US005995392A
`[11] Patent Number:
`[45] Date of Patent:
`
`5,995,392
`Nov.30, 1999
`
`5,420,780 5/1995 Berstein et al. ........................ 323/908
`5,574,632 11/1996 Pansier ........
`.... 363/49
`5,737,160 4/1998 Duffy ......
`.... 361/58
`5,737,161
`4/1998 Thomas ..................................... 361/58
`FOREIGN PATENT DOCUMENTS
`United Kingdom .
`United Kingdom .
`WIPO .
`WIPO .
`
`1375 605 11/1974
`2 300 982 11/1996
`WO 97/10635 3/1997
`WO 97/10637 3/1997
`
`[21] Appl. No.: 08/986,053
`[22] Filed:
`Dec. 5, 1997
`OTHER PUBLICATIONS
`[30]
`Foreign Application Priority Data
`Ellegård, Kristen, “Inrush Current Limiter,” Electronics
`World and W.W., Jul. 1994, p. 601.
`Dec. 6, 1996 [GB|
`United Kingdom ................... 96.25434
`Primary Examiner—Adolf Deneke Berhane
`[51] Int. Cl." … H02H 9/00
`Attorney, Agent, or Firm—Patterson & Keough, PA.
`[52] U.S. Cl. ................................ 363/49; 323/908; 361/58
`[58] Field of Search ........................ 363/49, 77; 323/294,
`[57]
`ABSTRACT
`323/908; 361/58, 91.5
`
`United States Patent [19]
`Turner
`
`[54] CURRENT LIMITER
`
`[75] Inventor: Michael James Turner, Leeds, United
`Kingdom
`
`[73] Assignee: Switched Reluctance Drives Limited,
`Harrogate, United Kingdom
`
`[56]
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`3,112,435 11/1963 Barney ...................................... 361/58
`3,935,511
`1/1976 Boulanger et al. ..
`... 317/20
`4,450,496
`5/1984 Doljack et al. ......
`... 361/58
`4,652,963 3/1987 Fahlen ....................................... 361/58
`4,769,752 9/1988 Rackowe ................................... 363/56
`5,187,653 2/1993 Lorenz ...................................... 363/89
`5,305,174 4/1994 Morita et al. .
`... 361/58
`5,350,997 9/1994 Ghotbi et al. ............................. 363/49
`
`
`
`A current limiter for a rectifier circuit includes a fixed
`resistor 50 and a positive temperature coefficient (PTC)
`resistor 100 connected in series across a control switch 70.
`The control switch is connected between a rectifier output
`and a smoothing capacitor 20. If the switch fails to operate,
`the PTC resistor heats up, increasing its resistance in the
`presence of an abnormal current. The increased resistance
`will limit the current supplied to the rectifier output termi
`mals 40.
`
`15 Claims, 3 Drawing Sheets
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`OV
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`Page 1 of 9
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`U.S. Patent
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`Nov.30, 1999
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`Sheet 1 of 3
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`5,995,392
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`(Prior Art)
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`Page 2 of 9
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`CHRIMAR 2049
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`U.S. Patent
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`Nov.30, 1999
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`Sheet 2 of 3
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`5,995,392
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`OV
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`Page 3 of 9
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`CHRIMAR 2049
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`

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`U.S. Patent
`
`Nov.30, 1999
`
`Sheet 3 of 3
`
`5,995,392
`
`
`
`20 &
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`S R Motor
`Converter
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`Page 4 of 9
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`CHRIMAR 2049
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`

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`1
`CURRENT LIMITER
`
`BACKGROUND OF INVENTION
`
`1. Field of Invention
`This invention relates to current limiters, and particularly,
`but not exclusively, to current limiters for rectified power
`supplies.
`2. Description of Related Art
`FIG. 1 shows a rectifier circuit for converting an alter
`nating current (a.c.) input into a rectified and smoothed
`direct current (d.c.) output. The circuit comprises a bridge
`rectifier 10 which produces a full-wave rectified output from
`the a.c. input which is applied across terminals 30. The full
`wave rectified output of rectifier 10 is smoothed by means of
`a capacitor 20 to provide the circuit output across terminals
`40. In a practical application, a load requiring a d.c. input
`would be connected across the terminals 40.
