Patent Publication Number: US-9839093-B1

Title: Self-healing overtemp circuits in LED lighting systems

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119 to provisional U.S. application Ser. No. 62/190,941, filed Jul. 10, 2015, which is hereby incorporated by reference in its entirety. 
    
    
     I. BACKGROUND OF THE INVENTION 
     The present invention generally relates to what will be referred to herein as “overtemp circuits”; namely, circuits included in electrical designs which remove or reduce power supplied to one or more components when a temperature (e.g., junction temperature, ambient temperature) exceeds a threshold. More specifically, the present invention relates to overtemp circuits in LED lighting systems, and apparatus, means, and methods for preventing or mitigating undesirable lighting effects that occur after the temperature threshold issue is resolved and power is returned to the one or more components of the lighting system. 
     It is well known that in recent years the reduction in cost and increase in luminous efficacy (lm/W) of LEDs has permitted their use beyond novelty and general purpose lighting and into areas of more specialized lighting. For specialized lighting applications such as wide area or sports lighting, often a large number of LEDs (e.g., many hundreds for a single tennis court) are required to provide uniform lighting that meets minimum requirements—see, e.g., Illuminating Engineering Society (IES) RP-06-01  Recommended Practice for Sports and Recreational Area Lighting  for examples of such lighting requirements. As is also well known in the art, high luminous efficacy—a primary selling point of LEDs—is only realized if LED junction temperature is kept low. Thus, it stands to reason that using many hundreds (if not thousands) of LEDs so to adequately light a sports field to one or more standards (as dictated by governing bodies, municipalities, or otherwise) cannot be done in a cost-effective manner unless measures are taken to control the temperature of said LEDs. 
     In the art of LED wide area or sports lighting there currently exist two approaches to controlling junction temperature: passive and active cooling. Passive cooling techniques are generally defined as means which do not require external forces or, to some extent, moving parts. An external fixture housing which is designed to promote airflow, formed from thermally conductive material, and includes a number of heat fins to increase surface area is an example of a passive cooling technique in LED lighting design; another is the inclusion of heat pipes or thermosyphons such as is discussed in U.S. Provisional Patent Application Ser. No. 62/118,675 incorporated by reference herein in its entirety. Active cooling techniques in the current art of LED lighting design typically center around forced air or fluid in, through, around, or generally proximate heat sources (e.g., LEDs); some examples are described in U.S. Pat. Nos. 8,651,704 and 9,028,115 which are incorporated by reference herein in their entirety. It is generally understood that active cooling techniques are more aggressive and remove or redistribute heat from an LED lighting system more effectively than passive cooling techniques. 
     If passive or active cooling techniques fail (e.g., power to a fan is disabled), one would expect the temperature of the LEDs to increase and efficacy to decrease. If said cooling techniques are applied system-wide, the temperature of other components (e.g., drivers) may increase in commensurate fashion upon such a failure. An increase in temperature of an LED lighting system, left unmitigated, could reduce cost effectiveness and damage parts. 
     It is logical, then, that if such a thermal runaway event occurred—as it will be called herein—a potential solution would be to temporarily terminate power to the LEDs and/or other temperature sensitive components until the situation is resolved and components cool (e.g., via natural convection). One can think of such a solution as similar to GFCI circuits common in other areas of electrical design; a safety feature that only triggers in extreme events. However, for specialty LED lighting applications such as wide area and sports herein lies a problem—the high voltages required for the large number of LEDs prevents implementation of a traditional overtemp circuit. There are no high voltage (e.g., 1000V) solutions to thermistors or bi-metallic switches that would permit detection of a thermal runaway event and act to open a circuit, thereby terminating power to the LEDs. Thermal fuses are terminal event devices—if a thermal fuse opens a circuit to mitigate a thermal runaway event, that fuse must be replaced before power to the LEDs can be restored. Given that wide area and sports lighting applications typically have the aforementioned hundreds (if not a thousand or more) LEDs mounted several dozens of feet in the air in an environmentally sealed housing, replacing thermal fuses in a luminaire (also referred to herein as a fixture) is highly impractical. 
