The LED bicycle light that we described in the July/August 2009 edition of Elektor has proved very popular. The author was particularly struck by the basic design, but as always, there is room for a little improvement! Below we describe two enhanced variations on the original theme. Both circuits are, like 2009 original, powered from a 6 V (rechargeable) battery, shown here as V1. The simpler of the two circuits, which consists of four transistors, is in function essentially the same as the original version. It takes the form of a boost converter with feedback provided by the voltage drop across a current sensing resistor, in this case, R2. A value of 6.2 Ω for R2 is suitable for use with four white LEDs at D4 to D7 and gives an LED current of approximately 20 mA. The 250 mW Zener diode D10 is provided to limit the output voltage in the case that the LED chain should go open-circuit, pulling the gate of the MOSFET to ground via T3, T1, and T2 if the output voltage exceeds the breakdown voltage of the Zener. A breakdown voltage of between 15 V and 24 V is recommended. L1 is a 100 µH coil with a current rating of at least 0.5 A and should have a very low DC resistance.
Transistor T1 provides a low-impedance source to charge the gate of MOSFET T5. Transistor T2 (the author used an SMD BC846S dual transistor) is wired as a diode and is responsible for discharging the gate of T5 via T3. This extension to the original circuit means that MOSFET T5 switches more quickly, which improves overall efficiency. As a side effect, the switching frequency also rises significantly. With a switching frequency of over 150 kHz ceramic or film, capacitors must be used at the input and output, as electrolytes will gradually fall off in effectiveness. In the original circuit a type NTD4815N MOSFET with an on-resistance RDS(on) of 15 mΩ (at VGS=10 V) was recommended, although almost any N-channel MOSFET with similar on-resistance characteristics will be equally suitable.
The second circuit employs five transistors and differs from the first in that it uses a secondary current regulation loop based around transistor T4. This makes the design more suitable for use at higher LED currents, giving greater current stability in the presence of power supply voltage fluctuations. The voltage drop across resistor R6 due to the current flowing through the LEDs turns transistor T4 on. Via T3 this in turns modulates the switching of T5, hence keeping the output current constant. Transistor T4 is a type BC856B; an alternative device in a leaded package is the BC556B. T3 is a BC546B. The BC846S dual SMD transistor used for T1 and T2 can be replaced by a BC546B (for T1) and a type 1N4148 diode (for T2).