Triac control circuit

1. A circuit for controlling a triac, comprising:

a bidirectional current-limiting active element connecting a triac gate to its opposite power electrode and excluding a resistor; and

a switch connecting the gate to the other power electrode of the triac.

2. The circuit of claim 1 , wherein the switch is of normally-on type.

3. The circuit of claim 1 , wherein said current-limiting element is formed of two current sources connected head-to-tail.

4. The circuit of claim 3 , wherein a diode is connected in parallel to each current source.

5. The circuit of claim 3 , wherein said current sources are formed of N-channel MOS transistors.

6. The circuit of claim 5 , wherein said transistors are depleted transistors.

7. A circuit for controlling a load intended to be supplied by an A.C. voltage of the type comprising a triac, comprising the circuit of claim 1 .

8. A circuit for controlling a triac having a gate and first and second terminals, the circuit comprising:

an active current source coupled between the gate and the first terminal of the triac; and

a switch coupled between the gate and the second terminal of the triac.

9. The circuit of claim 8 wherein the switch:

comprises a control terminal operable to receive a control signal; and

is in a conducting state in the absence of the control signal.

10. The circuit of claim 8 , wherein the triac is enabled when the switch is in a non-conducting state.

11. The circuit of claim 8 wherein the switch comprises a transistor.

12. A circuit for controlling a triac having a gate and first and second terminals, the circuit comprising:

an active current limiter coupled between the gate and the first terminal of the triac;

a switch coupled between the gate and the second terminal of the triac; and

wherein the active current limiter comprises a pair of current sources coupled to one another.

13. The circuit of claim 12 wherein the current sources are coupled in series opposition to one another.

14. The circuit of claim 12 wherein the active current limiter further comprises a pair of diodes each in parallel to a respective one of the current sources.

15. The circuit of claim 12 wherein the current sources comprise respective transistors.

16. The circuit of claim 13 wherein the active current limiter further comprises a resistor serially coupled between the current sources.

17. The circuit of claim 12 wherein each of the current sources comprises a respective MOS transistor having a drain and a source and an inherent diode coupled between the drain and the source.

18. A method of controlling a triac having a gate and having first and second terminals coupled to an alternating-current power supply, the method comprising:

activating the triac by supplying a current to the gate with an active current source coupled between the gate and the first terminal; and

deactivating the triac by coupling the gate to the second terminal.

19. A system, comprising:

first and second supply nodes;

a load having a first terminal coupled to the first supply node and having a second terminal;

a triac having a gate, a first terminal coupled to the second terminal of the load, and a second terminal coupled to the second supply node;

an active current source coupled between the gate and one of the first and second terminals of the triac; and

a switch coupled between the gate and the other of the first and second terminals of the triac.

20. The system of claim 19 wherein the load comprises an inductive load.

21. The system of claim 19 wherein:

the active current source is coupled between the gate and the first terminal of the triac; and

the switch is coupled between the gate and the second terminal of the triac.

22. A circuit for controlling a triac, comprising:

a bidirectional current-limiting active element connecting the triac gate to its opposite power electrode;

a switch connecting the gate to the other power electrode of the triac; and

wherein said current-limiting element is formed of two current sources connected head-to-tail.