     The art is at a loss. LEDs operated in large number under high voltage conditions—such as in wide area or sports lighting applications—are prime candidates for passive and active cooling techniques, and failure of said cooling techniques would likely result in thermal runaway thereby also making such lighting applications prime candidates for overtemp circuits. That being said, there are no adequate overtemp circuits commercially available to address high voltage LED lighting systems. Thus, there is room for improvement in the art. 
     II. SUMMARY OF THE INVENTION 
     In the current state of the art of LED lighting design it is well known that to make LEDs cost effective as compared to more traditional light sources for wide area and sports lighting (e.g., HID lamps) temperature control is critical. While passive and active cooling techniques exist for LED lighting systems, they are not impervious to failure. In the event of a thermal runaway event, the art may benefit from an overtemp circuit which could temporarily terminate or reduce power to the LEDs until the situation is resolved and/or LED temperature decreases. That being said, there are no adequate high voltage solutions to traditional overtemp circuits (e.g., those which rely upon mechanical means to open a circuit). Further, even if such overtemp circuits existed for LED lighting systems operating around 1000V, there is still the matter of restoring power—either from a reduced power or no power state—after the thermal runaway event has been resolved, and in a manner that mitigates undesirable lighting effects and potential component damage. 
     It is therefore a principle object, feature, advantage, or aspect of the present invention to improve over the state of the art and/or address problems, issues, or deficiencies in the art. 
     Envisioned is an overtemp circuit designed for LED lighting systems operating at high voltage, said overtemp circuit self-healing in the sense that once a thermal runaway event has been resolved, the overtemp circuit permits power to the LEDs to be reestablished without replacement of parts or intervention from an operator (for at least some configurations). Said envisioned overtemp circuit further includes apparatus or means for (i) dissipating excess voltage that builds up during the time in which there is no load on the driver, or (ii) cycling a driver to prevent startup at an excessive voltage. In either scenario, excess voltage could result in damage to the LEDs (or other components) when power is reestablished. 
     Further objects, features, advantages, or aspects of the present invention may include one or more of the following:
         a. apparatus, methods, and/or means to detect a thermal runaway event in an LED lighting system;   b. apparatus, methods, and/or means to reduce or terminate power to impacted LEDs and/or other components in response to detecting a thermal runaway event; and   c. apparatus, methods, and/or means to reestablish full or partial power to said LEDs and/or other components when the thermal runaway event has been resolved;   d. said apparatus, methods, and/or means:
           i. robust (i.e., not prone to breakage or fatigue);   ii. not formed from terminal event devices (i.e., devices such as fuses which are designed to fail and require replacement in accordance with a threshold);   iii. having multiple options for being supplied with power;   iv. having multiple options for signal communication;   v. having an option for overcurrent protection; and   vi. having at least some degree of selectivity so to adjust thresholds and define conditions for a thermal runaway event based, at least in part, on the characteristics of the load (e.g., wiring and number of LEDs).   
               

     A method according to aspects of the present invention generally comprises recognizing a thermal runaway event, sending a signal to a controller which instructs a driver to reduce (or remove) power provided to a load (e.g., an array of LEDs in series), and generating a second signal to instruct the driver to reestablish power provided to said load when (i) the thermal runaway event has been resolved and (ii) there is little to no risk of an inrush of excess voltage or current to the load. 
     An apparatus according to aspects of the present invention generally comprises an overtemp circuit including a temperature sensing circuit, a current sensing circuit, a voltage sensing circuit, and a processor. Said processor is adapted to provide instruction to a driver (with associated controller) in response to sensed temperature and current values using sensed voltage as guidance in determining driver characteristics prior to providing instruction. The sensed voltage provides, in essence, a feedback loop thereby ensuring that with proper thresholds in place, said apparatus only permits power to be reestablished when (i) the thermal runaway event has been resolved and (ii) there is little to no risk of an inrush of excess voltage or current to the load. 
