Introduction to the Control Technique
This circuit employs a clever method of controlling a flyback converter by utilizing the voltage developed on a current sensing resistor. Initially, when power is applied, only a small current flows to charge C4, resulting in insufficient voltage on R3 to activate T2. Simultaneously, D1 enables C2 to charge from the 6 V battery, allowing R1 to provide enough voltage to turn on T1. Consequently, T1 shunts the voltage across L1, initiating a rise in current through it.
Flyback Phase Initiation and Energy Utilization
Upon reaching a specific point, the returning current via R3 generates enough voltage to trigger T2. T2 then diverts the gate voltage from T1, causing T1 to switch off, initiating the flyback voltage from L1. This flyback pulse circulates through the circuit, charging C4 and powering the LEDs. As the return current passes through the current sensing resistor R3, it keeps T2 turned on and T1 turned off, ensuring the flyback phase is not clamped until it has released all its energy. Positive feedback is provided by capacitor C3 to maintain reliable oscillation and sharpen the switching edges.
Bootstrap Boost Circuit and Component Selection
Components D1, D2, and C2 create a bootstrap boost circuit for the MOSFET gate. Although it operates at logic level, it guarantees the specified RD-S(on) only at a Vg level of approximately 8 V. Interestingly, the combined Vf of four ultrabright red LEDs is around 8.8 V, which is the typical clamping value for the output. Specific component choices are vital; for T1, an n-channel MOSFET with a low RD-S(on) of 15 mΩ (at 10 V) is recommended. While a high ID rating (35 A) is not strictly necessary, it’s a suitable choice for this application.
Choosing Diodes and Safety Measures
Purists might consider Schottky barrier diodes for D2 and D4, but a close examination of the BAT85 data sheet reveals its 4 ns Trr is no faster than the 1N4148. The lower Vf is unlikely to yield noticeable differences. To safeguard against open circuit situations, Zener diode D5 has been added. When the flyback converter operates without a load, it can generate a high voltage, potentially damaging the MOSFET. D5 acts as a buffer, protecting components from excessive voltage. The zener’s value can range from 18 to 24 V, depending on the application.
Selecting Components and Resistors
L1, a 220 μH inductor with low DC resistance, is crucial. Avoid using small axial lead inductors disguised as resistors, as they have short lifespans. R3’s value depends on the LED configuration; for example, four white LEDs require about 6.8 Ω. R4 (1 Ω 1%) is temporary, aiding current measurement when setting the correct LED current by adjusting R3. The LED current affects efficiency and determines the switching frequency, crucial for component durability. For instance, at 10 mA (4 white LEDs), the prototype measured 170 kHz.
Construction and Housing Considerations
Constructing the circuit on stripboard is straightforward and can be adapted based on available lamp housings. Housings like Ever Ready or Ultralight, when modified, work well. Often, the hole for the bulb requires notches for LED placement, secured with hot melt glue. The battery and switch box can be challenging; using existing structures like bicycle baskets or ABS project boxes simplifies assembly. Battery boxes, such as a modified Halfords lamp, can securely hold a 6 V 1.3 Ah SLA battery, ensuring a reliable power source.
Legal and Safety Considerations for Bicycle Lighting
It’s essential to adhere to legal restrictions, traffic laws, and potential type approval requirements specific to each country. These considerations are crucial when designing and implementing bicycle lighting solutions.