Abstract:
A device and method for diagnosing shorts and fault conditions in electrical circuits such as heating, ventilation, and air conditioning (HVAC) control circuits. The device includes a positive temperature coefficient (PTC) member to protect the electrical circuit from overcurrent conditions during diagnosis. The device also includes a light emitting diode (LED) and resistor electrically connected in parallel across the PTC member. An overcurrent condition causes the resistance of the PTC member to rise dramatically, creating a potential difference across the PTC member and lighting the LED. The invention also includes a method for locating a short in an HVAC control circuit using the short locating device.

Description:
FIELD OF THE INVENTION 
   This invention relates generally to diagnostic tools for locating shorts and other fault conditions in electrical circuits, and in particular, to diagnostic tools for locating shorts while protecting the electrical circuits being diagnosed. More specifically, this invention relates to an electrical-circuit-protecting diagnostic tool for locating shorts or current fault conditions in heating, ventilation, and/or air conditioning (HVAC) control circuits. 
   BACKGROUND 
   One of the most common causes of electrical circuit or system failure is a short circuit. Shorts occur due to a variety of causes, including electrical component failure, damaged wiring, or damaged insulation. To restore the shorted electrical circuit to normal operation, the short must be located and the failed component, wiring, or insulation replaced. Precisely identifying the location and cause of a short circuit, however, is frequently a difficult and time-consuming task. 
   The challenge of locating shorts is one commonly faced by home appliance and HVAC service technicians. To verify the existence of a short, the service technician may replace a blown fuse with another fuse, or manually reset a tripped circuit breaker. However, if the short is drawing sufficient current, this will only result in another blown fuse or tripped circuit breaker, and the technician will not be aided in pinpointing the exact location of the short. In order to overcome these limitations, many service technicians jump the terminals of a blown fuse or tripped circuit breaker, and thereafter move wires around or probe different points in the local circuit in an effort to pinpoint the location of the short. Unfortunately, in the meantime, the appliance or HVAC transformer and other circuit components may overheat and become damaged. 
   Short locating tools are well known. For example, U.S. Pat. No. 4,353,026 to Edwards, Jr., discloses a short locating tool for high voltage applications utilizing two momentary push buttons, a relay circuit, and an audible horn that emits an audible alarm when a short is cleared. The short locating tool does not, however, include any short-circuit protection components to prevent current overloading. Rather, it provides a closed, essentially zero-resistance circuit between a power source and the shorted circuit&#39;s wire. While one can connect the diagnostic tool to one of the shorted circuit&#39;s own circuit breakers, if the short pulls enough current to trip the circuit breaker, the short locating tool is rendered useless. 
   Other short locating tools are described in U.S. Pat. No. 6,252,409 B1 to Iijima; U.S. Pat. No. 5,877,631 to Takahashi; U.S. Pat. No. 5,476,497 to Mower et al.; U.S. Pat. No. 5,463,322 to Kwasnick et al. These are all complicated and potentially expensive devices, require significant expertise to use, or have very specific applications. 
   What is needed is an inexpensive but reusable simple-to-use short locating tool that provides protection for the electrical circuit being diagnosed without requiring replacement of fuses or resetting of circuit breakers. This is particularly needed for home appliance and HVAC service technicians. 
   SUMMARY OF THE INVENTION 
   The invention is directed to, but not limited by, one or more of the following non-exhaustive objects, separately or in combination: 
   to provide a tool for diagnosing short circuits and overcurrent conditions; 
   to provide a short circuit diagnostic tool with automatically resettable overcurrent protection; 
   to provide a short circuit diagnostic tool with positive temperature coefficient (PTC) overcurrent protection; 
   to provide a short circuit diagnostic tool that is inexpensive and easy to use; 
   to provide a short circuit diagnostic tool for use in low-voltage circuits; 
   to provide a short circuit diagnostic tool designed, in use, to be temporarily mounted and moved from one branch circuit to the next; 
   to provide a short circuit diagnostic tool for use in circuits powered by a 24 volt, 60-Hertz power supply; 
   to provide a short-locating tool with PTC overcurrent protection that is amenable for use by technicians in servicing appliances and heating, ventilation, and/or air conditioning (HVAC) control circuits; and 
   to provide a safe method of diagnosing a short circuit in a HVAC control circuit. 
   In U.S. Pat. No. 6,075,689, to Mitchell, I described a “Device and Method for Protection of Heating Ventilation and Air Conditioning Control Circuits from Overcurrents,” the details of which are incorporated, in their entirety, by reference. In that patent, I described a device that included a polymeric PTC member for protecting HVAC control circuits that could be used as a substitute for a conventional fuse or circuit breaker. 
