The latter type of lamp features better efficiency (in converting electric power to light), longer lifetime and very robust construction - to name just a few advantages. However, unlike conventional filament lamps, high-power LEDs are far less forgiving in electical supply. If over-powered - be it for a split second - they tend to heat and burn. Then, the main issue is to regulate the (direct) current through the LED, by use of current-limiting electronics. Two popular simple versions are shown here.
In the first circuit, LM317 - an adjustable voltage regulator - powers the LED set in line with a resistor which creates a voltage drop. By construction, the LM317 chip includes a compensating circuit that keeps the voltage difference across its “GND” and “OUT” pins at a pre-set 1.25 Volt. Therefore, choosing the value of the resistor (say R1, in Ohm) sets the current (I1 in Ampere) at a constant value I1=1.25/R1.
The second circuit works in a similar way: the LED is driven by a power transistor Q1 which acts as a variable resistor. Its state (between pass and cut-off) is set by a second transistor Q2 which monitors the voltage drop on a resistor connected in line with the LED. In this version, Q2 saturates (thus driving Q1 into the cut-off region) around I1*R1=0.7 Volt. Again, choosing value of R1 sets the value of I1.To start with, bike electical systems typically include a dynamo, producing alternating current (AC). On the other hand, LEDs are polarised devices, operating on direct current (DC) - the one i used is a 1 Watt star rated at 350 mA. Both circuits above use a simple AC-DC converter, consisting of a full-wave rectifier bridge followed by a smoothing capacitor C1. The value of C1 relates to the frequency of the AC current produced by the dynamo (which, in turn, is proportional to the bike’s speed). I found out my bike’s hub dynamo (a 28-pole Shimano DH 3N70, rated at 6 Volt / 3 Watt) alternates at approximately 1,7 Hertz per km/h. For a typical speed of 10 km/h, the period is 60 ms, which - combined with an internal resistance of 8 Ohm leads to a rough-cut estimate of an ideal capacitor in the order of several milli-Farad. This is far beyond the space allowance of the fixture, thereby this simple learning: use the largest capacitor that mechanically fits. Point (and pray).
I decided to keep the original headlamp casing and mirror. I removed the original halogen lamp and squeezed the electronics (in this case, the LM317 circuit above) inside the body of the headlamp. For the actual LED element (a 1 Watt star, powered at 3V-330mA), to go through, i had to widen the hole of the plastic cup supporting the reflector. The largest capacitor fitting the fixture was an electrolytic 2200uF/25V.As expected, the light flickers at low speeds - however, once on the go, the white LED produces a cool bright beam.
