine operation resumes).

Description:
TECHNICAL FIELD  
         [0001]    This invention relates to motor vehicle electrical accessory loads, and more particularly to a circuit and method for supplying electrical power to an accessory load during operation of the vehicle engine.  
         BACKGROUND OF THE INVENTION  
         [0002]    Various electric accessory loads can be operated in a vehicle, using the vehicle electrical system to supply the electrical power requirements of the load. Such loads may be in the form of permanently installed devices, as in the case of factory-installed equipment, or portable devices, as in the case of consumer equipment that is powered through a vehicle accessory load connector or cigarette lighter plug. Although an engine-driven alternator develops current for both charging the storage battery and supplying power to electrical loads during vehicle operation, many electrical loads consume sufficient power to discharge the vehicle storage battery if left on during a prolonged period of vehicle inactivity. Accordingly, the electrical systems in some vehicles have been designed to automatically disconnect the accessory power supply after the engine has been turned off for a predefined interval and/or if the ignition switch is off and the battery voltage falls below a reference value. See, for example, the U.S. Pat. No. 4,493,001, which is assigned to General Motors Corporation, and the earlier issued patents mentioned therein. However, many vehicle electrical systems do not have such a safeguard against battery discharging, and what is needed is an accessory load power supply circuit that will protect against battery discharging in otherwise unprotected vehicles.  
         SUMMARY OF THE INVENTION  
         [0003]    The present invention is directed to an improved circuit and method for supplying electrical power to a vehicle accessory load, such as a load that is powered through a vehicle accessory load connector or cigarette lighter plug. The circuit of this invention utilizes battery voltage comparator and timer switching circuitry to interrupt the supply of power to the accessory load if the engine-driven alternator fails to charge the storage battery for at least a predefined time interval (i.e., when the vehicle engine is not running), and to automatically re-establish electrical power supply to the load when battery charging resumes (i.e., when engine operation resumes). 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0004]    The single drawing figure is a diagram of an accessory load electric power supply circuit according to this invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0005]    Referring to the drawing, the present invention is described in the context of a conventional motor vehicle electrical system including a 12-volt storage battery  10 , an engine-driven alternator and charging circuit (ALT)  12 , and an accessory load connector or cigarette lighter plug as signified by the connectors  14 ,  15 . Also, a conventional accessory load, such as a portable resistance heater, is designated by the reference numeral  16 , and the power supply circuit of this invention is designated by the reference numeral  20 . In general, the circuit  20  compares the terminal voltage (Vbatt) of battery  10  to a reference voltage (Vref), establishes (or re-establishes) a power connection to the load  16  when Vbatt is above Vref, and interrupts the power connection if Vbatt falls below the Vref for at least a predefined interval such as 30 seconds. The implementation suggested in the drawing is on in which the circuit  20  is coupled to the accessory load connectors  14 ,  15 , and the load  16  is separate from the circuit  20 . However, the circuit  20  may be integrated into the load  16 , if desired. Other implementations are also possible; for example, the circuit  20  may be interposed between the battery  10  and the accessory load connectors  14 ,  15 .  
         [0006]    The reference voltage Vref of circuit  20 , which may be 2.5 volts for example, is established at the junction  22  between resistor  24  and zener diode  26 , and a scaled down version of Vbatt is established at the junction  28  between resistors  30  and  32 . In particular, the resistors  30  and  32  are selected so that the voltage at junction  28  is equal to Vref when Vbatt is equal to a specified switching voltage (such as 13 volts, for example) in excess of the maximum nominal open-circuit terminal voltage of battery  10  but lower than a minimum voltage produced by the alternator  12  during engine operation. The voltage at junction  28  is coupled to the non-inverting input of operational amplifier  34  via resistor  36 , and Vref is coupled to the inverting input of amplifier  34 . The amplifier  34  is referenced to Vbatt as shown, so that its output voltage on line  38  switches from ground voltage to Vbatt when Vbatt rises above the specified switching voltage (13 volts). A further resistor  40  provides a feedback voltage to the non-inverting input of amplifier  34  so that Vbatt must fall somewhat below the specified switching voltage before the amplifier output voltage on line  38  switches from Vbatt to ground. This hysteresis, along with the filter function of capacitor  42 , prevents erratic switching of amplifier  34  when the voltage at junction  28  is subject to electrical noise. Thus, the output of amplifier  34  on line  38  can be considered as a digital “Power OK” indication having a high (near Vbatt) voltage when Vbatt is above the specified switching voltage (i.e., when the vehicle engine is running and the battery  10  is being charged), and a low (near ground) voltage when Vbatt is below the specified voltage (i.e., when the vehicle engine is off, or the engine is running but the alternator  12  is inoperative).  
