Circuit for headlight LED works, but I don't know why
February 20, 2011 11:22 AM Subscribe
It's been cold recently (-15C) and I burned out two bulbs in my dutch bike's headlamp in a short time. Since the bulbs are hard to find, I thought this was a good time to replace the bulb with a 1-Watt LED light. What I built works, but I don't quite know why.
posted by kamelhoecker to Technology (9 answers total) 4 users marked this as a favorite
The headlamp is dynamo powered, and from a bit of googling, it seems that the dynamo is 3W and produces 6V. That's too much to drive my 1 Watt (luxeon clone) LED directly, which has a max rating of 3.6V @ 350mA - so i built two super-basic "circuits" to drop down the voltage and current.
The first just uses a 7805 linear regulator, which limits the voltage to 5V. I thought this wouldn't limit the current and expected it to almost immediately blow the LED, but was able to ride around and the LED still works fine.
The second circuit uses an LM317 based on an article at Instructables. A 3.9 ohm resistor is connected between the adjustment and output, which limits the current to 320mA. (Apparently there is a 1.25V reference voltage running between the ADJ and OUT pins, so 1.25V/3.9 Ohm = 0.320A) I expected this circuit to also blow the LED since the current is limited, but not the voltage. But this setup also works fine.
Now, it's great that both of these setups work (unlike many other of my electronic projects) but I'm curious to know why and which design is better at providing the LED with power closer to the specifications (ie. is not overdriving it). And even more awesome would be tweaks to make either of these circuits less hard on the LED.
(One other detail might may be relevant: There appears to be a black (schottsky?) diode in the casing of the headlight. I don't know why it's there - but i added my circuits after it.)