Variable-impedance load for LED lamps
In various embodiments, a startup circuit for a low-voltage-lighting electronic transformer includes a low-impedance output circuit. At the beginning of an AC signal cycle, the low-impedance output circuit presents a low-impedance path to the output of the transformer, thereby causing a surge of current in the transformer and initiating oscillation therein.
1. A circuit for starting a transformer that powers an LED lamp, the circuit comprising:
circuitry for presenting a low-impedance path to an output of the transformer during a beginning of an alternating-current (AC) signal cycle,
wherein the low-impedance path causes a surge of current in the transformer during the beginning of the AC signal cycle, thereby initiating self-oscillation in the transformer and powering the LED lamp.
2. The circuit of claim 1 , wherein the transformer transforms an AC signal to a low-voltage signal.
3. The circuit of claim 1 , wherein the circuitry removes the low-impedance path during a remainder of the AC signal cycle.
4. The circuit of claim 3 , wherein the AC signal is at least one of a mains voltage or a dimmer voltage.
5. The circuit of claim 1 , wherein the circuitry for presenting the low-impedance path comprises a clamp circuit.
6. The circuit of claim 5 , wherein the clamp circuit comprises a capacitor, the charging of which provides the low-impedance path.
7. The circuit of claim 1 , wherein the circuitry for presenting the low-impedance path comprises a transistor-based switch.
8. The circuit of claim 7 , further comprising a sensing circuit for enabling the transistor-based switch during the beginning of the AC signal cycle and disabling the transistor-based switch during a remainder of the AC signal cycle.
9. A method for initiating self-oscillation in a transformer that powers an LED lamp, the method comprising:
presenting a low-impedance path to the output of the transformer during the beginning of an alternating-current (AC) signal cycle; and
stimulating a surge of current in the transformer during the beginning of an AC signal cycle in response to the low-impedance path, thereby powering the LED lamp.
10. The method of claim 9 , further comprising removing the low-impedance path from the output of the transformer during a remainder of the AC signal cycle.
11. The method of claim 9 , wherein the surge of current is generated at least in part by firing a diac.
12. The method of claim 9 , wherein presenting the low-impedance path comprises clamping an output voltage of the transformer.
13. The method of claim 9 , wherein presenting the low-impedance path comprises switching in a low-impedance load at the output of the transformer.
14. The method of claim 9 , further comprising initiating a self-oscillation in the transformer in response to the surge of current.
15. The method of claim 14 , wherein the self-oscillation in the transformer creates a bias voltage in an auxiliary winding of the transformer.
16. The method of claim 15 , wherein the bias voltage is a feedback control for a transistor bridge in the transformer.
17. An LED lamp circuit for use with a self-oscillating transformer, the LED lamp circuit comprising:
an LED lamp connected to an output of the transformer; and
circuitry for presenting a low-impedance path to an output of the transformer during a beginning of an alternating-current (AC) signal cycle,
wherein the low-impedance path causes a surge of current in the transformer during the beginning of the AC signal cycle, thereby initiating self-oscillation in the transformer and powering the LED lamp.
18. The circuit of claim 17 , wherein the circuitry comprises a clamp circuit.
19. The circuit of claim 17 , wherein the circuitry comprises a transistor-based switch.