Improvements / Modifications

Up to 3 LEDs can be fitted to the aluminium plate. The LED positions are shown in the mechanical drawing (figure 3), and the connections are shown in the schematic (figure 1). When using multiple LEDs each can be fitted with a different optic, this gives a composite beam. (Like both the Scurion and Sten light.)

I recommend that the total LED power is limited to an absolute maximum of 3.6W due to the heat dissipation. i.e. when using one LED it can be driven at up to 1A, and when using three LEDs each can be driven at up to 350mA. This will give 240 Lumens of light (slightly more with multiple LED due to their higher efficiency at the lower current) which is comparable with the new Speleo Technics Nova+. At such a high power the aluminium plate will get very hot; this will have an adverse effect on both LED life and efficiency.

With the design presented, if there is no desire to use alkaline batteries and a shorter battery life is acceptable, the main bean resistor value can be dropped to as low as 1.1Ω. This would give an LED current of almost 1A and a noticeable increase in brightness. The side effect is the battery would be discharged in 2.7 hours.

Resistor values can be calculated using the formula below:

Where I_LED is the desired LED current and V_LED is taken from the graphs on the datasheet.

The calculated resistor value should be rounded up to nearest standard value, the standard values are:

1, 1.1, 1.2, 1.3, 1.5, 1.6, 1.8, 2.0, 2.2, 2.4, 2.7, 3.0, 3.3, 3.6, 3.9, 4.3, 4.7, 5.1, 5.6, 6.2, 6.8, 7.5, 8.2, 9.1, 10, 11, 12, etc.

The battery life, resistor power dissipation and efficiency can then be calculated using the formulas shown in figure 2.

With a twin LED setup each LED could be switched separately, this may de desirable for photographers when one LED could have a narrow beam for normal caving, the other a wide beam for taking photos.

Note:

All results presented are based on typical values from the LEDs datasheet. The voltage drop across the LED has a wide range of values, and is dependant on both LED current and temperature. When higher currents are used it is safer to calculate the worst case LED voltage from the datasheet to ensure the LED is not damaged. Or a current meter and temperature probe should be used to ensure the LED is operated within its design parameters.