   A PTC device placed as a series element in a circuit protects the circuit by going from a low-resistance to a high-resistance value in response to an overcurrent. The distinguishing characteristic of a PTC device is that its resistance increases non-linearly as temperature increases, typically as a result of the material undergoing a phase change. 
   The PTC device, like any resistor, generates heat at a rate equal to the square of the current times power. In operation, the PTC device will reach a temperature of equilibrium where the heat is dissipated, by conduction or convection, as fast as it is generated. 
   At sufficiently low temperatures, corresponding to sufficiently low current levels, the resistance of the PTC device remains very low and increases at a very low rate with respect to temperature. In this stage, the resistance of the PTC device will generally be too low to have any significant influence on the operation of the circuit. 
   If, however, the current through the PTC device increases to a level, designated I trip , sufficient to cause the temperature of the device to exceed a breakpoint (corresponding to a phase change in the PTC material, typically 125° C. in commercially available PTC devices), the resistance of the PTC device begins to rise very rapidly. At this stage, a very small temperature increase results in a very large increase in resistance. The large resistance increase, in turn, correspondingly decreases the current flowing through the circuit. In this manner, the circuit is protected from overcurrent conditions (such as short circuits). 
   In U.S. Pat. No. 6,075,689, I described my invention of an automatically resettable current protection device for use in HVAC control circuits as a substitute for a fuse or circuit breaker. Here, the invention I describe comprises a tool and method for using the same for diagnosing, detecting, and locating overcurrent conditions, including short circuits, in an electrical circuit. The tool comprises a light-emitting diode (LED) in series with a resistor and a PTC member in parallel with the LED-resistor pair. The tool also includes a pair of connective members, arranged so that the PTC member, LED, and resistor are disposed therebetween and in electrical contact therewith. The connective members provide the necessary external electrical interface for the diagnostic tool. 
   One of the advantages of my invention is that the PTC member rapidly resets itself when the overcurrent condition is removed. Moreover, unlike fuses, the PTC device is not destroyed by the overcurrent condition. And unlike circuit breakers, the PTC material-based device does not require a service technician using the device to manually reset anything. Another advantage of the present invention is the PTC member stays in a “tripped” state for an extended period of time, but resets itself quickly when the overcurrent condition is removed. This gives a service technician ample time to move wires and probe circuit points while attempting to locate a short. 
   In view of the need for reusable, non-bulky, short circuit diagnostic tool for a HVAC control circuit that includes overcurrent protection that can be reset without the user having access to the device, the present invention has been designed and developed. The present invention includes features and components that have been invented and selected for their individual and combined benefits and superior performance. The invention includes a circuit protection device for protecting an HVAC control circuits from overcurrents and methods for employing the circuit protection device. 
   These and other aspects, objects, features, and advantages of the present invention will be readily apparent to those skilled in the art from the following detailed description taken in conjunction with the annexed sheets of drawings, which illustrate the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a circuit diagram illustrating one embodiment of the present invention configured to be used as a short locating tool for an electrical circuit, showing its placement within the electrical circuit. 
       FIG. 2  is a circuit diagram illustrating an alternative embodiment of the present invention configured to be used as a short locating tool. 
       FIG. 3  illustrates the electrical components of a preferred embodiment of the short locating tool of the present invention. 
       FIG. 4  is a perspective view of one embodiment of a short locating tool including a protective member at least partially covering the PTC member and connective members. 
       FIG. 5  is a simplified illustration of a HVAC control circuit, depicting a short locating tool connected across the terminals of a fuse holder. 
       FIG. 6  illustrates a method of locating a short in a HVAC control circuit using a short locating tool built in accordance with the present invention. 
       FIG. 7  is a front elevational view of the connective members of one embodiment of the short locating tool. 
       FIG. 8  is a side elevational view of the connective members depicted in  FIG. 7 . 
       FIG. 9  is a perspective view of the connective members of another embodiment of the short locating tool. 
   

   DETAILED DESCRIPTION 
   Based on the description and illustrations provided herein, the many benefits provided by the invented structure and methods of utilization are apparent. These described benefits, as well as those that are inherent to those skilled in the art, fall within the scope of the invention of the present patent application as limited only by the claims appended hereto. 