`When the a.c. supply is first switched on, the capacitor 20
`is uncharged and a very high current is initially drawn as the
`capacitor charges. This in turn may damage the components
`of the circuit, particularly the diodes of the rectifier 10. In
`addition, the current surge reflected into the supply is often
`unacceptable. In order to prevent this, it has been proposed
`to connect a switch in the circuit between the rectifier and
`the capacitor, with a resistor in parallel with the switch. Such
`a circuit is shown in FIG. 2. The switch 70 is held open when
`the a.c. supply is switched on, and the rectified current is
`then forced to flow through the resistor. Thus, only a
`proportion of the total e.m.f. available is dropped across the
`capacitor 20, thereby limiting the amount of current drawn
`as the capacitor charges. After a period, the voltage across
`the capacitor rises to some suitable level as the capacitor
`charges and the switch can be closed. This process is
`commonly called “soft starting’. A modification of this
`well-known arrangement is shown in U.S. Pat. No. 5,087,
`871, which is incorporated herein by reference.
`One problem with such a circuit is that if the capacitor 20
`is faulty, the charge it stores may never be sufficient to create
`a significant voltage drop across it. Alternatively, the load to
`which the circuit output at the terminals 40 is connected may
`malfunction, causing abnormally large currents to be drawn
`and preventing the normal working voltage across the
`capacitor from being established. In either case, the high
`current drawn through the resistor 50 will not decrease
`sufficiently to allow the switch 70 to be closed. Significant
`current will then be drawn through the resistor 50 for a
`prolonged period, causing it to get very hot and at least pose
`a fire risk if not actually to catch fire itself.
`A second problem is that the switch 70 may become stuck
`in the open circuit condition. This would also cause the
`normal load current to flow through the resistor for extended
`periods with the associated fire risk.
`A further problem is that users of the equipment may
`cause the resistor 50 to overheat by repeatedly discharging
`the capacitor, then charging again using this circuit. The
`resistor 50 will typically be chosen to comfortably accom
`modate the energy dissipated in one charging cycle.
`However, it then requires a significant period of time to cool
`before the charging sequence can be repeated. Repeated
`cycling (e.g. by the user switching the input on and off too
`rapidly and repeatedly) may cause the resistor to overheat.
`European published patent application number 667666,
`which is incorporated herein by reference, shows a circuit
`having detection means which detects if the switch is closed
`prior to switching on the a.c. supply to the rectifier 10. If it
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`is, the circuit is prevented from operating. Although this
`arrangement precludes high start-up currents from occurring
`accidentally, it does not address the more serious problem
`which arises when, for example, the capacitor is unable to
`store charge properly or a load draws excessive current, nor
`when the switch is unable to close in the first place.
`It is an object of the present invention to provide a circuit
`which at least alleviates these problems of the prior art.
`SUMMARY OF THE INVENTION
`According to embodiments of the present invention there
`is provided a current limiter comprising first and second
`terminals, a first resistor connected in series with a second
`resistor between the terminals, and switch means connected
`in parallel with the resistors between the terminals, charac
`terized in that the second resistor is a thermistor having a
`positive temperature coefficient, the thermistor being
`responsive to an abnormal current, above a working current
`through the resistors, while the switch means is open, to
`cause its resistance to increase, thereby limiting the magni
`tude of the current above the working current.
`It will be realized that this circuit uses the PTC thermistor
`in an unconventional way. Normally the PTC thermistor is
`expected to cycle over a resistance range in response to a
`working current. By contrast, in the circuit of embodiments
`of the present invention, the PTC thermistor is designed to
`pass the initial working current, which will include an initial
`surge current, without any significant change in resistance.
`It is only if a fault occurs in the components associated with
`the circuit, or if the circuit is operated in such a way that
`damage due to prolonged exposure to excessive currents
`occurs, that the PTC thermistor will react and reduce the
`current to a safe level.
