Control of a triac for the starting of a motor

1. A circuit for controlling a triac when the triac is 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:

a circuit for detecting a voltage representative of the voltage across a combination of said element and of the triac, and for comparing this voltage with respect to a threshold; and

a circuit for blocking a turning back on of the triac when said threshold has been exceeded.

2. The circuit of claim 1 , wherein said element is a resistor with a positive temperature coefficient.

3. The circuit of claim 1 , further comprising a circuit for controlling the triac at the voltage zero, controlled by said blocking circuit.

4. The circuit of claim 1 , wherein said blocking circuit stores the information that said threshold has been exceeded.

5. The circuit of claim 1 , wherein said detection and comparison circuit comprises:

a resistive dividing bridge receiving said voltage representative of the halfwave-rectified voltage across the combination of the triac and of said element; and

a zener diode having its threshold voltage setting the triggering of the blocking circuit.

6. The circuit of claim 5 , wherein 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.

7. The circuit of claim 6 , wherein said blocking circuit is sized to store the blocking for at least two halfwaves of the supply voltage.

8. The circuit of claim 1 , wherein the circuit for detecting the voltage comprises a capacitive divider for dividing the voltage across the series connection of said element and the triac.

9. The circuit of claim 1 , wherein the circuit for comparing the voltage with a threshold comprises a pair of zener diodes arranged to provide a two-way voltage reference.

10. The circuit of claim 9 , wherein the zener diodes have 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.

11. The circuit of claim 9 , wherein the anode of one and the cathode of the other of said pair of zener diodes is coupled to a node receiving the detected voltage.

12. The circuit of claim 1 , wherein 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.

13. The circuit of claim 12 , wherein 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.

14. A circuit for controlling an asynchronous motor having a main winding and 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, comprising a circuit for controlling the triac of claim 1 .

15. A method for controlling a triac when the triac is 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:

detecting a voltage representative of the voltage across a combination of said element and the triac;

comparing this detected voltage with a threshold; and

blocking a turning back on of the triac when said threshold has been exceeded.

16. The method of claim 15 , wherein the information that said threshold has been exceeded is stored for at least two halfwaves of the supply voltage to maintain the triac blocking.

17. A circuit for controlling a switch when the switch is coupled in series with a first element and a starting winding of an asynchronous motor, the circuit comprising:

a first circuit to detect a signal from the first element; and

a second circuit to prevent the switch from turning on when the signal exceeds a threshold.

18. The circuit of claim 17 , wherein the first element comprises a positive temperature coefficient resistor, a capacitive element or a resistive and capacitive element.

19. The circuit of claim 17 , wherein the switch comprises a triac.

20. The circuit of claim 17 , wherein the signal is temperature-dependent.

21. The circuit of claim 20 , wherein the signal is representative of a voltage across a series combination of the first element and the switch.

22. The circuit of claim 17 , wherein the first circuit comprises:

a diode coupled to the first element; and

a resistive voltage-dividing bridge coupled to the diode.

23. The circuit of claim 22 , wherein the diode is coupled to a terminal between the first element and the switch.

24. The circuit of claim 17 , wherein first circuit comprises:

a capacitive divider for dividing a voltage across a series connection of the first element and the switch.

25. The circuit of claim 17 , wherein the second circuit comprises a Zener diode that establishes the threshold.

26. The circuit of claim 25 , wherein the second circuit comprises a pair of Zener diodes arranged to provide a two-way voltage reference.

27. The circuit of claim 17 , wherein the second circuit comprises at least one node coupled to a control terminal of a first transistor and to a control terminal of a second transistor, the first and second transistors each having main current terminals coupled between a control node of the switch and a reference voltage level.

28. The circuit of claim 27 , wherein the second circuit further comprises a first capacitor coupled to the control terminal of the first transistor and arranged to store a 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 a voltage detected during a negative cycle of the asynchronous motor.

29. A method of controlling a switch coupled in a first branch including a first element and a starting winding of a motor, the method comprising:

detecting a signal from the first branch;

determining whether the signal exceeds a threshold; and

preventing the switch from turning on when the signal exceeds the threshold.

30. The method of claim 29 , further comprising:

storing an indication that the threshold has been exceeded, wherein the indication is stored for at least two halfwaves of a supply voltage that drives the motor.

31. The method of claim 29 , further comprising:

turning on the switch at zero crossings of the voltage across the switch.

32. The method of claim 29 , wherein detecting the signal comprises detecting a temperature-dependent signal from the first branch.

33. A compressor comprising:

an asynchronous motor comprising:

a winding coupled in series with a triac for starting an asynchronous motor for supply by an A.C. voltage; and

a resistive element of positive temperature coefficient or a capacitive element;

a circuit for controlling the triac when the triac is series-connected with the resistive element of positive temperature coefficient or the capacitive element, and the winding a circuit for detecting a voltage representative of the voltage across a combination of said element and of the triac, and for comparing this voltage with respect to a threshold; and

a circuit for blocking a turning back on of the triac when said threshold has been exceeded.