   Referring to  FIGS. 1 and 2 , each alternative embodiment of the short locating tool circuit  100 ,  200  comprises a PTC member  110 ,  210  having a first lead  112 ,  212  and a second lead  114 ,  214 ; a light emitting diode (LED)  120 ,  220  having an anode  122 ,  222  and a cathode  124 ,  224 ; and a resistor  130 ,  230  having a first lead  132 ,  232  and a second lead  134 ,  234 . In each of these alternative embodiments, the resistor  130 ,  230  is electrically connected in series to the LED  120 ,  220 , forming a resistor-LED pair; and the PTC member  110 ,  210  is electrically connected in parallel across the resistor-LED pair. In  FIG. 1 , the resistor  130  is electrically connected to the anode  122  of the LED  120 . In  FIG. 2 , the resistor  230  is electrically connected to the cathode  224  of the LED  220 .  FIG. 1  also differs from  FIG. 2  in that  FIG. 1  depicts an optional additional diode  150  having an anode  152  and a cathode  154  that is electrically connected in series with lead  132  of resistor  130 . Use of the additional diode  150  is believed to lengthen the performance life of the LED  120  when it is subjected to alternating currents. 
     FIG. 1  depicts the short locating tool circuit  100  in connection with the electrical circuit  180  being diagnosed. As depicted, electrical circuit  180  comprises an alternating current (AC) power source and load  170  in the electrical circuit. When the short locating tool circuit  100 ,  200  is used in a direct-current (DC) circuit, the lead  140 ,  240  should be connected to the current source side of a break in the DC circuit; and lead  142 ,  242  should be connected to the current drain side of the break in the DC circuit. In AC circuits, of course, leads  140 ,  240  and  142 ,  242  may be connected to either direction. 
   The LED  120 ,  220  is preferably a standard, low-cost, visible-light LED that is sufficiently bright to be easily detected in operation. To light a typical red LED, a voltage of at least about 1.7 volts must typically be applied across the LED&#39;s leads. Of course, other types and colors of LEDs, which typically require greater potential differences, may be used. The resistor  130 ,  230  preferably has a resistance sufficiently low, relative to the potential difference between leads  140 ,  240  and  142 ,  242 , to allow enough current to pass through the LED  120 ,  220  at make the emitted light easily visible in typical maintenance environments. Typically, a desirable current will be in the vicinity of 7–10 mA. At the same time, the resistor  130 ,  230  preferably has a resistance sufficiently high, relative to the potential difference between the leads  140 ,  240  and  142 ,  242 , to keep the flow-through current below the maximum rated current (typically, between 20 and 40 mA) of the LED  120 ,  220 . For typical HVAC control circuits that are supplied by a 24 volt secondary voltage RMS, the preferred value of the resistor  130 ,  230  is between about 500 ohms and 3000 ohms, and more preferably, between about 1000 and 1500 ohms. One embodiment that I have created uses a standard low-cost, 10% tolerance, 1200 ohm, ¼ or ⅛ watt resistor. It will, of course, be appreciated that different values of resistors  130 ,  230  may be preferable when diagnosing significantly higher or lower voltage systems. 
   In connection with the overcurrent protection device I described in U.S. Pat. No. 6,075,689, I suggested that a PTC device be selected whose I trip  characteristic (i.e., the current that will cause the material to “trip”) is less than the maximum current carrying capacity, I max , of an HVAC control circuit to be protected and greater than a combined load current, I load , drawn by all loads in the HVAC control circuit when functioning properly; where I max  is greater than I load . This continues to be preferred, if possible. However, in the current state of the art, there is a significant ratio (e.g., ˜2) between the maximum current at which the device will not trip (known as the hold current I hold ), and the minimum current at which the device will always trip (known as the trip current I trip ). In some HVAC circuits powered by a 24 volt, 40 VAC transformer, the ratio between I max  and I load  is smaller than the ratio between I trip  and I hold . In such cases it may be necessary to use a PTC device whose I trip  is greater than the HVAC control circuit&#39;s I max . 
   For example, a preferred embodiment of a short locating tool  100  for an HVAC control circuit utilizes a PTC member  110 ,  210  manufactured by Raychem® and designated as the RXE110 1.10A PolySwitch®, which, at the time of this writing, could be obtained in bulk at a cost of approximately $1.25 per PTC member. The RXE110&#39;s rated hold current I hold  at 20° C. is 1.10 amps. The RXE110&#39;s rated trip current I trip  at 20° C. is 2.20 amps. The RXE110&#39;s rated maximum time to trip at currents six (6) times the hold current I hold , is 8.2 seconds. As will be understood by those familiar with PTC devices, the time-to-trip is significantly lower for higher trip currents. While tripped, the RXE110 is rated to dissipate approximately 1.5 watts of power at 20° C. in a still air environment. The RXE110&#39;s rated initial resistance, prior to ever being tripped, is between 0.15 and 0.25 ohms. The RXE110&#39;s rated post-trip resistance (i.e., the maximum resistance to which the RXE110 will return one hour after the over-current condition is removed) is 0.38 ohms. The RXE110 is also rated to operate at a maximum voltage of 60 volts and a maximum current of 40 amps. 
