Abstract:
A method and a circuit for controlling a triac intended to be series-connected with a resistive element of positive temperature coefficient or a capacitive element, and a winding for starting an asynchronous motor, for supply by an A.C. voltage, the present invention including the steps of: detecting a voltage representative of the voltage across the series connection of the element and of the triac; comparing this detected voltage with respect to a threshold; and blocking a turning back on of the triac when the threshold has been exceeded.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a continuation-in-part of prior application Ser. No. 11/602,889, filed Nov. 21, 2006, entitled “Control Of A Triac For The Starting Of A Motor”, now allowed, which claims priority to French application serial number 05/53570, filed Nov. 23, 2005, entitled “Control Of A Triac For The Starting Of A Motor,” which are herein incorporated by reference in their entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention generally relates to circuits for starting asynchronous motors powered by an A.C. voltage and, more specifically, to a circuit for controlling a triac driving an auxiliary winding of an asynchronous motor for starting thereof. 
         [0004]    An example of application of the present invention relates to compressors which generally comprise such asynchronous motors powered by the A.C. mains voltage. 
         [0005]    2. Discussion of the Related Art 
         [0006]      FIG. 1  very schematically shows a conventional example of a circuit for controlling windings of an asynchronous motor. For simplification, the motor has been symbolized by a main winding Lm and an auxiliary winding Ls used for the starting. Main winding Lm is intended to be powered by an A.C. voltage Vac applied between two terminals  1  and  2 . A switch K (for example, controlled by a thermostat Th, by the user, etc.) is interposed in series with winding Lm between terminals  1  and  2 . 
         [0007]    To start an asynchronous motor, it is necessary to create torque by means of a phase shift or by injecting a current greater than the current absorbed by winding Lm. Such is the function of auxiliary winding Ls, connected in parallel with the main winding. 
         [0008]    This auxiliary or starting winding Ls is not intended to operate continuously. This is why it is generally associated with a resistive element  3  of positive temperature coefficient (PTC) having its resistance increasing along with temperature. Element  3  enables disconnecting the auxiliary winding once the motor has started, the current then flowing in the auxiliary winding being sufficient for the resistance of element  3  to be considered as opening the branch of the auxiliary winding. 
         [0009]    To avoid that element  3  continuously dissipates power in the circuit it is generally series-connected with a triac T. Thus, as illustrated in  FIG. 1 , auxiliary winding Ls in series with element  3  and triac T are connected in parallel with main winding Lm. The gate of triac T is connected to the junction point of a resistor R and a capacitor C, connected between terminals  1  and  2 , a rectifying diode D being interposed between terminal  1  and resistor R with its anode on the side of terminal  1 . 
         [0010]    The circuit of  FIG. 1  is described in document EP-A-0571956. 
         [0011]    When a voltage Vac is applied between terminals  1  and  2  and switch K is turned on, the current which flows in winding Ls helps provide a torque to the motor to start it. In parallel, triac T is turned on by the gate current provided thereto by diode D and resistor R. A circuit  6  is used to discharge capacitor C to turn off triac T after a given time, which disconnects winding Ls. This time set by circuit  6  corresponds to the starting time. The starting time (conduction of winding Ls) is set by the time constant brought by resistor R and capacitor C. In such a circuit, resistive element  3  of positive temperature coefficient is used as a security for the case where triac T would be defective. 
         [0012]    A disadvantage of the circuit of  FIG. 1  is that it is used only once, on powering-on of the assembly. Due to the direct connection of diode D to terminal  1 , it is no longer used when the thermostat turns off the motor, capacitor C remaining charged. 
         [0013]    Even if the starting circuit (diode D, resistor R, capacitor C) were connected downstream of switch K (anode of diode D connected between switch K and windings Lm and Ls), the absence of the discharge circuit of capacitor C would adversely affect the restarting of the motor, after a turning-off/turning-on of the thermostat. 