`It will be appreciated that the components of the circuit of
`FIG. 3, described in more detail below, will be rated
`according to an expected working current. The invention
`provides protection for the circuit in the presence of an
`abnormal current, for example a fault current or, in the event
`of abnormal operation by a user, an excessive current for a
`prolonged period.
`The thermistor provides a backup protection for the first
`resistor in the event that the switch fails to close when
`required to do so. Any prolonged exposure to the high
`current causes the thermistor to heat up such that its resis
`tance rises rapidly and limits the current flowing in the
`circuit. The thermal inertia of the thermistor may also assist
`in alleviating the problem of repeated cycling of the circuit.
`Multiple charging operations with insufficient cooling inter
`vals between them will cause the PTC thermistor tempera
`ture to rise to the point where its resistance increases sharply,
`thereby again protecting the resistor 50 from overheating.
`A PTC thermistor is typically a semiconductor device,
`including barium titanium oxide. Other types are also avail
`able. They are supplied by electronic component
`manufacturers, e.g. Philips Components Limited, London,
`England as the 2322 Series of PTC resistors. A PTC resistor
`is distinguished from a standard wire-wound or carbon
`resistor by having an initially relatively low resistance which
`rises rapidly with temperature.
`The switch may for example be a relay, a thyristor or other
`semiconductor switching device.
`Preferably, the switch is arranged to close once the
`magnitude of the current surge has subsided and the voltage
`across capacitor 20 has reached a predetermined value. Once
`the current surge has passed, closing the switch provides a
`short circuit for the current, substantially preventing it from
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`5,995,392
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`3
`flowing through the resistors which could otherwise over
`heat. The PTC thermistor acts to prevent overheating of the
`first resistor when the switch is open if there is a current
`surge of a magnitude, duration or frequency greater than that
`for which the resistor is rated, i.e. if the integral of the
`thermal energy associated with the current is excessive.
`Preferably, the thermistor is in a heat-exchange relation
`ship with the resistor, so that the heat from the resistor
`increases the responsiveness of the thermistor.
`The current limiter according to embodiments of the
`invention is suitable for many circuits, such as, for example,
`a rectifier circuit comprising a rectifier having an a.c. input
`and a rectified output, output terminals, and a capacitor
`connected in parallel with the output terminals. The current
`limiter is arranged between the rectifier and one of the output
`terminals. The other output terminal is connected with the
`other rectifier terminal. This circuit may supply a variety of
`loads from its output terminals, such as a switched reluc
`tance motor through an appropriate switching circuit.
`Also according to embodiments of the invention, there is
`provided a method of limiting an abnormal current above a
`working current in an electrical circuit comprising switch
`means connected in parallel with a first resistor between first
`and second terminals, the method comprising connecting a
`thermistor having a positive temperature coefficient in series
`with the first resistor between the first and second terminals,
`the thermistor being substantially unresponsive to the work
`ing current such that its resistance remains relatively low,
`and being responsive to the abnormal current through the
`first resistor to cause its resistance to increase, thereby
`limiting the magnitude of the current, above the working
`current, through the first resistor.
`BRIEF DESCRIPTION OF THE DRAWINGS
`The present invention can be put into practice in various
`ways, some of which will now be described by way of
`example with reference to the accompanying drawings in
`which:
`FIG. 1 shows a prior art rectifier circuit;
`FIG. 2 shows a known rectifier circuit with a current
`limiter;
`FIG. 3 shows a rectifier circuit having a current limiter
`according to an embodiment of the present invention; and
`FIG. 4 shows a current-limiting portion of a rectifier
`circuit, according to an embodiment of the present inven
`tion; and
`FIG. 5 shows output terminals connected to a switched
`reluctance motor converter, according to an embodiment of
`the invention.
`
`DETAILED DESCRIPTION OF PREFERRED
`EMBODIMENTS
`Referring to FIG. 3, a rectifier circuit is shown. Those
`components common to FIGS. 1–4 have been labelled with
`like numerals.
