Control of a triac for the starting of a motor

1. A circuit for controlling a triac intended to be series-connected with a resistive element of positive temperature coefficient or which is at least capacitive, 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 the series connection 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 series connection 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. 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 which is at least capacitive, and the auxiliary winding, comprising a circuit for controlling the triac of claim 1 .

9. A method for controlling a triac intended to be series-connected with a resistive element of positive temperature coefficient or which is at least capacitive, 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 the series connection of said element and of the triac;

comparing this detected voltage with a threshold; and

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

10. The method of claim 9 , 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.

11. A circuit for controlling a triac when the triac is coupled in series with a resistive element and a starting winding of an asynchronous motor, the resistive element having a resistance that increases as the temperature of the resistive element increases, the circuit comprising:

a first circuit to detect a temperature-dependent voltage produced by the resistive element; and

a second circuit to turn on the triac at zero crossings of the voltage across the triac and prevent the triac from turning on when the temperature-dependent voltage exceeds a threshold.

12. The circuit of claim 11 , wherein the first circuit comprises:

a diode coupled to the resistive element; and

a resistive voltage-dividing bridge coupled to the diode.

13. The circuit of claim 12 , wherein the diode is coupled to a terminal between the resistive element and the triac.

14. The circuit of claim 11 , wherein the first circuit further comprises a Zener diode that establishes the threshold.

15. The circuit of claim 11 , wherein the second circuit detects a voltage across the triac.

16. A method of controlling a triac coupled in series with a resistive element and a starting winding of an asynchronous motor, the resistive element having a resistance that increases as the temperature of the resistive element increases, the method comprising:

detecting a temperature-dependent voltage produced by the resistive element;

determining whether the temperature-dependent voltage exceeds a threshold;

turning on the triac at zero crossings of the voltage across the triac; and

preventing the triac from turning on when the temperature-dependent voltage exceeds the threshold.

17. The method of claim 16 , 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 asynchronous motor.

18. The method of claim 16 , further comprising:

reducing the magnitude of the temperature-dependent voltage to produce a reduced-magnitude temperature-dependent voltage; and

comparing the reduced-magnitude temperature-dependent voltage to the threshold.