         [0014]    Further, most often, a normally-on switch (not shown in  FIG. 1 ) used as a thermal protection (known under name KLIXON) is interposed between point  4  of interconnection of windings Ls and Lm and switch K. This protection switch is generally internal to the motor so that point  4  is, in practice, not accessible. In such a case, the circuit of  FIG. 1  does not enable automatically restarting the motor on turning-on of switch KLIXON, after having undergone a thermal protection opening. 
         [0015]    Assemblies (for example, from document U.S. Pat. No. 5,989,289) are also known in which a second resistive element with a positive temperature coefficient is provided to supply the triac gate. A disadvantage of this assembly is that the priming is late in the case where the second resistive element heats up, which generates electromagnetic noise. Another disadvantage of this type of assembly is a halfwave conduction. 
       SUMMARY OF THE INVENTION 
       [0016]    At least one embodiment of the present invention aims at overcoming all or part of the disadvantages of known circuits for controlling a triac used to disconnect an auxiliary winding of a motor once it has been started. 
         [0017]    At least one embodiment of the present invention more specifically aims at providing a re-triggerable solution, that is, enabling successive startings of the motor without generating an excessive heat dissipation in a resistive element. 
         [0018]    At least one embodiment of the present invention also aims at providing an integrable solution. 
         [0019]    At least one embodiment of the present invention also aims at providing a solution compatible with the operation of a thermal protection making one of the terminals of the motor winding not directly accessible. 
         [0020]    To achieve all or part of these as well as other objects, at least one embodiment of the present invention provides a circuit for controlling a triac intended to be series-connected with a resistive element with a positive temperature coefficient or a capacitive element, and a winding for starting an asynchronous motor for supply by an A.C. voltage, comprising: 
         [0021]    a circuit for detecting a voltage representative of the voltage across the series connection of said element and of the triac, and for comparing this voltage with respect to a threshold; and 
         [0022]    a circuit for blocking a turning back on of the triac when said threshold has been exceeded. 
         [0023]    According to an embodiment of the present invention, said element is a resistor with a positive temperature coefficient. 
         [0024]    According to an embodiment of the present invention, the circuit further comprises a circuit for controlling the triac at the voltage zero, controlled by said blocking circuit. 
         [0025]    According to an embodiment of the present invention, said blocking circuit stores the information that said threshold has been exceeded. 
         [0026]    According to an embodiment of the present invention, said detection and comparison circuit comprises: 
         [0027]    a resistive dividing bridge receiving said voltage representative of the halfwave-rectified voltage across the series connection of the triac and of said element; and 
         [0028]    a zener diode having its threshold voltage setting the triggering of the blocking circuit. 
         [0029]    According to an embodiment of the present invention, the blocking circuit comprises a switch selected from among a MOS transistor, a cathode-gate thyristor, a bipolar transistor, to ground the gate of the triac. 
         [0030]    According to an embodiment of the present invention, said blocking circuit is sized to store the blocking for at least two halfwaves of the supply voltage. 
         [0031]    According to an embodiment of the present invention, the circuit for detecting the voltage comprises a capacitive divider for dividing the voltage across the series connection of said element and the triac. 
         [0032]    According to an embodiment of the present invention, the circuit for comparing the voltage with a threshold comprises a pair of zener diodes arranged to provide a two-way voltage reference, the zener diodes for example having their anodes or cathodes coupled together and are coupled via their other terminals between a node receiving the detected voltage and an input node of the blocking circuit, or the anode of one and the cathode of the other of the pair of zener diodes being coupled to a node receiving the detected voltage. 
         [0033]    According to an embodiment of the present invention, the blocking circuit comprises at least one input node coupled to the control terminal of a first transistor and to the control terminal of a second transistor, the first and second transistors each having main current terminals coupled between a control node of the triac and a reference voltage level. 
         [0034]    According to an embodiment of the present invention, the blocking circuit further comprises a first capacitor coupled to the control terminal of the first transistor and arranged to store the voltage detected during a positive cycle of the asynchronous motor, and a second capacitor coupled to the control terminal of the second transistor and arranged to store the voltage detected during a negative cycle of the asynchronous motor. 
