Bleed Circuit Extends LED Dimming Range
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Solution Overview
Problem
Conventional dimmers struggle to effectively reduce light output of solid state lighting loads, such as LEDs, at low dimming levels, leading to high minimum light output and compatibility issues with phase chopping dimmers, which can result in low end dropout, triac misfiring, and high end flicker.
Innovation Solution
A bleed circuit connected in parallel with the solid state lighting load, comprising a resistor and a transistor, is controlled by a digital control signal to adjust its effective resistance, reducing light output by increasing bleed current as the dimmer phase angle decreases below a predetermined threshold, thereby meeting minimum load requirements of phase chopping dimmers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If a phase chopping dimmer is used to control solid state lighting load, then the dimmer can reduce light output, but the minimum light output remains relatively high (15-30% of maximum) and cannot be reduced further
Solution Approach 1:
A bleed circuit is introduced as an intermediary component connected in parallel with the solid state lighting load. This bleed circuit provides an alternative current path that allows the dimmer to achieve lower light output levels by diverting excess current away from the LEDs, thereby extending the dimming range below the conventional 15-30% minimum threshold.
Solution Approach 2:
The invention changes the electrical parameters of the circuit by adding a bleed circuit with variable resistance. By adjusting the resistance of the bleed circuit in parallel with the LED load, the system can alter the current distribution and achieve lower effective light output levels than the dimmer alone could provide, thus expanding the dimming range.
2Illumination intensity
If the dimmer phase angle is reduced to achieve lower light output, then light output decreases, but the solid state lighting load may cause triac misfiring and low end dropout
Solution Approach 1:
The bleed circuit acts as a mediator between the dimmer and the solid state lighting load. It provides a minimum load path that prevents triac misfiring and low end dropout by ensuring the dimmer always has sufficient load, while still allowing the LEDs to be dimmed to very low levels by diverting current through the bleed circuit.
Solution Approach 2:
The bleed circuit provides beforehand cushioning by pre-establishing a minimum load path that prevents problematic conditions (triac misfiring, low end dropout) from occurring. This protective mechanism is always in place and becomes active when the dimmer phase angle reduction would otherwise cause reliability issues.
3Illumination intensity
If a bleed circuit is added to extend dimming range, then low end light output is reduced, but the device complexity increases
Solution Approach 1:
The bleed circuit is a relatively simple intermediary component consisting of basic electrical elements (resistor, transistor, or other switching element) connected in parallel with the existing LED load. This adds minimal complexity compared to alternative solutions while achieving the desired extension of dimming range to very low light output levels.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution effectively reduces light output at low dimming levels, addressing the high minimum light output issue and ensuring compatibility with phase chopping dimmers, while maintaining compliance with minimum load requirements, thereby enhancing the dimming range and efficiency of solid state lighting systems.
Implementation Method 1
The transistor turns on and off in response to the duty cycle of the PWM control signal
Implementation Method 2
decreasing an effective resistance of the bleed circuit as the dimming level decreases
Data Source
Figure 1~3
Figure 2
Figure 4
AI summary
A device for controlling levels of light output by a solid state lighting load at low dimming levels includes a bleed circuit connected in parallel with the solid state lighting load. The bleed circuit includes a resistor and a transistor connected in series, the transistor being configured to turn on and off in accordance with a duty cycle of a digital control signal when a dimming level set by a dimmer is less than a predetermined first threshold, decreasing an effective resistance of the bleed circuit as the dimming level decreases.