     These and other objects, features, advantages, or aspects of the present invention will become more apparent with reference to the accompanying specification and claims. 
    
    
     
       III. BRIEF DESCRIPTION OF THE DRAWINGS 
       From time-to-time in this description reference will be taken to the drawings which are identified by figure number and are summarized below. 
         FIG. 1  illustrates a typical prior art sports lighting application including generic LED lighting fixtures. 
         FIG. 2  illustrates the generic lighting system of  FIG. 1  modified according to at least some aspects of the present invention. 
         FIG. 3  illustrates one possible method of practicing the invention according to a first embodiment. 
         FIG. 4  illustrates, in wiring diagram form, a self-healing overtemp circuit for use in a lighting system such as that illustrated in  FIG. 2  in accordance with a method such as that illustrated in  FIG. 3 . Electrical symbols denoting resistors having 0Ω illustrates jumpers for single-sided boards and electrical symbols denoting “TP” illustrate test points used in troubleshooting. 
         FIG. 5  illustrates the power supply circuit for the self-healing overtemp circuit according to a first embodiment. 
         FIG. 6  illustrates the voltage sensing circuit of the self-healing overtemp circuit according to a first embodiment. 
         FIG. 7  illustrates the temperature sensing circuit of the self-healing overtemp circuit according to a first embodiment. 
         FIG. 8  illustrates the current sensing circuit of the self-healing overtemp circuit according to a first embodiment. 
         FIG. 9  illustrates the switching circuit of the self-healing overtemp circuit according to a first embodiment. 
         FIG. 10  illustrates the controller circuit of the self-healing overtemp circuit according to a first embodiment. 
     
    
    
     IV. DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     A. Overview 
     To further an understanding of the present invention, specific exemplary embodiments according to the present invention will be described in detail. Frequent mention will be made in this description to the drawings. Reference numbers will be used to indicate certain parts in the drawings. Unless otherwise stated, the same reference numbers will be used to indicate the same parts throughout the drawings. Likewise, frequent mention will be made to various components and circuits. Reference numbers will be used to indicate certain portions of circuits. For simplicity, all circuit portions are referred to herein as “circuits”, regardless of whether said referenced portion comprises a complete circuit in and of itself. Further, reference is made herein to “user(s)”, “designers”, and/or “operators”. It is to be understood that these terms are used for convenience, and in no way place limitations on who may practice the invention, or benefit from aspects thereof. Further, it should be noted that the terms “luminaire”, “fixture”, and/or “lighting fixture” are used interchangeably herein; these terms are used for convenience, and in no way place limitations on load characteristics, circuit design, or other aspects of the present invention. Lastly, communications between electrical components are referred to herein as “pulses”, “communications”, “signals”, and/or “instructions”. It is be understood that communications could take a variety of forms and that the aforementioned terms—used interchangeably for convenience—in no way limits the invention to a particular communication mode, bandwidth, or the like. 
     The exemplary embodiments envision an overtemp circuit formed from solid state components designed to disable or reduce power to a load when a threshold indicative of a thermal runaway event is exceeded, or in some cases approached, and then reestablish power at a desired level after the thermal runaway event has been resolved (and/or when temperature is below some threshold and the driver has been cycled). As envisioned, the overtemp circuit is formed from components which permit selectivity of the threshold based, at least in part, on characteristics of the load. In practice, the overtemp circuit could be applied to a number of electrical systems and many different kinds of loads; though the following discusses an LED lighting system, the present invention is not limited to such. 
