White LED Specifications and Voltage Compatibility
Typically, it’s widely accepted that white LED function at approximately 3.6 V and require a current of around 20 mA, and this assumption is quite accurate. Interestingly, Lithium-Ion cells happen to provide a voltage of precisely 3.6 V, which appears to be quite convenient. However, directly connecting an LED to a voltage source like the cell is not advisable because it could lead to an excessive current that might damage the LED. Hence, white LEDs are commonly operated using a current source, despite the fact that some energy is inevitably lost in the process.
Operating White LEDs with a Current Source
To effectively power white LEDs, a current source is often employed, ensuring that the LED operates within safe current limits. Nevertheless, it’s worth noting that while this approach is necessary to protect the LED, it results in a loss of energy.
Simplifying Current Source for LED Operation
Creating a proper current source becomes challenging in situations where a few volts need to be dropped, which isn’t the case here. But do we really need a precise current source? The brightness of an LED is primarily determined by the current flowing through it, but our eyes can be easily deceived. While it’s noticeable when comparing two distinct LEDs side by side, slight variations in brightness between different instances are often imperceptible to the human eye. Consequently, the specific current (be it 10, 20, or 30 mA) doesn’t significantly impact our perception. Thus, an accurate current source isn’t essential; a ‘rough’ current source that limits the current to safe levels suffices. In this simple yet efficient design, the current source comprises a resistor of a few ohms in conjunction with the LED’s internal resistance, approximately 10 ohms at 20 mA.
Flexible LED Configurations with Parallel Branches
In practical applications, parallel branches can be added as desired. Securing a single 3.6 V cell might be challenging, but camcorder battery packs with two cells (7.2 V) are readily accessible. Even at 7.2 V, the circuit remains straightforward: two LEDs connected in series with a current-limiting resistor about double the value. To determine the appropriate value for the current-limiting resistor, reference the graph illustrating the relationship between a white LED’s operating voltage and current.
For instance, considering a LED current of 20 mA. The calculation for the current-limiting resistor would be: (3.6-3.44) / 0.02 = 8 Ω. At 3.6 V. The current is 20 mA, reaching approximately 27 mA at 3.7 V and around 16 mA at 3.5 V. In practice, the specified values of 8 Ω at 3.6 V and 18 Ω at 7.2 V can be slightly adjusted; Values like 15 Ω and 33 Ω respectively are still effective.