   Notably, the RXE110 has a maximum rated voltage that is well below the typical voltage (120V) supplying most HVAC transformers. If there is a short from the 120 volt power supply across the transformer, the RXE110 may detect this unusual occurrence by being destroyed by the overvoltage condition. This has the important advantage of alerting the technician of an unusual and potentially dangerous condition and saving the technician time that might otherwise be wasted attempting to isolate the problem in some other part of the HVAC circuit. 
   In operation, the PTC member  110 ,  210  will provide a path of very low resistance (less than one to three ohms) if no overcurrent condition exists. In a properly functioning HVAC branch circuit, this resistance should be far less than the load impedance, resulting in a negligible voltage drop between lead  140 ,  240  and lead  142 ,  242 . This, in turn, limits the current flowing through LED  120 ,  220 , as well as the voltage drop across LED  120 ,  220 , thus preventing it from becoming lit. On the other hand, the existence of a short will cause the PTC member  110 ,  210  to heat up, and in turn increase the PTC member&#39;s resistance, resulting in a large voltage drop between lead  140 ,  240  and lead  142 ,  242 . This, in turn, increases the current flowing through LED  120 ,  220 , as well as the voltage drop across LED  120 ,  220 , to levels sufficient to light it. In this manner, the short locating tool of the present invention both signals the technician to the existence, if any, of a short while simultaneously protecting the circuit and transformer from excessive current. 
   There are, of course, many possible adaptations of the short locating tool circuit  100 ,  200 . For example, other signaling apparatuses, such as regular visible light bulbs, buzzers, electromagnetic signal transmitters (digital or analog), or any of the foregoing in combination with a secondary signal-transmitting circuit, may be used in place of LED  120 , the LED-resistor pair, or in place of all three of the series-connected LED  120 , resistor  130  and diode  150 . Of course, with any of these alternative signaling apparatuses, different values of resistor  130  (if used) may be required in order to permit sufficient current to activate the signaling device when a fault is detected. Nevertheless, the preferred mode of the invention utilizes an LED-resistor pair because LEDs are less expensive and/or require less current than most alternative signaling apparatuses. 
   In a yet further alternative embodiment (not shown), a second resistor-LED pair, with the anode and cathode of the second LED oriented opposite the LED of the first resistor-LED pair, is connected in parallel with the PTC member, so that one of the LEDs will light up to signal an overcurrent condition regardless of the direction in which the leads  140 ,  142  and  240 ,  242  are connected in a DC circuit. 
   It will also be appreciated that for AC circuit applications, other impedance members, such as capacitors and inductors, may be substituted for or used in combination with the resistor  130 ,  230  to limit the current flowing through the LED  120 ,  220 . It will also be appreciated that a bridge rectifier could be combined with the circuit  100 ,  200 . 
   It should be understood that any of the aforementioned adaptations, whether or not directly illustrated in the drawings, are within the scope of the invention and may be encompassed within the scope of one or more of the claims. 
     FIG. 3  provides a physical illustration of the electrical components of a preferred embodiment of a short locating tool  300 . The short locating tool  300  comprises a radially leaded PTC member  310  disposed electrically between a pair of connective members  340  and  342 . The short locating tool  300  also comprises a LED  320  with a pair of leads, one of which is electrically connected to connective member  340 , the other of which is electrically connected in series with a resistor  330 . The resistor  330  is electrically connected to connective member  342 . Connective members  340  and  342  may comprise clamps such as alligator clips. 
     FIG. 4  provides a perspective view of one embodiment of a short locating tool  400  including a protective member  410  at least partially covering a PTC member and resistor (neither of which are shown) and connective members  440  and  442 . The LED  420  protrudes from the top of the protective member  410 . The protective member  410  comprises insulating material such as rubber or molded plastic and serves to protect a user from contact with electrical hazard when the tool  400  is installed and to protect the device  400  from electrical and mechanical hazard. 
