3 Watt LED Bike Light
February, 2008, Rev c
Prototype 1 - Resistor Controlled Prototype
This experiment was to see how well a simple resistor can be used to adjust the circuit current for a bike light. This is the easy way to power LEDs and is the cheap method used by all LED flashlights that require specifically 3 AAA batteries. I chose this unknown Chinese brand single LED module to get the brightness without extra work assembling multiple LEDs. The LED module was epoxied (JB Weld) on the bottom of a surplus heat sink form a TO-3 transistor power supply. This heat sink was used only because I had one in my electronics junk pile. It is a double heat sink which is why it has external and internal fins. The two pieces are mated with thermal paste and screwed together. The reflector is from a Mag-lite, trimmed at the bottom to fit the LED module. Two 5W, 2 ohm, resisters are in parallel to drop the 12 volt lead-acid battery so that the LEDs will not be over driven.
module DX SKU-5876 rated at 10 watts ( All
items with designated with DX were obtained from the dealextreme.com
and is a modular configuration (traces can be easily seen with a
magnifying glass) of 3x3 series-parallel 1 watt LEDs. Since 3 LEDs in
series would have a Vf (forward voltage) of 3 x 3.7vf/led = 11.1
volts, it was convenient to use a 12 volt battery to power the light.
The current through the module is supposed to be about 1 watt per
each parallel path, so the total current is about 3 watts, which is
why it is advertised as a 10 watt LED. The resistor needs to absorb
an additional 1.4 volts if the battery is putting out 12.5 volts. W =
I˛ x R , so R = W / I˛ = 10 watts / 3˛ = 10/9= about 1
ohm. So the LED module will operate at about 10 watts with the
battery at 12.5 volts. Of course the light will be brighter with a
full charge on the battery and dimmer as the battery discharges. But
it is a simple circuit. The result is a prototype that gets quite hot
and need a substantial breeze over the unit to keep from getting to
hot. This design needs a larger heat sink.
After verifying the concept of using a resistor, I attached a reflector just to make the unit useful. I happened to have one from a Mag-lite in my junk box so that is what was used. The Mag-lite reflector is a parabola for a point source but the LED module is flat as seen above, so the beam pattern was full of rings and actually projected the LED pattern. A piece of translucent plastic notebook cover was used to diffuse the light to an acceptable pattern. The beam is very broad and suitable for trail riding, but not on a roadway. On coming bikers or cars would have to face significant glare.
The problem of clamping the lights to the handle bars had been solved some time ago when I built several “Cheap 20 watt bike lights”. The Rockland Tool Co. plastic 4” clamps, which have a very powerful spring, work very well on any handlebar. I have clipped the clamp tips in the photos below to make clearance for brake cables. Bolts or cap screws were used to attach the lights to the clamps, as seen below. A serrated lock washer was used between the clamp and the heat sink to prevent lateral twisting. A piece of electrical tape wrapped around the handle bars gives the clamp a non-slip surface, absolutely preventing any movement. This design has the advantage of being portable between different bikes, and can be removing from the bike to prevent theft. These specific clamps are made for Ocean State Job Lot (a New England surplus and overstock “stuff” chain). I did not find useable plastic clamps at Home Depot, Lowes Stores, or Sears. Ace Hardware and Aubuchon Hardware may have usable ones. Two Internet sources that seem to be the same are Lee Valley and Bargain Smart. For comparison, the larger clamps I used are 4 ˝ “ long, and the smaller ones are 3” long.