Abstract:
A varistor failure detector includes one or more surge detector in communication with a varistor, to detect surges shunted by the varistor and a processor, in communication with the at surge detector(s). The processor is programmed to count surges shunted by the varistor, as indicated by the surge detector (s) and store at least one count representing a cumulative count of surges shunted by the varistor. An indicator of the count may be provided to an operator to indicate that the varistor should be replaced, to avoid catastrophic failure of the varistor.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to varistors, and more particularly, to a varistor failure detector that may anticipate varistor failure. 
       BACKGROUND 
       [0002]    Varistors are often used to protect the flow of increased current in the presence of excess voltage. As will be understood, a varistor is a voltage-dependent resistor. As such, varistors are often included in a circuit to shunt current created by high voltage away from sensitive components. 
         [0003]    For example, a varistor may be used in power supplies that are supplied by AC mains (e.g. 110 or 220 volts AC). The varistor is usually placed downstream of the power supply fuse, between the AC mains live conductor and neutral. In the presence of a high transient voltage, the varistor clamps the voltage and shunts any resulting current. 
         [0004]    A typical varistor is formed of a bulk material (such as ceramic) between two conducting plates. The bulk material contains grains of a conducting material (such as small amount of zinc, bismuth, cobalt or manganese) which acts to form diode junctions that allow current to flow only in one direction in the presence of a moderate voltage. Only small currents flow through the diode junctions, caused by reverse leakage. The presence of a large applied voltage, on the other hand, causes the diode junctions to break down and the varistor to conduct. 
         [0005]    Unfortunately, after repeated breakdowns caused by applied high voltages the varistor may fail. Such a failure may be catastrophic to downstream components, and the varistor itself. 
         [0006]    Accordingly, varistor circuits that allow for the prevention and/or detection of possible catastrophic failures are desirable. 
       SUMMARY 
       [0007]    According to an aspect, there is provided a method of operating a power supply comprising supply lines and a varistor interconnected with the supply lines to shunt surges on the supply lines. The method comprises sensing a surge on the supply lines; protecting downstream components from the surge by clamping the surge by way of the varistor; sensing each the surge, and maintaining a cumulative count of surges shunted by the varistor; and providing an indicator of a count reflecting surges shunted by the varistor, as indicated by the cumulative count. 
         [0008]    According to another aspect, there is provided a varistor failure detector comprising: at least one surge detector in communication with a varistor, to detect surges shunted by the varistor; a processor, in communication with the at least one surge detector, programmed to count surges shunted by the varistor, as indicated by the at least one surge detector; memory storing at least one count representing a cumulative count of surges shunted by the varistor. 
         [0009]    According to a further aspect, there is provided a varistor module comprising a varistor and the varistor failure detector described above. 
         [0010]    According to yet another aspect, there is provided a varistor module for interconnection with an electronic circuit, the varistor module comprising a substrate, a connector extending from the substrate, and a varistor electrically interconnected with the connector to allow solderless removal and replacement of the varistor module from the electronic circuit to allow the varistor to shunt surges on a supply line to a power supply of the electronic circuit. 
         [0011]    Other features will become apparent from the drawings in conjunction with the following description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    In the figures which illustrate example embodiments, 
           [0013]      FIG. 1  is a schematic diagram of a circuit including a varistor and varistor failure detector; 
           [0014]      FIG. 2  is a schematic diagram of components of the circuit of  FIG. 1 ; 
           [0015]      FIGS. 3A and 3B  depict voltage waveforms in the presence of a voltage surge; 
           [0016]      FIG. 4  is a rating curve for a typical varistor; 
           [0017]      FIG. 5  is a schematic diagram of varistor failure detector of  FIG. 1 ; and 
           [0018]      FIG. 6  is a flow charts of blocks performed by a processor of  FIG. 5 ; 
           [0019]      FIGS. 7 and 8  are perspective views of a varistor module, exemplary of an embodiment of the present invention 
       
    
    
     DETAILED DESCRIPTION 
       [0020]      FIG. 1  illustrates a typical protected circuit  10 , protected with a varistor failure detector  100 , exemplary of an embodiment of the present invention. As illustrated, in circuit  10 , an alternating current (AC) supply voltage is provided by way of supply lines including live conductor line  12  and a neutral line  14 . Varistor  20  is placed between live and neutral lines  12 ,  14  that provide primary power to downstream components  18 . A fuse  16  is placed in series with live conductor line  12  and downstream electric/electronic components  18  to protect against over-current. 
         [0021]    Example varistor  20  may be a metal oxide varistor (MOV). Downstream components  18  may include active or passive electronic components, a further regulating power supply, or the like. As such, live and neutral lines  12 ,  14  may provide a primary AC voltage that is converted into a regulated secondary DC voltage. Voltage conversion/regulation may be performed by a switching converter, transformer or the like. 
         [0022]    In the example embodiment, varistor failure detector  100  may be in indirect communication with lines  12  and  14 , by interconnection with downstream components  18 , on the secondary side of the power supply. 