         [0035]    At least one embodiment of the present invention also provides, a compressor comprising an asynchronous motor comprising a winding coupled in series with a triac and a resistive element of positive temperature coefficient or a capacitive element, and the above circuit arranged to control the triac. 
         [0036]    At least one embodiment of the present invention also provides a circuit for controlling an asynchronous motor provided with a main winding and with an auxiliary starting winding, comprising at least one supply switch in series with said windings, and a triac in series with a resistive element of positive temperature coefficient, or a capacitive element, and the auxiliary winding, the motor control circuit comprising a circuit for controlling the triac. 
         [0037]    At least one embodiment of the present invention also provides a method for controlling a triac intended to be series-connected with a resistive element of positive temperature coefficient or a capacitive element, and a winding for starting an asynchronous motor, for supply by an A.C. voltage, comprising the steps of: 
         [0038]    detecting a voltage representative of the voltage across the series connection of said element and of the triac; 
         [0039]    comparing this detected voltage with a threshold; and 
         [0040]    blocking a turning back on of the triac when said threshold has been exceeded. 
         [0041]    According to an embodiment of the present invention, the information that said threshold has been exceeded is stored for at least two halfwaves of the supply voltage to maintain the triac blocking. 
         [0042]    The foregoing and other objects, features, and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0043]      FIG. 1 , previously described, shows a conventional example of a circuit for controlling a triac used to start an asynchronous motor of the type to which the present invention applies; 
           [0044]      FIG. 2  very schematically shows in the form of blocks an embodiment of a circuit for controlling a triac in an asynchronous motor start circuit according to the present invention; 
           [0045]      FIG. 3  is a detailed electric diagram of an example embodiment of the control circuit of  FIG. 2 ; 
           [0046]      FIG. 4  shows a variation of the circuit of  FIG. 3 ; 
           [0047]      FIG. 5  is an electric diagram showing a circuit for controlling a triac in an asynchronous motor start circuit according to a further embodiment of the present invention; 
           [0048]      FIG. 6  is an electric diagram showing a circuit for controlling a triac in an asynchronous motor start circuit according to yet a further embodiment of the present invention; and 
           [0049]      FIG. 7  shows a compressor according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0050]    The same elements have been referred to with the same reference numerals in the different drawings. For clarity, only those elements that are necessary to the understanding of the present invention have been shown in the drawings and will be described hereafter. In particular, the details constitutive of an asynchronous motor have not been described in detail, the present invention being compatible with any conventional asynchronous motor comprising an auxiliary winding used for its starting. 
         [0051]      FIG. 2  very schematically shows in the form of blocks an embodiment of a circuit  10  for controlling a triac T used to start an asynchronous motor  5 . In  FIG. 2 , motor  5  is symbolized by its main winding Lm, its secondary winding Ls, and a thermal protection switch Kth (KLIXON). Switch Kth connects a common node  4  of windings Lm and Ls to a terminal  51  intended to be connected, via a switch K (for example, controlled by a thermostat Th), to a terminal  1  of application of an A.C. supply voltage Vac between terminals  1  and  2 . The other ends of windings Lm and Ls define terminals  52  and  53  accessible from the outside of motor  5 . Terminal  52  is intended to be directly connected to the other terminal  2  of application of voltage Vac. As previously, terminal  53  is connected to terminal  2  by means of a resistive element  3  with a positive temperature coefficient (PTC) in series with triac T. 
         [0052]    According to this embodiment of the present invention, a measure (block  11 , LEVEL DET) of a voltage V 53  present between terminal  53  and the ground is used, this voltage being representative of voltage V 3  across resistive element  3  and triac T in series, to be compared with a threshold. This measurement is performed, for example, by means of a resistive dividing bridge formed of two resistors R 1  and R 2  in series between terminal  53  and ground M, with an interposed rectifying diode D 1  having its anode connected to terminal  53 . A first capacitor C 1  is optionally used to filter possible disturbances. 
         [0053]    When voltage V 53  reaches a determined threshold, the corresponding information is latched (block  12 , LATCH) or stored. Such a function is made necessary by the fact that the signal is variable with the periodicity of the supply voltage (generally, the mains). 
