Spectral shift control for dimmable AC LED lighting
Apparatus and associated methods involve operation of an LED light engine in which a relative intensities of selected wavelengths shift as a function of electrical excitation. In an illustrative example, current may be selectively and automatically diverted substantially away from at least one of a number of LEDs arranged in a series circuit until the current or its associated periodic excitation voltage reaches a predetermined threshold level. The diversion current may be smoothly reduced in transition as the excitation current or voltage rises substantially above the predetermined threshold level. A color temperature of the light output may be substantially changed as a predetermined function of the excitation voltage. For example, some embodiments may substantially increase or decrease a color temperature output by a solid state light engine in response to dimming the AC voltage excitation (e.g., by phase-cutting or amplitude modulation).
1. A method of conditioning current in a light engine steps comprising:
providing a pair of input terminals that receive a periodic excitation voltage;
receiving a current of equal magnitude and opposite polarity into each one of the pair of terminals, said current flowing in response to the excitation voltage;
providing a plurality of light emitting diodes (LEDs) arranged in a first network, said first network arranged to conduct said current in response to the excitation voltage exceeding at least a forward threshold voltage associated with the first network;
providing a plurality of LEDs arranged in a second network in series relationship with said first network;
providing a bypass path in parallel with the second network;
dynamically increasing an impedance of the bypass path as a substantially smooth and continuous function of said current in response to a current amplitude increasing in a range above a threshold current value; and
diverting the current from the first network with dimming conditioning circuitry as a function of the waveform of the periodic excitation voltage;
wherein a resistor within the dimming conditioning circuitry dynamically increases the impedance of the bypass path as the substantially smooth and continuous function of said current in response to said current amplitude increasing in the range about a above the threshold current.
2. The method of claim 1 wherein the current is diverted as the waveform of the periodic excitation voltage approaches zero volts.
3. The method of claim 1 wherein the dimming conditioning circuitry comprises a shunting path through which current flows as the waveform of the periodic excitation voltage approaches zero volts.
4. The method of claim 3 further comprising the step of dynamically increasing an impedance of the shunting path as a substantially smooth and continuous function of said current amplitude.
5. The method of claim 1 wherein the dimming conditioning circuitry comprises at least one transistor in series with a resistor.
6. The method of claim 5 wherein the dimming conditioning circuitry further comprises at least one diode in series with the resistor.