`As with the prior art circuits of FIGS. 1 and 2, a.c. input
`terminals 30 are connected with the input to a full-wave
`rectifier 10 in the form of a diode bridge. One of the outputs
`of the rectifier bridge 10 is connected directly to one circuit
`output terminal 40. The other output of the rectifier is
`connected to the other circuit output terminal 40, via serially
`connected first and second resistors 50, 100. In parallel with
`these two resistors is a switch 70, which is typically a
`semiconductor device, such as a bipolar transistor or
`thyristor, or an electromechanical switch, such as a relay.
`
`4
`A capacitor 20 is connected across output terminals 40 for
`smoothing the output voltage of the rectifier 10. In this
`embodiment the rectifier 10 is capable of handling 8 A at
`230V from an a.c. supply, producing a rectified and
`smoothed d.c. output of about 5 A at about 320 volts. The
`capacitor is typically about 4700 uf, 350 volts rating.
`The first resistor 50 is a wire-wound resistor or the like of
`about 47G2. The second resistor 100 is a positive temperature
`coefficient (PTC) thermistor or other PTC device exhibiting
`the appropriate temperature/resistance response. Typically
`the thermistor is a 2322 661 4111 type manufactured by
`Philips Components Limited, having a resistance of about
`392 at about 25° C. and having a trip current threshold of
`about 615 mA at about 25°C. Under normal circumstances,
`the circuit operates substantially as described above in
`connection with FIG. 1 as far as rectification is concerned.
`However, when the a.c. supply 30 is first switched on, the
`capacitor 20 is initially uncharged and, at this time, the first
`resistor 50 has a much larger resistance than the second
`resistor 100. It is predominantly the first resistor 50,
`therefore, that limits the current drawn by the capacitor 20
`and, thus, the amount of current drawn from the supply.
`Provided none of the components are faulty, the switch 70
`is closed as soon as a sufficient charge is stored in the
`capacitor 20, thereby creating a short circuit around the
`resistors 50, 100.
`If the switch 70 fails to close at this time when, for
`example, the capacitor is faulty, or the current drawn at the
`circuit output 40 is excessive or the switch is faulty, a
`non-negligible current will instead continue to flow through
`the pair of resistors 50, 100. The current flowing through the
`PTC thermistor 100 causes it to heat up, thereby increasing
`its resistance significantly. The change in resistance of the
`PTC thermistor 100 for even moderate resistive heating is
`sufficiently high to decrease the current being drawn through
`the resistors to a safe value.
`Even if the switch 70 is damaged and unable to close, the
`PTC thermistor 100 is able to withstand the steady-state
`current passing through it for prolonged periods without risk
`of catching fire. The self heating this causes increases the
`PTC thermistor’s resistance to a high enough level that the
`voltage dropped across the first resistor 50 is small. Thus the
`risk of this first resistor catching fire is also substantially
`reduced. Repeated charging operations, which would other
`wise cause the first resistor 50 to overheat, likewise result in
`heating of the PTC thermistor 100, which again protects the
`current by increasing its resistance, and preventing further
`high charging currents from flowing until both the resistor
`50 and (by implication) the PTC thermistor 100 have cooled
`to a safe temperature.
`The switch 70 should ideally be closed when the voltage
`across capacitor 20 equals the peak of the rectified supply
`voltage and the current through the soft-start resistor 50 has,
`in the absence of any load current, dropped to zero. A
`number of methods for closing the switch are possible, each
`having different approximations to the ideal.
`The closing of the switch 70 can simply be timed, by any
`known method, from the moment current is drawn between
`the terminals. Electronic and electromechanical timers could
`be used. Where the switch 70 is a relay, a relay with a built-in
`timer for energizing the coil can be used. This approach is
`simple to implement but does not cater for any extended
`charging time required by the capacitor 20.