         [0054]    Preferably, triac T is made conductive on each zero crossing of voltage V 53  by means of a block  13  (ZVS) to limit electromagnetic disturbances by the turning-on of triac T. When voltage V 53  reaches the determined threshold, circuit  12  deactivates circuit  13  and thus prevents triac T becoming conductive on each zero crossing. 
         [0055]    An advantage which already appears from the functional representation of  FIG. 2  is that the circuit of the present invention automatically reactivates in case of a disappearing of supply voltage Vac across windings Lm and Ls, be it via control thermostat Th or by thermal security Kth integrated to the motor. 
         [0056]      FIG. 3  shows the detailed electric diagram of a first example of embodiment of circuit  10  of  FIG. 2 . According to this example, control circuit  13  of triac T at the voltage zero comprises a cathode-gate thyristor Th 1  having its anode connected, via a resistor R 3 , to a positive output terminal  131  of a fullwave rectifying bridge formed of diodes D 3 , D 4 , D 5 , and D 6 , the cathode of thyristor Th 1  being connected to a second rectified output terminal  132  of the bridge. A first A.C. input terminal  133  of the bridge (anode of diode D 3  and cathode of diode D 5 ) is connected to junction point  14  of resistive element  3  and triac T. Second A.C. input terminal  134  of the bridge is connected to the gate of triac T. The gate of thyristor Th 1  is connected to the junction point of a resistor R 4  and of a MOS transistor M connected between terminal  53  and ground M (corresponding to the second rectified output terminal  132  of the bridge). 
         [0057]    Assuming transistor M to be turned off (non-conducting), as soon as voltage V 53  starts increasing at the beginning of the halfwave while resistive element  3  is cold, a current flows both through this element  3  and through resistor R 4  (via the rectifying bridge) to trigger thyristor Th 1 . Once said thyristor has been triggered, the current flowing through element  3  and through resistor R 3  is used to trigger thyristor triac T via two of the diodes of the rectifying bridge, of resistor R 3 , and of thyristor Th 1 . Thyristor Th 1  is selected to be sensitive with respect to triac T and resistor R 4  is selected to be greater than resistor R 3  to reduce losses in the blocked state of triac T. 
         [0058]    When resistive element  3  is hot, assuming that triac control circuit  10  does not turn off said triac once the motor has started, auxiliary winding Ls is disconnected due to the high resistance of element  3 . The security brought by element  3  is thus preserved. 
         [0059]    Level detection circuit  11  comprises the resistive dividing bridge formed of resistors R 1  and R 2 , capacitor C 1 , and a zener diode DZ 1  having its threshold value selected according to the desired triggering threshold. 
         [0060]    Junction point  15  of resistors R 1  and R 2  is connected to the emitter of a first PNP-type bipolar transistor B 1  having its collector connected to the base of a second NPN-type bipolar transistor B 2 , the base of transistor B 1  being connected to the collector of transistor B 2  and also, via a resistor R 5 , to point  15 . Transistors B 1  and B 2  and resistor R 5  form an anode-gate thyristor of the detection circuit. The anode of diode DZ 1  is grounded while its cathode is connected to the base of transistor B 1 . As soon as the voltage between terminal  15  and ground M exceeds the threshold voltage of diode DZ 1  (neglecting the voltage drop in resistor R 5 ), transistor B 1  turns on, which turns on transistor B 2  which provides a locking of the conduction of transistor B 1 . Diode D 1  provides a halfwave rectification for the voltage measured by bridge R 1 /R 2 . 
         [0061]    Transistors B 1  and B 2  block as soon as the current in diode D 7  disappears, when the voltage across capacitor C 1  becomes lower than that across capacitor C 2 . Locking circuit  12  is required to store the detection performed by circuit  11  to turn on transistor M and prevent the restarting of circuit  13  by short-circuiting the gate and the cathode of thyristor Th 1 . 
