Preventing LED Failure
Ensuring the current through an LED doesn’t exceed its limit is vital in preventing failure. Anyone who has blown up a few LEDs would understand this well. A straightforward solution involves placing a resistor in series with the LED. This method is not only effective but also incredibly economical, requiring just one additional inexpensive component.
Minimizing Power Loss
However, there’s a caveat. As the power supply voltage rises, more power is dissipated in the resistor. This power loss becomes significant, especially in battery-powered devices. To address this, a power-saving version can be devised, albeit at the cost of a few extra components.
Utilizing Commonly Available Components
The circuit presented here utilizes components commonly found in most electronics enthusiasts’ inventories, with the exception of the small coil. In contemporary switching power supplies, monitoring current is a standard practice. When the current or power exceeds the safe limit, components tend to fail. This circuit employs a smart technique to avoid this scenario. Resistor R5 gauges the current flowing through the coil, while T2 ensures it doesn’t surge beyond safe limits. This design ensures that L1 never saturates, preventing T3 from malfunctioning.
Current Control and Efficiency Improvement
When the current flowing through R5 reaches approximately 25 mA, T2 conducts while T1 and T3 block. Consequently, the current seeks an alternative path, passing through LED D1, causing it to illuminate. Placing D1 in this position effectively acts as a free-wheeling diode, enhancing efficiency. When the current decreases, T2 blocks, allowing T3 to conduct. The inclusion of R6 introduces a slight hysteresis, preventing the switching frequency from unnecessarily increasing, maintaining it at around 50 kHz.
Versatility Across Voltage Levels
This circuit operates efficiently with a power supply voltage of about five volts, contingent on the LED’s forward voltage. Notably, at around 9 V, a noticeable enhancement in efficiency becomes apparent. The circuit accommodates various LED types, including blue and white ones requiring 3.5 V. The voltage generated by the coil adjusts automatically, making it adaptable to different scenarios. The maximum allowable power supply voltage is 24 V. The selection of the coil is relatively flexible; values like 3.9 mH, 6.8 mH, or even 10 mH can be used, especially with power supply voltages surpassing 9 V.
Coil Selection Considerations
The choice of the coil primarily depends on its ability to handle at least 25 mA. Physical dimensions, like length (minimum 15 mm) and diameter (7 mm), usually provide an estimation of its suitability. Modern advancements have introduced smaller SMD-coils capable of managing high currents, although their availability beyond 1 mH is limited.