Improved Variations of the LED Bicycle Light Circuit
The LED bicycle light featured in the July/August 2009 edition of Elektor gained significant popularity due to its design. Despite its success, there was room for enhancement. In this article, we present two improved versions of the original design, both powered by a 6 V rechargeable battery denoted as V1. The first enhanced circuit, comprising four transistors, retains the core functionality of the original. It operates as a boost converter with feedback facilitated by the voltage drop across a current sensing resistor, R2 (with a suitable value of 6.2 Ω).
This setup is ideal for driving four white LEDs (D4 to D7) with an LED current of around 20 mA. To prevent damage in case of an open-circuit LED chain, a 250 mW Zener diode, D10, is included. This Zener diode limits the output voltage, ensuring it does not exceed a specific range (recommended between 15 V and 24 V). The gate of the MOSFET is grounded via transistors T3, T1, and T2 if the output voltage surpasses the Zener’s breakdown voltage. Additionally, L1, a 100 µH coil with low DC resistance and a minimum current rating of 0.5 A, is utilized in this configuration.
Improving MOSFET Switching Efficiency
In this modified circuit, Transistor T1 acts as a low-impedance source, charging the gate of MOSFET T5. Simultaneously, Transistor T2, configured as a diode, discharges T5’s gate through T3. This alteration enhances T5’s switching speed, thereby boosting overall efficiency. An unintended consequence is the significant increase in the switching frequency, surpassing 150 kHz. Consequently, ceramic or film capacitors are necessary at the input and output, as electrolytes lose effectiveness at such high frequencies. The original design suggested employing an NTD4815N MOSFET with an on-resistance (RDS(on)) of 15 mΩ at VGS=10 V. However, any N-channel MOSFET with similar on-resistance characteristics can be used, ensuring compatibility.
Enhanced Current Regulation Circuit
The second circuit distinguishes itself with the integration of five transistors and introduces a secondary current regulation loop centered around Transistor T4. This modification renders the design particularly suitable for higher LED currents. Ensuring stable current output despite power supply voltage fluctuations. When current flows through the LEDs, creating a voltage drop across resistor R6, Transistor T4 activates. Through T3, this activation modulates the switching of T5, maintaining a constant output current. Transistor T4, denoted as BC856B, can alternatively be replaced by a BC556B in a leaded package. T3, a BC546B, plays a crucial role in this regulation. Additionally, the BC846S dual SMD transistor used for T1 and T2 can be substituted with a BC546B for T1 and a 1N4148 diode for T2, offering flexibility in component selection.