         [0023]      FIG. 2  is a schematic block diagram of selected components of downstream components  18  interconnected with varistor failure detector  100 . Varistor failure detector  100  is in communication with varistor  20  to detect surges shunted by varistor  20 . In the depicted embodiment, varistor failure detector  100  is interconnected with an unregulated DC bus  50 , which feeds a DC-DC converter  48  that produces a regulated DC voltage V CC  for use by downstream components. DC voltage on DC bus  50  may be produced from lines  12 ,  14  by way of transformer  52 , and rectifying diodes  54 , further filtered by capacitor  56 . 
         [0024]    Surges and spikes on live and neutral lines  12 ,  14  may threaten downstream components  18 , causing current and/or voltage surges. Varistor  20  protects downstream component from such surge, by clamping the voltage surge. To that end,  FIGS. 3A and 3B  illustrate the typical effect of a power surge on supply voltage on lines  12  and  14  as well as the resulting voltage on components protected by a varistor, such as the varistor  20 . As illustrated, the typical AC mains voltage in North America is sinusoidal having a period of 1/60 th  of a second. A voltage surge in interval S may last for only a fraction of such a period. In the presence of a surge above a threshold voltage, varistor  20  assumes a low resistance state and conducts current between live line  12  and neutral line  14  to shunt current that might otherwise be provided to protected components  18 . In the depicted  FIGS. 3A and 3B  this threshold voltage is  169  V pp . 
         [0025]    Unfortunately, each shunted surge may cause varistor  20  to become less effective for future failures, and increases the likelihood of a catastrophic failure. 
         [0026]    In particular,  FIG. 4  illustrates a pulse rating curve for a conventional MOV. As illustrated, the varistor&#39;s surge protection capability decreases in the presence of repetitive surges. The amplitude (i.e. voltage) as well as duration of each surge will impact on the effectiveness of the varistor&#39;s ability to protect against future surges of varying amplitude and duration, and increases the likelihood of the next surge causing failure of the varistor. For example, a varistor may be rated for only a single surge of 1000 A having a duration of 20 uS, and  1000  surges of 75 A having a duration of 20 uS. 
         [0027]    Conveniently then, varistor failure detector  100  may sense surges and count the number of surges that varistor  20  shunts. Varistor failure detector  100  may provide an indicator of a potential future failure of varistor  20 , as further detailed below. 
         [0028]    An example varistor failure detector  100  is schematically illustrated in  FIG. 5 . As illustrated, the varistor failure detector  100  may include one or more surge detectors  102 , each including an isolator  106   a ,  106   b  (individually and collectively isolators  106 ), exemplified as opto-couplers tuned by tuning resistors  110   a ,  110   b  (individually and collectively tuning resistors  110 ). In  FIG. 5 , only two surge detectors are illustrated. However, failure detector  100  may have any number of surge detectors, each corresponding to a tuned threshold surge voltage. Surge detectors  102  are coupled to a processor  108 —for example in the form of a microprocessor, or microcontroller, by way of isolators  106 , which are in turn coupled to DC supply line  50 . Processor  108  may thus count the number of surges above a threshold as sensed through the isolators  106 . Memory  110  in the form of a suitable combination of persistent and dynamic memory, may store processor readable instructions, adapting processor  108  to operate as described herein, as well as the described surge count or counts. Memory  110  may for example be a suitable combination of static RAM, dynamic RAM, registers and read-only-memory. 
         [0029]    Resistors  110  may be tuned to allow an associated detector  102  to detect surges of specific amplitude. That is, failure detector  100  is interconnected to unregulated DC bus  50  of circuit  10 . Circuit  10  may be placed under test, and the effect of example surges of particular on lines  12  and  14  may be measured on this DC bus  50 . Resistors may be tuned so that each resistor causes its associated detector  102  to sense a voltage surge on lines  12  and  14  above a particular threshold, causing its isolator  106  to assume a logic high in the presence of a surge voltage in excess of this tuned voltage. As will thus be appreciated, in this configuration, the higher the surge voltage and resulting spike on DC bus  50 , the more detectors  102  will provide a logic high output. The logic outputs of the multiple detectors  102  may be provided to processor  108 , by way of a suitable interface. Processor  108  may, in turn, count the number of surges at a particular voltage, and store the count in memory  110 . 
         [0030]    In operation, processor  108  performs blocks S 600  depicted in  FIG. 6 . As illustrated, in block S 602 , processor  108  determines that one or more detectors  102  is providing a logic high signal. In block S 604  processor  108  determines the threshold voltage represented by the detected surge and increments the count stored in memory  110  associated with that voltage threshold in block S 606 . The threshold voltage may, for example, equal the greatest threshold voltage for which an associated tuned detector  102  is providing a logic high signal. 
         [0031]    In block S 608  processor  108  may optionally form a weighted sum of surge counts, based on the various individual surge counts stored in memory  110 . The sum of counts may, for example, be calculated as a weighted count of all the counted surges. This may be accomplished by weighting each detected and counted surge stored in memory  110  by a value proportional to its voltage and maintaining a sum of detected surges, so weighted. 