         [0062]    The locking circuit comprises a capacitor C 2  grounding the gate of transistor M in parallel with a resistor R 7  of high value. The emitter of transistor B 2  is connected to the gate of transistor M by a diode D 7 , the anode of diode D 7  being on the emitter side of transistor B 2 . Capacitor C 2  is used to store the information detected by circuit  11  to turn on transistor M. The time constant of the resistive and capacitive cell C 2 -R 7  is selected according to the period of the A.C. supply voltage (and thus to voltage V 53 ) to store the information for at least one period. For example, for a 50-Hz A.C. voltage, a time constant in the order of 20 milliseconds will be selected. Resistor R 7  is used to discharge capacitor C 2  to enable resetting of the circuit when voltage V 53  disappears for a sufficiently long time indicating a need to restart the motor. 
         [0063]    Optional capacitor C 1  of circuit  11  enables accelerating the charging of capacitor C 2  at each halfwave and is used to filter possible disturbances present on voltage V 53 . 
         [0064]    As a specific example embodiment, an assembly such as shown in  FIG. 3  is formed with components having the following values: R 1 =510 kiloohms, R 2 =30 kiloohms, R 3 =620 ohms, R 4 =R 7 =1 megaohm, R 5 =10 kiloohms, C 1 =C 2 =10 nanofarads, DZ 1 =15 volts, Vac=220 or 230 volts—50 Hz. 
         [0065]    As a variation, transistor M is a bipolar transistor and the circuit is adapted for a current triggering. 
         [0066]      FIG. 4  shows an alternative embodiment of starting circuit  10 ′ in which thyristor Th 1  is no longer blocked by means of a transistor M but by means of a cathode-gate thyristor Th 2  connecting the gate of thyristor Th 1  to ground (terminal  132 ). A zener diode DZ 2  connects the gate of thyristor Th 2  to node  15  (anode of diode DZ 2  on the side of thyristor Th 2 ). As soon as the voltage at node  15  becomes greater than the threshold voltage of zener diode DZ 2 , a current flows through the gate of thyristor Th 2  to trigger it. Preferably, a capacitor C 2  between the gate of thyristor Th 2  and the ground stores sufficient power to maintain thyristor Th 2  on for two halfwaves while current is only injected one halfwave out of two by the halfwave rectification performed due to diode D 1  (not shown in  FIG. 4 ). 
         [0067]      FIG. 5  illustrates, in circuit diagram form, an alternative embodiment of a circuit  10 ″ for controlling a triac T used to start an asynchronous motor  5 . 
         [0068]    Many elements of the circuit of  FIG. 5  are the same as those of  FIG. 2 . These elements have been labeled with like reference numerals and will not be described again in detail. In particular, the main winding Lm, secondary winding Ls and thermal protection switch Kth of the motor  5 , as well as the further switch K are the same as those of  FIG. 2 . As with the circuit of  FIG. 2 , terminal  53  of the asynchronous motor  5  is coupled to terminal  2  by means of a resistive element  3  having a positive temperature coefficient (PTC) coupled in series with a triac T. Furthermore, as with the circuit of  FIG. 2 , an A.C. supply voltage Vac is applied between terminals  1  and  2 . 
         [0069]    In circuit  10 ″, a capacitor C 3  and a resistor R 8  are coupled in series with each other between node  53  and a node  54 , node  54  providing a control signal to triac T. Node  53  is also coupled to a node  55  via a resistor R 9  coupled in series with a capacitor C 4 . Node  55  is further coupled to terminal  2  via a capacitor C 5 . Capacitors C 4  and C 5  form a capacitive divider, providing at node  55  a voltage part-way between the voltages at node  53  and terminal  2 . Node  55  is coupled to a node  56  via a pair of zener diodes DZ 3  and DZ 4  coupled anode to anode to provide a two-way voltage reference. Alternatively, zener diodes DZ 3  and DZ 4  could be coupled cathode to cathode to provide the two-way voltage reference. Node  56  is further coupled to a node  57  via a diode D 8 , and to a node  58  via a diode D 9 , diodes D 8  and D 9  having their anodes coupled to nodes  56  and  58  respectively. Node  57  is coupled to terminal  2  by a resistor R 10  and a capacitor C 6  coupled in parallel with each other, and node  58  is coupled to terminal  2  via a resistor Rh 1  and a capacitor C 7  coupled in parallel with each other. Node  57  is also coupled to the control node of a transistor Q 1  via resistor R 12 , while node  58  is coupled to the control node of a transistor Q 2  via a resistor R 13 . Each transistor Q 1 , Q 2  is coupled via its main current terminals between node  54  and terminal  2 . In this example, transistors Q 1  and Q 2  are bipolar junction transistors, and Q 1  is for example of NPN type, while Q 2  is of PNP type. 