       FIG. 1  illustrates a generic LED lighting system in which a pole  10  elevates one or more LED lighting fixtures  1001 , said fixtures oriented and aimed so to project light generally towards a target area  20  (e.g., a sports field); lighting fixtures could be designed in accordance with U.S. Pat. No. 8,789,967, or otherwise. Generally, power to the LEDs (i.e., the load) is supplied along power wiring from a distribution source  60  at some voltage and amperage regulated by national, state, or local codes (e.g., 480 VAC, 60 Hz). Said codes may also require equipment grounding, distribution panels, a main power disconnect, or the like; for brevity, no such functionality is illustrated. Power wiring generally enters hollow pole  10 , then enclosure  40 —where it is regulated by some combination of controllers  50  and drivers  30  (here, one driver per fixture, though this is by way of example and not by way of limitation)—before continuing along pole  10  to said fixtures  1001 . 
     Power from a distribution source  60  is often ill-suited for direct application to a load. Incoming power may be three-phase alternating current (AC) whereas an LED requires direct current (DC). The forward voltage drop across an LED (e.g., any of the XLamp series of LEDs available from Cree, Inc., North Carolina, USA) may only be on the order of 3V, but in large number (e.g., as may be required in the example of  FIG. 1 ) may greatly exceed incoming power. Considerations such as these dictate the type and number of drivers. Likewise, in a typical lighting system such as that illustrated in  FIG. 1  there may be a need to alter duty cycle (e.g., to manage temperature), adjust current (e.g., in response to some sensor input), or a modification to some other aspect of operation to effectuate a lighting change (e.g., provide dimming). Considerations such as these dictate the type and number of controllers. While a particular combination of controller and driver may be such as is described in U.S. Provisional Patent Application Ser. No. 62/040,741 incorporated by reference herein in its entirety, it is to be understood that it is impractical to discuss every configuration of power control in LED lighting systems, and that a thorough understanding of each is not necessary to practice the invention. 
     Specialty lighting systems such as the sports lighting system of  FIG. 1  typically rely on several hundreds of LEDs so to provide adequate illumination of a target area to one or more standards; often this requires running one or more long strings of LEDs in series for each associated driver (e.g., so to ensure all light sources within a single luminaire act in unison and emit uniformly). To accommodate a long string of series-connected LEDs (e.g., several hundreds with a forward voltage drop on the order of 3V each) given distribution power at 480V, 277V, or even 208V (depending on the geographic area, local codes, and the like), a boost-type driver is needed. It is assumed one of ordinary skill in the art of circuit design, and more specifically LED lighting circuit design and/or power supply design, is well aware of how boost-type converter circuits operate, and so for brevity any discussion of such is omitted. 
     With respect to the aforementioned boost-type LED drivers, it is well known that when the load (e.g., lighting fixture  1001 ) is removed from the boost-type driver circuit—as might be the standard protocol for a thermal runaway event—the MOSFET (or similar switching device) associated with the boost-type driver circuit is, in essence, switched off, thereby acting as a current source to an infinite load (an open circuit) until the output voltage reaches a maximum. What may not be well known is that when the load is returned and the circuit closed, there is an inrush of voltage as energy is supplied to the load; this may be harmful to the LEDs (i.e., the load). Initial experiments have shown that for boost-type drivers designed for a maximum 700V output, an inrush voltage of 800V is actually supplied under these conditions—almost instantaneously. Results have shown a best case scenario to be a disabling bright flash of light and a worst case scenario to be a damaging of parts (e.g., the LEDs themselves). 
     Presented herein are apparatus, methods, and means for dissipating the excess charge that stores in an LED boost-type driver when the circuit is opened in response to a thermal runaway or overcurrent event; according to one example, by dimming down to 0% (i.e., reducing driver output to 0V or near 0V). Also presented herein are apparatus, methods, and means for closing the circuit and reestablishing power to the load once both (i) the thermal runaway or overcurrent event has been resolved and (ii) driver output will not damage the LEDs or other components of the lighting system. This is achieved, in one example, by a combination of sensing circuits, logic, and pulse generator. 
     Communications between the drivers and controller at the base of the pole (see, e.g., reference nos.  30  and  50  of  FIG. 1 , respectively) and envisioned overtemp circuit components at the top of the pole rely, in one example, upon a combination of existing power wiring and the residual potential in the driver at 0% dimming; this ensures no extra wiring is needed to add the self-healing overtemp functionality to the lighting system, though of course, this is by way of example and not by way of limitation. 
