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;
diverting the current from the first network with dimming conditioning circuitry as a function of the waveform of the periodic excitation voltage; and
wherein a resistor within the dimming conditioning circuitry dynamically increases an impedance of the bypass path as a substantially smooth and continuous function of said current amplitude in response to said current amplitude increasing in the range 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 at least one transistor in series with a resistor.
4. The method of claim 3 wherein the dimming conditioning circuitry further comprises at least one diode in series with the resistor.