Patent Publication Number: US-8124918-B2

Title: Positive temperature coefficient light emitting diode light

Description:
REFERENCE TO PRIOR APPLICATIONS 
     This application is a continuation of application Ser. No. 11/612,886 filed Dec. 19, 2006, now U.S. Pat. No. 7,633,037. 
    
    
     BACKGROUND 
     The present application relates generally to lighting devices. While it finds particular application to lighting devices employing one or more light-emitting diodes (LED). 
     Light-emitting diodes (LEDs) have been used in various light devices. In one such application, a flashlight has included a plurality of batteries connected electrically in series with a fixed, current-limiting resistor, an LED, and a switch that opens and closes the circuit. With the circuit so configured, the diode forward current varies as a function of both the battery voltage and the diode forward voltage. 
     However, batteries are generally characterized by a sloping discharge curve, with their output voltage decreasing as the batteries discharge. While the value of the resistor can be selected to provide a desired diode forward current when the batteries are fully charged, the current will decrease as the batteries discharge, and energy that could otherwise be used to produce useful illumination is dissipated in the resistor. The value of the resistor can also be selected to provide the desired forward current at a point relatively lower on the discharge curve. While doing so tends to reduce the power dissipated in the resistor, the diode forward current will be greater than desired when the batteries are more fully charged. Such an approach is likewise relatively inefficient, and can result in greater than desired diode power dissipation. 
     According to another approach, a switching regulator circuit configured as a current regulator has been used to drive one or more LEDs at a substantially constant forward current. While such an approach can provide improved current regulation compared to the use of a fixed current-limiting resistor, it also tends to be relatively expensive, and the switching regulator circuit and its associated circuitry can be bulky. Moreover, losses in the switching regulator circuit can have a deleterious effect on the overall efficiency. 
     SUMMARY 
     Aspects of the present application address these matters, and others. 
     In one aspect, an apparatus includes a light source, a substrate, a temperature-based controller and an insulator. The light source is mounted to the substrate. The temperature-based controller is electrically coupled to the light source and causes the light source to provide a relatively constant light output. The insulator is positioned proximate the temperature based controller. 
     In one aspect, an apparatus includes electrical contacts coupled to a LED. The apparatus further includes a positive temperature coefficient resistor in operative thermal communication and electrically in series with the LED. A resistance of the PTC resistor varies as a function of a temperature of the LED. 
     In another aspect, an apparatus includes a power receiving region, at least one LED, and a temperature-based, closed-loop controller that varies in resistance as a temperature of the at least one LED varies. 
     In another aspect, a method includes applying a forward current to a LED, whereby the forward current causes the LED to heat, sensing a temperature of the LED, and using the sensed temperature to vary a resistance of a positive temperature coefficient (PTC) resistor electrically in series with the LED to reduce the fluctuations in the forward current. 
     In another aspect, an apparatus includes a means for receiving power used to energize an LED and a means in operative thermal communication and electrically in series with the LED for reducing forward current variations of a forward current of the LED based on a temperature of the LED. 
     Those skilled in the art will recognize still other aspects of the present application upon reading and understanding the attached description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present application is illustrated by way of example and not limitation in the figures of the accompanying drawings, in which like references indicate similar elements and in which: 
         FIG. 1  is a cross-sectional view of a light emitting diode (LED) light device. 
         FIG. 2  is a schematic diagram of an electric circuit. 
         FIG. 3  depicts a block diagram of an exemplary light device. 
         FIG. 4  depicts a method of operating the LED light device. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  depicts an exemplary battery powered light  100 . As illustrated, the light  100  is configured as a handheld flashlight having a generally cylindrical housing  104 , one or more LEDs  108 , and a light management system  112 . The housing  104  defines a battery-receiving region  116 , which includes first and second electrical contacts  106 ,  110  and receives first  120   1 , second  120   2 , and third  120   3  generally cylindrical batteries. The light management system  112  includes a generally parabolic reflector  124  and a lens  128  that cooperate to direct light generated by the light source  108  so as to form a generally unidirectional light beam. A user operated switch  132  allows the user to control the operation of the light  100 . 
     With ongoing reference to  FIG. 1 , the light  100  also includes a positive temperature coefficient (PTC) resistive element  136 , a thermally conductive substrate  140 , and an optional series resistor  144  (see  FIG. 2 ). A first major surface  148  of the substrate  140  is mounted for thermal communication with the LED  108 , while a second major surface  152  of the substrate  140  is mounted for thermal communication with the PTC resistive element  136 . Consequently, the PTC resistive element  136  is in operative thermal communication with the LED  108  so that changes in the temperature of the LED  108  cause a change in the resistance of the PTC resistive element  136 . 
     In one implementation, the batteries  120  are C-size, D-size, or other batteries that each produce a nominal open circuit voltage of approximately 1.5 volts direct current (VDC). The LED  108  is a single 1 Watt (W) white LED having a nominal forward voltage threshold of approximately 3.4 VDC (with specification limits typically ranging from roughly 3 to 4 VDC) and a nominal forward current rating of about 350 milliamperes (mA). 