     A more specific exemplary embodiment, utilizing aspects of the generalized example described above, will now be described. 
     B. Exemplary Method and Apparatus Embodiment 1 
       FIG. 2  illustrates the sports lighting system of  FIG. 1  modified according to aspects of the present invention. As can be seen, the lighting system includes some kind of active (or passive) cooling—in this example, fans  200  which are indicated generically by blocks—located proximate the temperature-sensitive components of the lighting system (e.g., the LEDs). Active cooling could be powered from a battery or line power, could be in accordance with the aforementioned incorporated references (or otherwise), or could even cool other components of the lighting system (e.g., drivers); the exact configuration may vary as is needed for the thermal management demands of the load. Most pertinent to the present invention is simply that a failure or absence of cooling means will cause an undesirable rise in temperature of one or more components of the system (at least under some operating conditions). 
       FIG. 2  also illustrates an exemplary self-healing overtemp circuit  1000 , details of which are illustrated in  FIGS. 4-10 . As is illustrated in the figures, according to the present embodiment circuit  1000  is installed in the fixture itself; as envisioned, on the same board as the LEDs contained within the fixture. While this does permit highly accurate readings from temperature sensing circuit  1005  which can be used to determine LED junction temperature, it is to be understood some or all of the functionality of self-healing overtemp circuit  1000  could be installed elsewhere in the lighting system; additional options are later discussed. 
       FIG. 3  illustrates one possible method of practicing the invention with respect to  FIG. 2 . According to method  500 , a thermal runaway event occurs (step  501 ); overcurrent conditions are later discussed. Step  501  could comprise a failure of passive or active cooling component  200 , some kind of weather event, structural failure of one or more components of the lighting system—any number of events or conditions which causes an undesirable temperature increase in the LEDs or other temperature-sensitive parts of the lighting system. A temperature sensing circuit detects an increase in value and in response to approaching or exceeding some limit a controller circuit sends a signal to a switching circuit to open the lighting circuit (step  502 ). In response to opening the lighting circuit there is a corresponding rise in the voltage output of the driver (step  503 ). A voltage sensing circuit detects the increase in voltage and in response to approaching or exceeding some limit, a communication circuit sends a signal to the driver to dim down to 0% (step  504 ). The output of the driver decreases (step  505 ). At some point the temperature of the system will decrease (step  506 ). It is preferred if a decrease in measured temperature is the result of the thermal runaway event being resolved, though the invention is not limited to such. Once both the temperature (as indicated by the aforementioned temperature sensing circuit) and the output voltage of the driver (as indicated by a high voltage temperature sensing circuit at the fixture) are below defined thresholds, the controller circuit sends a signal to the switching circuit to close the circuit (step  507 ). Simultaneously, or after the circuit is closed, the communication circuit sends a signal to the driver to re-establish power to the load. Power is reestablished by sending an instruction to the LED driver to increase dimming to some level (e.g., to 100%, to the dimming level set just before the thermal runaway event), which causes a commensurate increase in light output of the LEDs (step  508 ). It is of note that only some LED drivers are adapted to receive such instruction directly, whereas other designs of driver may require an associated controller. 
     Specific details and functionality of self-healing overtemp circuit  1000  is illustrated in  FIGS. 4-10 . As envisioned, circuit  1000  is formed from solid state, non-terminal event components; this ensures a robust design which requires little to no intervention from a user. Further, and as will be discussed, a number of components of circuit  1000  are selectable either in terms of quantity or value. This selectivity permits a designer significant flexibility in developing the various thresholds and triggers mentioned above in the description of method  500 . Additional features and options are later discussed. 