         [0070]    As a specific example, an assembly such as shown in  FIG. 5  is formed with components having the following values: R 8 =R 9 =100 ohms, R 10 =R 11 =100 kilo ohms, R 12 =R 13 =5.1 kilo ohms, C 3 =C 4 =150 nanofarads, C 5 =1 microfarad, C 6 =C 7 =10 microfarads, DZ 3 =DZ 4 =20 volts, D 8 =D 9 =30 volts, Vac=230 volts—50 Hz. 
         [0071]    In operation, during the positive cycle, while transistors Q 1  and Q 2  are non-conducting, triac T is controlled to be on by the voltage at node  53  providing a current to the control node  54  via capacitor C 3  and resistor R 8 . The circuitry delimited by the dashed line  60  comprising resistor  9 , capacitors C 4  and C 5 , and zener diodes DZ 3  and DZ 4 , provides a level detector, which provides a positive, zero, or negative voltage at node  56  based on the voltage at node  55 . In particular, when the resistance of the PTC resistive element  3  increases to over a certain value, the voltage V 53  across the triac T and the PTC resistive element  3  will also increase, and cause the voltage at node  55  to exceed a threshold determined by the zener diodes DZ 3  and DZ 4 . The zener diodes DZ 3  and DZ 4  will thus conduct, and increase the voltage at node  56 . The circuitry delimited by dashed line  62  comprising diodes D 8  and D 9 , resistors R 10  to R 13 , capacitors C 6  and C 7  and transistors Q 1  and Q 2  forms a blocking circuit that blocks the triac based on the level detected by the level detector  60 . This output of the level detector at node  56  is applied to the gate node of transistor Q 1  via diode D 8  and resistor R 12 , such that transistor Q 1  becomes conducting, coupling node  54  to terminal  2 . This counteracts the effect of the capacitor C 3  and resistor R 8 , and the results in the current from node  54  to terminal  2  falling, thereby turning off triac T. 
         [0072]    During the negative cycle, the circuit will operate in a similar fashion, except that the voltage at node  56  will be negative by conduction of capacitor C 7  and diodes DZ 3 , DZ 4  and D 9 , and transistor Q 2  rather than transistor Q 1  will be turned on via the resistor R 13  and diode D 9  when the resistance of the PTC resistive element  3  exceeds a certain level. 
         [0073]    The purpose of capacitors C 6  and C 7  is to introduce a time constant into the control of the triac. In particular, when the Zener diodes DZ 3  and DZ 4  pass a voltage to node  56 , it will be stored at nodes  57  and  58  by the capacitors C 6  and C 7 , but will slowly discharge via resistors R 10  and R 11 . 
         [0074]    With respect to the embodiments of  FIGS. 2 and 3 , the embodiment of  FIG. 5  allows the zero voltage crossing (ZVS) block  13 , and thus the rectifying diode bridge formed by diodes D 3  to D 6 , to be removed. 
         [0075]      FIG. 6  illustrates in circuit diagram form a further embodiment of a circuit  10 ′″ for controlling a triac T used to start an asynchronous motor  5 . 
         [0076]    Many features of the circuit  10 ′″ are the same as those of circuit  10 ″ of  FIG. 5 . These features have been labeled with like reference numerals, and will not be described again in detail. 