         [0007]    A timer circuit comprising an interconnected free running oscillator  50  and ripple counter  52  controls the operation of a power switch  54  coupled between ground voltage and one terminal of electrical load  16 . In general, the switch  54  is activated to the depicted state coupling load  16  to battery ground when the carry bit (CB) output of ripple counter  52  on line  56  has a low logic level voltage, and otherwise assumes the opposite state isolating load  16  from battery ground. Although depicted in a mechanical implementation, it is obvious that the switch  54  may be implemented electronically with a transistor or the like. The CB output of counter  52  is fed back to an inhibit (INH) input of oscillator  50  via line  58  so that oscillator  50  ceases producing clock pulses on line  60  when the CB output achieves a high logic level voltage. Additionally, the “Power OK” signal produced on line  38  by amplifier  34  is applied to a reset (RST) input of counter  52  for resetting the count of counter  52  to zero so long as the Power OK signal is high. Thus, the switch  54  couples load  16  to battery ground to enable operation of the load whenever Vbatt is above the specified switching voltage (13 volts, for example), but disconnects load  16  from battery ground to prevent further discharging of battery  10  by load  16  if Vbatt falls below the switching voltage long enough for the clock pulses of oscillator  60  to produce a high logic level voltage at the CB output of counter  52 . When the CB output goes high, further operation of the oscillator is inhibited, latching switch  54  in its power interrupt state. When Vbatt subsequently rises above the specified switching voltage, the Power OK signal on line  38  holds the count of counter  52  (and the CB output on line  56 ) at zero, and the switch  54  returns to the depicted state to enable operation of the load  16 . The clock frequency of oscillator  50  and the count capacity of counter  52  are designed so that the interval between a high-to-low transition of the Power OK signal and a low-to-high transition of the CB output of counter  52  is sufficient to prevent drop-out of the load  16  in response to battery voltage transients due to transient load conditions or even stalling and re-starting of the vehicle engine. As mentioned above, an exemplary interval for most applications is about 30 seconds.  
         [0008]    Of course it will be recognized that some or most of circuit  20  may be implemented by a suitably programmed microprocessor, which would be particularly advantageous in applications where the circuit  20  is integrated into the load  16  or some other control module that already includes a microprocessor. In such case, the functionality of oscillator  50  and counter  52  could be replaced by a variable that is periodically incremented whenever the Power OK signal is low, with the variable being reset to zero if the Power OK signal goes high, and the switch  54  being activated to the depicted state when the variable reaches a specified count. Obviously, the microprocessor could perform the function of amplifier  34  and other circuit elements as well.  
         [0009]    In summary, the power supply circuit of this invention effectively prevents discharging of a vehicle storage battery due to the use of electric accessory loads without causing nuisance interruptions, and automatically restores power to the loads as soon as possible without causing further discharging of the battery. While described in reference to the illustrated embodiment, it is expected that various modifications in addition to those mentioned above will occur to those skilled in the art. For example, simple RC timers or a microprocessor with built-in voltage comparators and A/D converters may be utilized. Thus, it will be understood that circuits incorporating these and other modifications may fall within the scope of this invention, which is defined by the appended claims.