Has anyone tried this? If so, have you tried anything other than Martin's ubiquitous circuits?
I've got myself a single mini XPG on my commuter bike with a simple rectifier and nothing else which is great for 100% road and town miles and an MCE on the MTB which has a smoothing capacitor and low speed boost. They're so good I wonder what I've been doing with batteries all this time.
Next step is standlight, I was wondering if anyone had implemented one, if so, roughly what components have you used and how much time can I expect to get? I was planning a single 5.5V 1.5F super capacitor, discharging through at 50ohm resistor, which should give me a half decent level of brightness. Is it as simple as:
E = 1/2 x C x V^2
for the energy storage, and then trying to get the V of the capacitor as close to its rated value as possible, i.e charge the capacitor over two LEDs, using a pair of diodes to both drop the voltage from the approx 6.4V (measured) to 5.0V, assuming dropping 0.7V over each diode, these diodes can then be serve the double function to make sure I don;t discharge through the two LEDs but through the resistor over a single LED. If that makes sense.
So, I can expect the LED, using V=IR, where the LED Vf = 3.2V and R = 50ohms, to run at 0.064A, which, at 3.2V is 0.2W. I can expect the capacitor to store 1/2 x C x V^2, where C=1.5, V=3.2, 7.68W or energy, giving me 7.6/0.2 = 40 seconds of light before the thing starts to dim?
Please correct me if I'm wrong, which I fully expect to be somewhere!
From memory, to double this time I can run two capacitors in parallel to double the capacitance, but maintain the same voltage rating? Similarly halving the resistor value should double the brightness (but halve the burn time)
Cheers for your help/comments/advice
Rob