`A second method is to measure the voltage across the
`capacitor 20 and wait for it to reach a predetermined
`threshold. When the threshold voltage is met, the closing of
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`the switch is triggered. It is possible to use the capacitor
`voltage directly to operate the coil of a relay which consti
`tutes the switch 70. This second method is simple but does
`not cater for wide variations in supply voltage and, if the
`supply is particularly high, results in premature closure of
`the switch while charging current is still flowing.
`A third method overcomes the drawbacks of the first two
`methods by combining them and allowing a sensed voltage
`to trigger a subsequent timed period.
`It would be possible to measure the current in the capaci
`tor 20 and to close the switch 70 when charging current falls
`below a predetermined value. While it is possible to sense
`the voltage across the resistor 50 as a measure of capacitor
`current, this measurement does not differentiate between
`capacitor and load current. If significant load current is
`flowing, then the voltage across the resistor 50 may never
`fall to the threshold value and the switch 70 will not be
`closed. It is therefore preferable to measure the true capaci
`tor current, but if this is done by inserting a resistor in series
`with the capacitor across the DC link, there is an ongoing
`power loss in the resistor. A non-invasive current transducer
`must therefore be preferable for accuracy.
`FIG. 4 illustrates an automated switching arrangement
`based on the circuit of FIG. 3 in which the capacitor current
`is monitored by a Hall-effect device 112. When the current
`value, as relayed by the device 112, passes a suitable
`predetermined threshold, a threshold detector 114 outputs an
`actuation signal to a delay timer 116. The timer 116 relays
`the actuation signal to close the switch 70 after the prede
`termined delay period.
`In an alternative embodiment, the two resistors 50, 100
`can be physically coupled together, the resistive heating of
`the resistor 50 then combining with the resistive heating of
`the PTC thermistor 100 to raise the resistance of the latter
`more quickly. Further, a diode can be placed in series with
`the two resistors 50, 100, also in parallel with the switch 70,
`to prevent the capacitor 20 discharging in the opposite
`direction to the current flow.
`A rectifier circuit incorporating such a current limiter will
`operate with any load attached to the circuit output 40, but
`is particularly suited to a switched reluctance motor
`converter, as shown schematically in FIG. 5. Other suitable
`loads are a switched mode power supply or a variable
`frequency inverter.
`While the invention has been described with reference to
`rectifier circuits, it will be understood that the current limiter
`could find application in many different circuits where it is
`desirable to limit current spikes, either on start-up or during
`normal operation. In the latter case the switch 70 would be
`opened once the current being drawn from the supply
`exceeded a predetermined value, and would not be closed
`again until the current dropped below this value once more.
`Those skilled in the art will recognize that various modi
`fications and changes may be made to the present invention.
`Accordingly, the above description of several embodiments
`is made by way of example and not for purposes of limita
`tion. The present invention is intended to be limited only by
`the spirit and scope of the following claims.
`What is claimed is:
`1. A d.c. current limiter for producing d.c. output, the d.c.
`current limiter comprising first and second terminals, a first
`resistor connected in series with a second resistor between
`the terminals, and switch means connected in parallel with
`the resistors between the terminals, characterized in that the
`second resistor is a thermistor having a positive temperature
`coefficient, the thermistor passing current such that it is
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`6
`responsive to an abnormal current, above a working current
`through the resistors, while the switch means is open, to
`cause the resistance of the thermistor to increase, conse
`quently limiting the magnitude of the d.c. current between
`the terminals above the working current, the thermistor also
`passing initial working current, including an initial surge
`current, without significant change in resistance.
`2. A current limiter as claimed in claim 1, in which the
`switch means comprises at least one of a relay and a
`semiconductor device.
`3. A current limiter as claimed in claim 1, in which the
`switch means is arranged to close once the magnitude of the
`current has dropped to a predetermined value.
`4. A current limiter as claimed in claim 1, in which the
`second resistor is in a heat-exchange relationship with the
`first resistor.
`5. A current limiter as claimed in claim 1, wherein the
`thermistor has a resistance of about 392 at about 25° C.
`6. A current limiter as claimed in claim 5, wherein the
`thermistor has a trip current threshold of about 615 mA at
`about 25° C.
