Drive Electronics

The brightness of an LED is determined by the forward current. The simplest designs use a resistor to limit this current. The resistor value is calculated by taking the maximum battery voltage, subtracting the minimum LED junction voltage, then dividing by the desired LED current. Multiple resistors can be used to give different brightness levels. The disadvantage of using a resistor in this way is that as the battery discharges it voltage will fall, and hence, so will the LED current and brightness. Also, some power will be dissipated in the resistor; this is wasted energy. The efficiency is inversely proportional to the difference between the battery voltage and the LED junction voltage. i.e. the higher the battery voltage, the lower the efficiency.

A switching regulator can be used to tightly control the LED current whilst maintaining full brightness until the battery is depleted. The switching regulator can also be designed to allow multiple brightness settings. There are several types of switching regulator: A step-up or boost regulator will allow the LED to be powered from a battery voltage below the LED junction voltage (typ 3.4v) i.e. 2.4V from the Speleotecnics FX2 battery; A step-down or buck regulator will allow the LED to be powered from a battery voltage above the LED junction voltage i.e. 6V from the Speleotechnics FX5 or Kirby Kidney Pack; If the battery voltage is very close to that of the LED i.e. from the Speleotechnics FX3 or FX-Headlite, a Buck-Boost, Inverting or Isolated Topology is required. A step-down converter is typically more efficient than a step-up converter due to lower switching currents. However with careful design all topologies can achieve electrical efficiencies of around 90%.

It is outside the scope of this report to discuss the design of these switching converters, however, for the amateur light builder, Luxdrive, Konlux and others offer a range of pre-built and tested converters specifically for driving LEDs. Unfortunately, most of these converters only offer one level of brightness and plunge the user into darkness with no warning when the battery is depleted.