Phase controlled dimming system with active filter for preventing flickering and undesired intensity changes

An load controller comprising:

• a) wires for connecting the controller to a load and an AC line, the AC line having an AC fundamental component with a fundamental frequency;
• b) an active filter receiving the AC line having a substantially linear phase delay characteristic of less than one-half of a period of the fundamental frequency and having a frequency gain characteristic such that substantially a selected one of the AC fundamental component and the second harmonic frequency of the AC fundamental component is passed to an output of the filter and such that third order harmonics and greater of the fundamental frequency are substantially attenuated relative to a gain of the AC fundamental component;
• c) a zero cross detector coupled to receive the output of the filter and providing indications of zero crossings thereof;
• d) a controllably conductive device coupled to one of the wires for controllably applying an AC line voltage to the load in response to trigger signals applied to a control terminal associated with the controllably conductive device, the trigger signals being generated in response to the indications provided by the zero cross detector and rendering the controllably conductive device conductive for at least a portion of a cycle of the AC line.

A system for controlling an AC lighting load powered by an AC line 10 having dynamic components of noise, distortion and frequency variation, the AC line having an AC fundamental component with a fundamental frequency, comprising:

• a) a thyristor coupled to the AC line for controlling an RMS voltage to be applied to the load in response to trigger signals applied to a control electrode associated with the thyristor;
• b) an active, low pass Bessel filter receiving the AC line in one of a rectified and unrectified form and providing a filtered output substantially free of noise and distortion present on the AC line, and substantially free of frequency components that are substantially equal to third order harmonics and greater of the fundamental frequency, the filtered output being phase delayed relative to the AC line, the frequency of the filtered output varying linearly with frequency variations of the fundamental frequency of the AC line;
• c) a zero crossing detector for providing indications of zero crossings of the filtered output;
• and,
• d) a microprocessor responsive to the zero crossing indications and to user adjustable intensity settings for generating the trigger signals;
• the zero crossing detector indicating the zero crossings in the filtered output in the simultaneous presence of noise and distortion on, and variations in the fundamental frequency of, the AC line.

An AC load control system for receiving an AC line and providing an AC output, the AC line having an AC fundamental component with a fundamental frequency and further having harmonic frequencies thereof, comprising:

• a) a full wave rectifier for producing a full wave rectified DC output;
• b) an active low pass filter receiving the full wave rectified DC output, the filter interposing a substantially linear phase delay of less than one-half of a period of the fundamental frequency and having a substantially flat frequency gain characteristic up to about twice the fundamental frequency and having a rapidly decreasing gain characteristic thereafter such that at least third order and greater harmonics of the fundamental frequency are substantially attenuated, the filter providing a sinusoidal output signal that is a substantial representation of the second harmonic frequency of the unrectified AC fundamental component;
• c) a zero crossing detector that provides indications of zero crossings of the filter output signal;
• d) a control circuit responsive to the indications provided by the zero crossing detector for controlling the level of AC output power delivered to a load.