     FIG. 5  is a simplified illustration of a HVAC electrical circuit  500 , depicting a short locating tool  510  of the present invention connected across the terminals of an overcurrent protection receptacle  515 . The HVAC electrical circuit  500  illustratively comprises a 40 volt-amp transformer  520  having a 24 volt secondary voltage; a standard HVAC terminal bus  525  having terminal connections for power (R), common ground (C), the fan circuit (G), the heating circuit (W), and the compressor circuit (Y); a thermostat  550 ; an overcurrent protection receptacle  515  (illustratively comprising a fuse holder and/or a fuse); and a HVAC control board  530 . The HVAC control board  530  comprises a plurality of branch circuits  532 ,  534 , and  536 . By way of illustration, the first branch circuit  532  may comprise a fan control circuit, the second branch circuit  534  may comprise a heating control circuit, and the third branch circuit  536  may comprise a cooling control circuit that increases the fan speed. 
     FIG. 6  illustrates a method of isolating a fault condition in a HVAC control circuit using a short locating tool built in accordance with the present invention. As illustrated in block  605 , power to the control circuit is disabled or disconnected. Also, as depicted in block  610 , the technician ensures that the thermostat is off and the fan switch is in the auto mode. In block  615 , the terminals or connectors of the control circuit&#39;s receptacle (e.g., fuse holder) for its overcurrent protection device (e.g., fuse) are exposed (e.g., if the device is a fuse, by removing it). In block  620 , the connective members of the short locating tool are connected in series across the terminals or connectors of the circuit&#39;s overcurrent protection device or receptacle/holder. In block  625 , power to the control circuit is restored. In block  630 , the technician looks at the LED on the short locating tool to learn whether it is signaling a fault condition. If the LED lights up, this indicates, as depicted in block  635 , that a short is located outside the HVAC&#39;s branch circuits. The short, for example, may be located between one of the wires going to the thermostat and ground. 
   If the LED did not light up, then, as depicted in block  640 , the technician jumps the power terminal (depicted as “R” in  FIG. 5 ) to the lead connect (depicted as “G,” “W,” and “Y”) of each branch circuit, one at a time, until the LED lights up. If the LED lights up, the particular branch circuit being jumped is the problem circuit. 
   It will be understood that the actions depicted in  FIG. 6  do not have to be done in the order depicted. For example, the order of blocks  605  and  610  are interchangeable. Additional actions, not depicted, may also be desirable. For example, if there is a time delay circuit or relay in the condensing unit or in a cooling circuit, the technician may want to use a jumper to electrically bypass the time delay circuit or relay while searching for the short. This is because a short condition is likely to trigger the time delay and interfere with or delay the circuit&#39;s diagnosis. 
   One advantage of the methods described above is that they allow the technician to very quickly isolate a short to one of four problem areas. Another advantage of this method is that the technician can leave the short locating tool in the HVAC control circuit while testing wires and circuit components at or near the condensing unit, without fear of having a short circuit damage or destroy the HVAC circuit. 
   Finally,  FIGS. 7–9  depict various alternative configurations for the connective members  440 ,  442  of the short locating tool. While in  FIG. 4  the connective members were depicted as spring-biased clamps, different physical configurations for the connective members may be preferable in some situations.  FIGS. 7 and 8  depict connective members  720 ,  730 ,  820 ,  830  disposed within a housing  740 ,  840  and configured as a pair of parallel coplanar blades electrically and mechanically compatible with a blade-type fuse receptacle.  FIG. 9  depicts connective members  920 ,  930  separated by fuse-shaped member  940 , wherein the connective members  920 ,  930  are electrically and mechanically compatible with a cartridge-type fuse and configured to be temporarily inserted into a cartridge-type fuse receptacle. In each of these embodiments, the connective members  720 ,  730 ,  820 ,  830 ,  920 ,  930  are electrically connected to leads  750 ,  760 ,  850 ,  860 ,  950 ,  960  of the PTC member (not shown) of the short locating tool. 
   Although the foregoing specific details describe various embodiments of the invention, persons reasonably skilled in the art will recognize that various changes may be made in the details of the method and apparatus of this invention without departing from the spirit and scope of the invention as defined in the appended claims. Therefore, it should be understood that, unless otherwise specified, this invention is not to be limited to the specific details shown and described herein. 
   Furthermore, it should be appreciated that continuation, divisional, and continuation-in-part applications from this specification may be pending at the time this patent issues, the claims of which may encompass embodiments and applications that are broader than the appended claims. Accordingly, if there are any embodiments disclosed in the specification that are not literally claimed in the appended claims, such embodiments or elements should not be presumed to be dedicated to the public.