`7. A rectifier circuit comprising a rectifier having an
`alternating current (a.c.) input and a rectified output, a pair
`of output terminals, a capacitor connected across the output
`terminals, and a current limiter connected between the
`rectifier output and one of the output terminals, the current
`limiter comprising a first resistor connected in series with a
`second resistor between the terminals and a switch con
`nected in parallel with the resistors between the terminals,
`characterized in that the second resistor is a thermistor
`having a positive temperature coefficient, the thermistor
`passing current such that it is responsive to an abnormal
`current, above a working current through the resistors, while
`the switch is open, to cause the resistance of the thermistor
`to increase, consequently limiting the magnitude of the
`current above the working current, the thermistor also pass
`ing initial working current, including an initial surge current,
`without significant change in resistance.
`8. A circuit as claimed in claim 7, in which the output
`terminals are connected to a switched reluctance motor
`COnverter.
`9. A circuit as claimed in claim 7, further comprising a
`monitoring device for monitoring capacitor current.
`10. A circuit as claimed in claim 9, wherein the monitor
`ing device comprises a Hall-effect device.
`11. A circuit as claimed in claim 9, further comprising a
`threshold detector operably coupled to the monitoring
`device to indicate when capacitor current exceeds a prede
`termined threshold.
`12. A circuit as claimed in claim 11, further comprising a
`delay timer operably coupled to the threshold detector for
`closing the switch once a predetermined delay period has
`passed after the capacitor current exceeds the predetermined
`threshold.
`13. A method of limiting an abnormal current above a
`working d.c. current in a d.c. electrical circuit comprising
`switch means connected in parallel with a first resistor
`between first and second terminals, the method comprising:
`connecting a thermistor having a positive temperature
`coefficient in series with the first resistor between the
`first and second terminals, the thermistor being sub
`stantially unresponsive to the working d.c. current such
`that its resistance remains relatively low, and passing
`current such that it is responsive to the abnormal
`current through the first resistor to cause its resistance
`to increase, the thermistor also passing initial working
`current, including an initial surge current, without
`significant change in resistance; and
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`5,995,392
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`7
`consequently limiting the magnitude of the current, above
`the working d.c. current between the terminals, through
`the first resistor.
`14. A rectifier circuit comprising a rectifier having an
`alternating current (a.c.) input and a rectified output, a pair
`of output terminals connected to a switched reluctance
`motor converter, a capacitor connected across the output
`terminals, and a current limiter connected between the
`rectifier output and one of the output terminals, the current
`limiter comprising a first resistor connected in series with a
`second resistor between the terminals and a switch con
`nected in parallel with the resistors between the terminals,
`characterized in that the second resistor is a thermistor
`
`8
`having a positive temperature coefficient, the thermistor
`passing current such that it is responsive to an abnormal
`current, above a working current through the resistors, while
`the switch is open, to cause the resistance of the thermistor
`to increase, consequently limiting the magnitude of the
`current above the working current, the thermistor also pass
`ing initial working current, including an initial surge current,
`without significant change in resistance.
`15. A circuit as claimed in claim 14, wherein the working
`current is a working d.c. current.
`
`10
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`UNITED STATES PATENT AND TRADEMARK OFFICE
`CERTIFICATE OF CORRECTION
`
`PATENT NO. : 5,995,392
`DATED
`: November 30, 1999
`INVENTOR(S) : Michael J. TURNER
`
`It is certified that error appears in the above-identified patent and that said Letters Patent is hereby
`corrected as shown below:
`Item [56],
`Title page of the patent, the following references should appear
`in the "References Cited"
`
`US PATENT DOCUMENTS
`
`5,087,871 2/1992 Losel.
`
`FOREIGN PATENT DOCUMENTS
`0 667 666 A1 8/1995 Europe.
`
`Signed and Sealed this
`Twenty-fifth Day of July, 2000
`
`32%–
`
`Q. TODD DICKINSON
`Director of Patents and Trademarks
`
`Attesting Officer
`
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