       FIG. 4  illustrates (in wiring diagram form) self-healing overtemp circuit  1000 . As envisioned, overtemp circuit  1000  is installed in fixture  1001 —preferably on the same board as the LEDs to ensure more accurate temperature readings—though as will be discussed, the invention is not limited to such. After a thermal runaway event has occurred (step  501 ,  FIG. 3 ) the temperature of the LEDs is expected to rise. This temperature rise is detected by a temperature sensing circuit  1005 , shown in detail in  FIG. 7 . As can be seen from  FIG. 7 , temperature sensing circuit  1005  generally comprises an analog temperature sensor, amplifiers, and a number of standard capacitors and resistors. In practice, different configurations of lighting fixtures (i.e., loads) are thermally cycled and three values measured: temperatures at the LED solder point, temperature at the board where temperature sensing circuit  1005  resides, and input voltage. Knowing the solder point temperature, input voltage, and LED characteristics such as thermal resistance (commonly available from the LED manufacturer) one can readily calculate junction temperature of the LEDs using well known formulas. In this manner, a temperature at temperature sensing circuit  1005  (i.e., at the temperature sensor) is correlated to an LED junction temperature. Operationally, a lighting designer can select a high temperature reading which correlates to a high junction temperature to use as a trigger for opening the circuit, and a low temperature reading which correlates to a low junction temperature to use as a trigger for closing the circuit. 
     Temperature readings from temperature sensing circuit  1005  are fed to controller circuit  1007  of  FIG. 10 . As can be seen from  FIG. 10 , controller circuit  1007  generally comprises a microprocessor and a number of standard capacitors and resistors. Controller circuit  1007  acts as the hub of self-healing overtemp circuit  1000 —as can be ascertained from the number of inputs and outputs illustrated. Controller circuit  1007 , and components of overtemp circuit  1000  on the whole, is powered by a power circuit  1002  (see  FIG. 5 ). As can be seen from  FIG. 5 , power circuit  1002  comprises a depletion mode MOSFET, amplifier, transistor, fast switching diode, and a number of standard capacitors and resistors. Additional power supply options are later discussed. 
     When temperature readings from temperature sensing circuit  1005  correlate to a thermal runaway condition (i.e., a high junction temperature) as determined by the settings of controller circuit  1007 , controller circuit  1007  sends a signal to switching circuit  1008  of  FIG. 9  which, in turn, opens the circuit (step  502 ,  FIG. 3 ). In practice, a temperature reading of 50° C. at the temperature sensor of circuit  1005  might correlate to an LED junction temperature of 80° C. for a particular design of lighting fixture, and may necessitate circuit  1007  sending a control signal to circuit  1008  to open the circuit. As can be seen from  FIG. 9 , switching circuit  1008  generally comprises a number of MOSFETs (some in enhancement mode), transistors, and a number of standard capacitors and resistors. 
     Once the circuit is open due to the thermal runaway event, voltage output of the boost-type LED driver climbs (step  503 ,  FIG. 3 ). Voltage increase is measured by voltage sensing circuit  1004  of  FIG. 6 . As can be seen from  FIG. 6 , circuit  1004  generally comprises an amplifier and a number of standard capacitors and resistors. In practice, voltage sensing circuit  1004  is different than temperature sensing circuit  1005  insomuch that the signal is already existing and does not need to be generated (i.e., the output of the driver already exists), requiring only conditioning (see e.g., the string of resistors in  FIG. 6 ); but voltage sensing circuit  1004  is the same as temperature sensing circuit  1005  insomuch that the input provided to controller circuit  1007  triggers an instruction based, at least in part, on characteristics of the load. For example, for some loads a voltage input into controller circuit  1007  correlating to 50V may indicate a deleterious condition, but if the load is different (e.g., number of LEDs in a series-connect string are varied), that same 50V may no longer indicate a deleterious condition. 
