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Bulb-2-LED Bicycle Light Conversion Schematic Circuit Diagram

Global Reach of Elektor: Originating from Holland

Elektor, with its roots in Holland, has established a global presence, reaching far and wide. Similar to the enduring legacy of bicycles, Elektor has become a ubiquitous presence across the globe, transcending geographical boundaries. From the scenic Åland Islands at N60 E20 (a location worth exploring), Anders Gustafsson shared his story: “Frustration brewed as my bicycle light consistently disappointed. Commuting to and from work daily, the light’s performance dwindled rapidly due to battery drainage.

Curiosity led me to ponder the potential of LEDs. The original Osram PR2 bulb, rated at 2.4 V and 0.5 A, promised a modest 10 lm. In contrast, a Cree XP-E LED, operating at 320 mA (slightly lower than its maximum), boasted a remarkable 114 lm output. To ensure consistent performance, I powered the LED using a constant-current switcher, maintaining a steady current even as the battery voltage dipped to 1.5 V. Achieving the desired output, I utilized a Khatod KLCP 20CR lens with a 6-degree angle.”

Bulb-2-LED Bicycle Light Conversion Schematic Circuit Diagram

Ingenious Voltage Step-Up Converter Design

Anders’ Innovative Approach: A Visual Insight

Anders shared a schematic detailing a straightforward voltage step-up converter, featuring the Linear Technology LT1618 chip [1]. Alongside this insightful diagram, he generously provided photographs showcasing his revamped bicycle light. While these visuals serve as inspiration, the core of his innovation lies in utilizing the LT1618, a versatile chip capable of functioning as a constant-current, constant-voltage source. Operating within an input voltage range of 1.6 V to 18 V, this fixed-frequency, current mode switcher boasts a high frequency of 1.4 MHz, enabling the use of compact inductors and capacitors. In this application, a constant-current mode configuration powers the converter using two robust 1.5 V batteries, maintaining a remarkably stable LED current of around 320 mA.

Precision in Design: Calculations and Performance

To achieve a desired output voltage of Vout = 4.64 V, Anders set the values of R2 and R3 according to the equation: R2=R3(Vout/1.263 – 1). While the values might seem arbitrary, they establish essential limits, ensuring a predetermined constant current of approximately 320 mA, allowing the Cree LED to shine brilliantly even under challenging conditions in places like the remote Åland Islands. With careful consideration, the circuit’s constants were fine-tuned, maximizing the LED’s luminosity.

Extensive testing at Elektor Labs corroborated the converter’s excellent performance, particularly when powered from two dry cells (source = 3.0 V) or rechargeable batteries (source = 2.4 V). Notably, the converter’s startup response was meticulously examined through LTSpice simulations, focusing on lower input voltage scenarios (1.8 V) and the nominal value (3.0 V). This analysis proved crucial, especially given the tendency of similar converters to falter with depleted batteries. The simulations underscored the intricate oscillation patterns within the LT1618 at 1.4 MHz.

Space-Efficient Design Choices and Practical Considerations

Considering the limited space within a bicycle headlight, Anders opted for Surface Mount Device (SMD) components, a pragmatic choice ensuring compactness. Consequently, a minuscule circuit board was meticulously designed and fabricated. As a word of caution, anyone experimenting with this circuit on their workbench should prioritize LED cooling. Fitting a heatsink to the LED, even a U-style finned heatsink designed for TO3 devices, is essential for preventing thermal damage and ensuring the LED’s longevity.

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