     The substrate  140  is fabricated from a thermally conductive material such as aluminum, copper, or the like. It should also be noted that, depending on the construction and characteristics of the LED  108 , the substrate  140  may also function as a heat sink that dissipates thermal energy generated by LED  108 . The substrate  140  may also be omitted. 
     An optional insulator may also be provided to reduce the influence of ambient temperature on the PTC resistive element  136 . Such insulator may be positioned next to and in relatively close proximity with one or more of the surfaces of the PTC resistive element  136 , which are not in thermal communication with the substrate  140 . 
     Turning now to  FIG. 2 , the switch  132 , batteries  120 , resistor  144 , PTC resistive element  136 , and LED  108  are connected electrically in series in a circuit  200 . The thermal relationship between the LED  108  and the PTC resistive element  136  is indicated by the dashed line  204 . 
     The forward current I F  through the LED  108  can be expressed as follows: 
                       I   F     =         V   Batt     -     V   F           R   Series     +     R   PTC           ,           Equation   ⁢           ⁢   1               
where V Batt  is the voltage produced by the batteries  120 , V F  is the forward voltage of the LED  108 , R series  is the resistance of the resistor  144 , and R PTC  is the resistance of the PTC resistive element  136 .
 
     As can be seen from Equation 1, the forward current I F  and hence the LED  108  power dissipation are a function of the battery voltage V Batt  and the diode forward voltage V F . As the temperature of the LED  108  is a function of its power dissipation, its temperature tends to decrease as the batteries discharge. Because the PTC resistive element  136  is in operative thermal communication with the LED  108 , the resistance of the PTC resistive element  136  likewise decreases, thus tending to increase the forward current I F . Thus, the circuit can be viewed as acting a temperature-based, closed-loop controller that tends to reduce or otherwise compensate for changes in diode forward current I F  that would otherwise occur as the batteries discharge. The circuit  200  similarly compensates for changes in the diode forward voltage V F , as may occur, for example, as the LED temperature changes or due to piece-to-piece or lot-to-lot variations in the LEDs. 
     Suitable values of R Series  and R PTC  in one example, can be determined according to the electrical and thermal characteristics of a particular light  100 , the desired efficiency, and similar factors. For instance, R Series  and R PTC  may be chosen to drive the LED  108  at about its maximum rated current level to maximize the brightness of the emitted light. In another instance, R Series  and R PTC  may be chosen to drive the LED  108  at a lower forward current to relatively improve efficiency and extend the life of the batteries  120 , although the nominal light output will be dimmer. In one such implementation, the nominal forward current is established at or near the LED&#39;s maximum luminous efficiency. 
     In one example embodiment, the PTC resistive element  136  is a polymeric PTC (PPTC) device. Such devices are also sometimes referred to as thermally resettable fuses, thermostats, or non-linear thermistors. A PPTC device generally includes a matrix of crystalline organic polymer with dispersed conductive carbon black particles. These particles change their physical properties as a function of temperature, which changes their electrical properties to be less or more electrically conductive. By way of example, if the current passing through the PPTC device exceeds an electrical current threshold, the PPTC device heats and expands, which causes the carbon particles to separate, breaking conductive pathways and, thus, causing the resistance of the device to increase. As the PPTC device cools, it contracts and its resistance decreases. 
     A non-limiting example of a suitable PTC device is discussed in U.S. Pat. No. 5,985,479 to Boolish, et al. (filed Nov. 14, 1997), which is incorporated herein by reference. 
     By employing the PTC element  136  as described herein, variations in the LED forward current can be reduced for a relatively wide range of supply voltages. By way of example, the PTC element  136  is especially well-suited for applications utilizing 1.5 VDC alkaline batteries (e.g., Zn/MnO 2 ) or other battery chemistries with similar voltage discharge properties. The voltage discharge curve of such batteries is generally characterized as non-linear with a relatively rapid and steep drop off, which tends to be relatively steeper when the batteries are fully charge or discharged, and the slope of the curve increases as the current is increased. Using the PTC element  136  to reduce forward current variations or fluctuations as described herein with such batteries can be used to provide a relatively more constant light output relative to a configuration without the PTC element  136  in which the light output follows and dims with the discharging voltage of the batteries. 
     The battery voltage range may also be due to using different battery chemistries. For example, Carbon Zinc (CZn), lithium iron disulfide (LiFeS 2 ), alkaline (zinc-manganese dioxide), nickel-cadmium (NiCd), and nickel metal hydride (NiMH) chemistries are generally physically interchangeable. However, CZn, LiFeS 2  and alkaline chemistries have a nominal open circuit voltage of about 1.5 VDC, whereas NiCd and NiMH have a nominal open circuit voltage of about 1.2 VDC. Thus, using three alkaline batteries provides an aggregate nominal open circuit voltage of 4.5 VDC, whereas using three NiMH batteries provides an aggregate nominal open circuit voltage of 3.6 VDC. Without the PTC element  136 , these voltage differences may result in relatively large forward current differences, depending on the battery chemistry. However, the PTC element  136  can be used to compensate for these voltage differences as described above, thus tending to reduce performance variations that may result from the use of batteries having different chemistries. In addition, R Series  and R PTC  can be selected to accommodate a range of battery voltages. 