     When voltage readings from voltage sensing circuit  1004  correlate to an excessively high driver output voltage due to open circuit as determined by the settings of controller circuit  1007 , controller circuit  1007  sends a communication to the driver to dim to 0% (step  504 ,  FIG. 3 ). This communication can be sent wirelessly; communication circuit  1006  ( FIG. 4 ) could be completed by mating to a wireless controller board (e.g., Bluetooth Low Energy (BLE) board) at the top of the pole (possibly in the lighting fixture) which could send the communication wirelessly to a complementary Wi-Fi controller board at the driver (or controller for the driver) at the bottom of the pole. This wireless option may be preferable in situations where wireless communications are permitted and cost is not the prevailing concern; two other signal/communication options are later discussed. 
     In practice, LED driver output voltage will likely decrease to below some threshold—which is defined by controller circuit  1007  ( FIG. 10 )—before the temperature of the LED fixture decreases below the temperature threshold defined by controller circuit  1007 ; this is indicated by steps  505  and  506  of method  500 , respectively. Again, thresholds may be varied by a designer in accordance with preferences, characteristics of the load, or otherwise. Further, because upper and lower thresholds are defined at controller circuit  1007 , the circuit is self healing; self-healing in the sense that no intervention from an operator is required to reestablish power to the lighting circuit after a fault related to temperature or voltage (step  507 ,  FIG. 3 ). That being said, as previously discussed, certain types of drivers (e.g., boost-type drivers) are prone to supplying an inrush voltage far exceeding what is safe and/or required for a load when power is reestablished under the conditions described. Therefore, as part of ramping light back up (step  508 ,  FIG. 3 ) power may need to first be cycled. This could happen automatically or semi-automatically (e.g., implemented as part of standard driver controller protocol) depending on the type of driver and associated controller. Also, as will be discussed, the nature of the communications means may impact how power is cycled pursuant to step  508 . 
     1. Power Supply Options 
     As previously stated, power for self-healing overtemp circuit  1000  is provided by power supply circuit  1002  of  FIG. 5 . Other power supply options are possible, and envisioned. For example, power for self-healing overtemp circuit  1000  may be pulled from line voltage; this eliminates the need for additional wiring in the interior of pole  10  ( FIGS. 1 and 2 ). The interior space of a lighting pole is typically already filled with wires, wire harnesses, and the like—which may impact an airflow path for some active cooling techniques—and so minimizing the impact to the already crowded space within the interior of such a pole may yield multiple benefits. Of course, line voltage (e.g., 700-800 VDC post-driver on its way to the top of the pole) is ill-suited to the components of self-healing overtemp circuit  1000 , and so some conditioning may be required. 
     As a further alternative, power for self-healing overtemp circuit  1000  could be provided by a battery system; such a system might be similar to that in U.S. Pat. No. 8,946,991, or otherwise. 
     2. Signal/Communication Options 
     As previously stated, communication from controller circuit  1007  of  FIG. 10  so to facilitate method  500  of  FIG. 3  is wireless; i.e., communicated to communication circuit  1006  ( FIG. 4 ) which is operationally connected to a commercially available wireless controller (not illustrated), which communicates wirelessly with a complementary wireless controller (not illustrated) at the bottom of the pole. Another option is to use existing power wiring as the communications means. 
     A wired configuration relying on powerline communications would require a nominal potential at the driver to carry a signal; therefore step  505  of  FIG. 3  would be modified insomuch that the driver would not dim to 0% but to some higher percentage that was still below the excitation voltage of the LEDs (e.g., 1-5%). With a signal path in place, controller circuit  1007  could provide communication directly to the driver or controller associated with the driver along existing wiring. If the microprocessor of controller circuit  1007  did not have capability to send such a communication pulse, controller circuit  1007  could be modified to include a pulse generator circuit (e.g., including a 555 timer IC). 