         [0077]    In the embodiment of  FIG. 6 , node  53  of the asynchronous motor  5  is coupled to the control node of triac T via resistor R 9 , capacitor C 4  and capacitor C 5  coupled in series. Node  55  is at a point between capacitors C 4  and C 5 , and in this embodiment is coupled to the cathode of zener diode DZ 3  and to the anode of zener diode DZ 4 . Zener diode DZ 3  is coupled anode to anode with diode D 8 , and the cathode of diode D 8  is coupled to node  57 , while zener diode DZ 4  is coupled cathode to cathode with a diode D 9 , and the anode of diode D 9  is coupled to node  58 . As with the arrangement of  FIG. 5 , zener diodes DZ 3  and DZ 4  provide a two-way voltage reference. In alternative embodiments diodes DZ 3  and D 8  could be coupled cathode to cathode and/or diodes DZ 4  and D 9  could be coupled anode to anode. 
         [0078]    As with the embodiment of  FIG. 5 , node  57  is coupled to the gate node of transistor Q 1  via resistor R 12  and node  58  is coupled to the gate node of transistor Q 2  via resistor R 13 . Node  57  is also coupled to terminal  2  via resistor R 10  and capacitor C 6  in parallel, and node  58  is coupled to terminal  2  via resistor R 11  and capacitor C 7  in parallel. 
         [0079]    The circuit  10 ′″ of  FIG. 6  operates in much the same way as circuit  10 ″ of  FIG. 5 . The circuitry delimited by dashed line  60 ′ in  FIG. 6  comprising the diodes DZ 3 , DZ 4 , D 8  and D 9 , the capacitors C 4  and C 5 , and the resistor R 9 , forms the level detector that detects a level of the voltage across the PTC resistive element  3  and triac T. The circuitry delimited by dashed line  62 ′ comprising resistors R 10  to R 13 , capacitors C 6  and C 7 , and transistors Q 1  and Q 2 , forms a blocking circuit that blocks the triac T when the level detected at node  53  increases above a certain level during the positive cycle, or below a certain level during the negative cycle. 
         [0080]    For the turning on of the triac, capacitors C 4  and C 5  play the equivalent role of capacitor C 3  of  FIG. 5 , and for the voltage detection, capacitors C 4  and C 5  provide a capacitor divider equivalent to capacitors C 4  and C 5  of  FIG. 5 . 
         [0081]    As a specific example, an assembly such as shown in  FIG. 6  is formed with components having the following values: R 9 =150 ohms, R 10 =R 11 =100 kilo ohms, R 12 =R 13 =5 kilo ohms, C 4 =150 nanofarads, C 5 =1.5 microfarads, C 6 =C 7 =20 microfarads, DZ 3 =DZ 4 =20 volts, D 8 =D 9 =40 volts, Vac=230 volts—50 Hz. 
         [0082]      FIG. 7  illustrates an apparatus  700  comprising a compressor  702  comprising the asynchronous motor  5  coupled to the PTC resistive element  3  and triac T in series, and the control circuit  10 , which could be any of the circuits  10 ,  10 ′,  10 ″ or  10 ′″ described herein. In this example, thermal switch K is coupled to terminal  2  rather than terminal  1 . Apparatus  700  is for example a fridge, air conditioning unit, dehumidifier, or other apparatus comprising a compressor. 
         [0083]    An advantage of at least one embodiment of the present invention is that the control circuit preserves a setting to the on state of triac T to the voltage zero. 
         [0084]    Another advantage of at least one embodiment of the present invention is that the circuit automatically reactivates in case of disappearance of the supply voltage. 
         [0085]    Another advantage of at least one embodiment of the present invention is that is preserves the security brought by resistive element  3  of positive temperature coefficient in case of a failure of the triac. 
         [0086]    Another advantage of at least one embodiment of the present invention is that it preserves the operation of the thermal motor protection. 
         [0087]    Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. In particular, the sizing of the different circuit components is to be adapted to the application and especially to the motor and supply voltage features. 
         [0088]    Further, although the present invention has been described in relation with a positive temperature coefficient resistor, it also applies to the starting circuit in which this element is replaced with a capacitive element or a resistive and capacitive element. 
         [0089]    Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.