     Variations are also contemplated. 
     While the above discussion has focused on a light  100  having three batteries, other battery configurations are contemplated herein. For instance, the battery-receiving region  116  may be alternatively configured to accept only a single battery  120 , two batteries  120 , or more than three batteries  120 . In one example, the light  100  is configured to accept two (2) AA size batteries, and the one or more LEDs  108  includes three (3) 72 milliwatt (mW) LEDs. 
     The battery-receiving region  116  may also be configured to receive lithium-ion (Li Ion) or other battery chemistries. Thus, in addition to receiving batteries having a nominal open circuit voltage of 1.2 VDC and 1.5 VDC as noted above, the light  100  receives batteries having nominal open circuit voltages of 1.8 VDC or 3.6 VDC, as well as other voltages. 
     Other wattages of LEDs may also be provided, as may colors other than white. Examples of suitable colors include cyan, green, amber, red-orange, and red. 
     Suitable LEDs also include LEDs that emit radiation having a wavelength outside of the visible light portion of the electromagnetic spectrum, including radiation having wavelengths within the infrared (IR) and ultraviolet (UV) portion of the electromagnetic spectrum. 
     Two or more of the LEDs may also be connected electrically in series or parallel. In one implementation, two or more LEDs are mounted to the same substrate, and the substrate is thermally coupled to a single PTC resistive element  136  as described herein. In another instance, each of a plurality of LEDs is mounted to its own corresponding substrate. With this configuration, a single PTC element  136  may be thermally coupled with only one of the LEDs  108  as described above so that the PTC element  136  responds to temperature changes in the thermally coupled LED  108  or a different PTC element  136  may be thermally coupled to each of the LEDs  108  as described herein so that each PTC element  136  responds to a corresponding one of the LEDs  108 . 
     The light  100  may also include more than one independently controllable LED  108 , batteries  120 , and/or circuits  200 . For example, one LED  108  may provide a light beam while another serves as an area light. 
     The illustrated embodiment is discussed with respect to a flashlight emitting a unidirectional light beam. However, the light  100  may also be configured otherwise, for example, as an area light, a lantern or a headlamp. The light  100  may also include one or more flat surfaces which facilitate placement thereof on surface. It may also include suitable clamps, brackets, cut and loop fasteners, magnets, or other fasteners for selectively attaching the light device  100  to an object. 
       FIG. 3  depicts a block diagram of an exemplary light  300  having an electrical power interface  304 , a switch  308 , a positive temperature coefficient resistive element  312  such as the PTC resistive element  136 , and a light source  316  such as the one or more LEDs  108 . Power for energizing the light source  316  is received via the electrical power interface  304 , which may receive power from various power sources including but not limited to a battery source, an alternating current source, an external power source. The switch  308  is used to open or close an electrically conductive path electrically connecting the electrical power source  304  and the light source  316 . 
     The positive temperature coefficient resistive element  312  is in operative thermal communication with the light source  316 , and the resistance of the positive temperature coefficient resistive element  312  changes as a function of the temperature of the light source  316 . In one instance, the positive temperature coefficient resistive element  312  is configured so that its resistance changes in a manner so as to reduce variations in the current flowing through the light source  308  for a relatively wide range of supply and light source  316  voltages. Optionally, a thermally conductive substrate  320  such as the thermally conductive substrate  140  is disposed between and in thermal communication with the temperature coefficient element  312  and the light source  316 . 
     The lights  100  and  300  can be used in various light applications. For example, the light  300  may be used as a domestic, industrial, or commercial lights, including but not limited to a flashlight, a floor lamp, a head lamp, a desk lamp, an interior light, an exterior light, an automotive vehicle light, a safety lamp, an under the counter light, a recessed light, as well as other lights. In addition, the lights  100  and  300  may be included in hand-held devices such mobile phones, personal data assistants (PDAs), gaming systems, and the like, and other applications such as motor vehicles (having a 12 VDC battery), domestic appliances, and industrial appliances. 
     The PTC element  136  can similarly be employed in applications that receive power from power sources other than batteries. In such applications, the PTC element  136  can be used as described herein to compensate for voltage ranges and variations in such power sources and LED forward voltage variations when using such voltage sources. 
     Operation of the lights  100  and  300  is now described in relation to  FIG. 4 . 
     At  404 , a forward current is supplied to the light LED. 
     At  408 , the forward current causes the LED to heat. 
     At  412 , the temperature of the LED is sensed. 
     At  416 , the sensed temperature varies a resistance of a positive temperature coefficient (PTC) resistor electrically in series with the LED so as to reduce variations in the forward current supplied to the LED. 
     The invention has been described with reference to the preferred embodiments. Of course, modifications and alterations will occur to others upon reading and understanding the preceding description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims and the equivalents thereof.