     Alternatively, self-healing overtemp circuit  1000  could exist as a standalone option; namely, with no communication means to reestablish power to the driver. In such a scenario a user would likely ascertain when a lighting fixture had likely cooled, and could manually flip a circuit breaker to reset the AC input to the driver (i.e., cycle power to the driver). The standalone option would not permit a slow ramp-down of light—the driver would immediately go to 0%—but a standalone version might be preferential for situations where an overcurrent condition is suspected, as it requires a manual override from a user who, presumably, would be equipped to troubleshoot driver failures. Alternatively, if self-healing overtemp circuit  1000  is in operative communication with remote control means for the lighting system, the remote control means may be used to cycle power. One possible example of remote control means having functionality for cycling power upon input from self-healing overtemp circuit  1000  may be as is described in U.S. Pat. No. 7,209,958 incorporated by reference herein in its entirety. 
     Overcurrent Protection 
     Overcurrent is a situation where excessive current is provided to the load. In traditional electrical systems, an overcurrent condition is associated with a grounding fault or a short in the circuit. In the specialty LED lighting system of  FIGS. 1 and 2 , though, an overcurrent condition can be associated with failure of a particular component within the aforementioned driver. An overcurrent condition can arise when said component—an optocoupler—fails and the driver no longer has an accurate reading of output current. Results have shown that left unchecked, output current goes to the maximum rated for the driver, which in turn increases heat and damages parts (particularly the LEDs). If desired, self-healing overtemp circuit  1000  could include a current sensing circuit such that method  500  of  FIG. 3  could also detect an overcurrent condition. As can be seen from  FIG. 8 , current sensing circuit  1009  generally comprises a current sensing resistor, amplifier, and a number of standard capacitors and resistors. In practice, current sensing circuit  1009  is similar to voltage sensing circuit  1004  insomuch that the signal is already existing and does not need to be generated (i.e., the output of the driver already exists), requiring only conditioning; and similar insomuch that that the input provided to controller circuit  1007  triggers an instruction based, at least in part, on characteristics of the load. In practice, method  500  would proceed similarly, but taking into account the additional current input to controller circuit  1007 . 
     C. Options and Alternatives 
     The invention may take many forms and embodiments. The foregoing examples are but a few of those. To give some sense of some options and alternatives, a few examples are given below. 
     Exemplary embodiments have addressed electrical circuits in terms of a load comprising LEDs, a thermal runaway event indicative of failure of active or passing cooling, an overcurrent condition indicative of failure of a driver component, and triggering circuits based on (i) output voltage of a boost-type LED driver, (ii) output current of said driver, and (iii) LED temperature. It is to be understood that there are a number of options and alternatives that could be explored without departing from at least some aspects according to the present invention. For example, while there is a benefit to measuring temperature of the LEDs—since efficacy is so closely tied to junction temperature in LEDs—temperature could be measured with respect to other parts of the system (e.g., the driver or controller). Temperature sensing circuit  1005  could be installed anywhere, regardless of whether LED temperature or some other temperature was being measured. This is likewise true for voltage measurements. If a different power supply was used which exhibited different characteristics but still resulted in an undesirable effect, voltage may no longer be a relevant or convenient triggering metric. For example, assume a driver (if overheated) no longer provides a predictable dimming profile (i.e., a command from the controller no longer produces an expected output to the load). Fluctuations in output voltage may render the metric unreliable and so a self-healing overtemp circuit such as that described herein could rely upon photocell input for method  500 . This approach could be extended to non-catastrophic or terminal events such as the thermal runaway or overcurrent events previously described. For example, aspects according to the present invention could be applied to normal driver operation so to effectuate normal dimming profiles (e.g., to run LEDs at a lower output when photocells indicate an abundance of ambient light, to extend LED life by running LEDs at a lower output when they (for any reason) are “running too hot”). 
     As a few additional examples of options and alternatives to those already described herein, the load could comprise other light sources (e.g., HID light sources) or non-light source loads; one possible example being a flow control system wherein a thermal runaway event results in an undesirable change in pressure or viscosity of a substance. Multiple temperature or voltage thresholds could be developed: to permit some low level light output once LEDs have cooled some (but before the thermal runaway is resolved), to permit a ramping down of light to make the change less abrupt, to more proactively identify an impending thermal runaway or overcurrent event and provide preemptive power reduction, etc.