         [0032]    Appropriate weights used by processor  108  may be determined empirically and stored in memory  110 , depending on the failure contribution of each surge. This may, for example, be mathematically determined from the failure curves in  FIG. 4 . Higher voltage surges may be given a greater weighting than lower voltage surges, so that a combination of many smaller voltage surges may be given equal weighting as one high voltage surge—thereby allowing processor  108  to effectively integrate the cumulative effect on varistor  20  of multiple surges of different magnitude. 
         [0033]    As will be appreciated, one or more indicator of the count or counts maintained by processor  108  and some indicator of the count or counts may be provided, by way of a display—in the form of one or more light emitting diodes (LEDs), a liquid crystal display (LCD) or the like (not specifically illustrated). Possibly, the indicator could provide an identification of the number of surges shunted by varistor  20 . Such identification could include presenting a numeric count, or other indicator of count. Additionally or alternatively, once the count (or counts) cumulatively exceeds (or exceed) some threshold, as determined by processor  108  in block S 610 , the indicator of count may reflect likely future failure of varistor  20  (e.g. a warning) that may be output in block S 612 . 
         [0034]    Blocks S 600  may be repeated at the occurrence of each surge, as detected by failure detector  100 . 
         [0035]    In some embodiments, the cumulative count may be the described weighted sum of counts. Alternatively, the cumulative count of surges may be maintained as a collection of individual counts, and the indicator of likely future failure of varistor  20  may be provided if any of the individual counts exceeds some threshold (e.g. 50% of the rated counts for any given threshold voltage). Optionally, each of the multiple maintained counts may be made available to a user through a suitable LED or LCD interface. 
         [0036]    The indicator of likely future failure output in block S 612  may prompt a user/maintainer of circuit  10  to replace varistor  20 . 
         [0037]    Other techniques for assessing a cumulative count of surges affecting the life of varistor  20  will be apparent to persons of ordinary skill. 
         [0038]    In alternate embodiments, processor  108  may also track the duration of each surge, and may weight the occurrence of a surge by its amplitude and duration in forming the cumulative count. 
         [0039]    As will now be appreciated, processor  108  could be used for other purposes in circuit  10 , and could, for example, be a processor used in the overall operation of the downstream circuit formed by components  18 . In this way, varistor failure detector  100  could be added to existing circuit designs that already include a processor, at minimal cost—using only detectors  102 . Processor readable instructions allowing such processors to function as described may be easily added to the firmware or operating system governing the operation of such processor. 
         [0040]    In yet other alternate embodiments detectors  102  may be replaced by one or more suitable digital to analog converters (DACs), allowing processor  108  (suitably programmed) to detect amplitude and duration of surges. In further embodiments, the detectors  102  may be interconnected with an AC voltage produced from lines  12  and  14 —for example directly across lines  12  and  14 , or on the secondary side of a transformer like transformer  52  ( FIG. 2 ). 
         [0041]    As will be appreciated, once varistor  20  has been assessed to have shunted a number of surges as determined by failure detector  100 , the replacement of varistor  20  may be advisable. As noted, suggested replacement may be prompted by detector  100 , or simply assessed by a user from the indicatioln of count provided by detector  100 . Possibly, varistor  20  may be replaced without replacing other components of protected circuit  10 , and solderlessly—without soldering or removing solder. 
         [0042]    To that end,  FIGS. 7 and 8  illustrate a varistor module  120 , hosting varistor  20 , mounted on a printed circuit board  128 . Module  120  further includes a connector  126  interconnected with varistor  20 , to allow varistor  20  to be removably connected to a power supply (such as the power supply of circuit  10  of  FIG. 1 ) of an electronic device  200  ( FIG. 8 ). Varistor  20  may be mounted on a surface of a substrate—such as printed circuit board  124 , and may further be thermally protected by a heat sink  122  that is in thermal communication with varistor  20 . Varistor module  120  may be inserted into a complementary opening  202  in device  200 . Opening  202  may house a connector complementary to connector  126  for interconnection of varistor module  120  with circuit  10 . A lid  204  may cover opening  202 . 
         [0043]    Optionally, in such an embodiment, varistor failure sensing circuit  100  may be combined with varistor  20  on printed circuit board  124  to form replaceable varistor module  120 , complete with varistor  20 , processor  108 , surge detectors  102 , memory  110  and a suitable indicator, used to provide the indicator of the count of surges shunted by varistor  20  (not specifically shown in  FIGS. 7 and 8 ). Optionally, lid  204  may be transparent or translucent to allow viewing of the indicator without removal of lid  204 . Again, in the case of failure, this entire varistor module  120  could be replaced. 
         [0044]    Of course, the above described embodiments are intended to be illustrative only, and in no way limiting. The described embodiments are susceptible to many modifications of form, arrangement of parts, details and order of operation. 
         [0045]    The invention is intended to encompass all such modification within its scope, as defined by the claims.