LED's ,the lowdown



There has been a lot of discussion about LED based lighting systems in these forums of late, since they are considerably more efficient than incandescent light bulbs
(2to4 times as energy efficient for the same light output) However LED's are very different devices,for instance applying an ac voltage can destroy them instantly therefore it would appear worthwhile to review their properties and application potential.
LED stands for light emitting diode,it is a particular type of junction diode,which when forward biased,emits photons,with energy levels (color that is) dependent on the bandgap of the junction material.If the diode has a reverse voltage applied to it, no current flows and no light is emitted and if this reverse voltage is large enough, reverse breakdown occurs, resulting in instantaneous destruction of the device.That voltage is usually around
3 V,in the same range as the normal forward bias voltage The moral of this story, don't apply ac voltages to LED's unless the reverse voltage is limited (by another diode or LED).
Conduction properties:LED diodes behave like normal diodes in that respect,above a certain voltage threshold the current increases rapidly with increasing voltage and the device will be destroyed due to thermal breakdown unless the current is limited somehow,for instance by connecting a series resistor between the voltage source and the diode.The increased voltage drop across the resistor will keep the the current from increasing too rapidly.This of course depends on the expected voltage variation and on the size of the resistor (the larger the better),however this is pretty wasteful since a lot of power would be wasted in that voltage dropping resistor.Ideally LED's should be operated from a "current source",that is a source that supplies a constant preset current regardless of the load,for instance it puts out 0.1 Amp and 3V with one LED,with two in series 6 V. but still at 0.1 Amp.
Lead acid or Nicad batteries behave more or less like constant voltage sources,that is the output voltage does not decrease much with increasing load,as the battery gets fully discharged the internal resistance increases and the output voltage under load goes down rapidly,so you can probably get away with using those type of sources and dispense with the power robbing resistor.
The operating voltage of LED's varies quite a bit depending on color and manufacturer.For white LED's it in the 3-3.5 V range.They should NOT be put in parallel,the threshold voltage can vary somewhat between individual diodes resulting in uneven currents and light output,they can be strung in series all sharing the same current.
Heat sinking,high temperatures are the death knell for semiconductor devices and LED's are no exception, therefore heatsinking of high power devices is essential.Apart from thermal runaway high operating Temp. can lead to rapid device aging,but under normal temp conditions they will practically last forever though.In hot regions of this country this is even more of a concern
LED's are instantaneous on off devices they turn off in a fraction of a microsecond,this leads to a flicker effect if pulsed at low frequencies.
Yeah LED's are the bomb. Once completed all the lighting on my bike will be LED based (Daytime running lamp, tail/brake light, directionals, plate light) except for the headlight. Theres just no beating halogen for the price at the moment. Maybe in the future I'll upgrade to a HID or Luxeon LED headlight but thats once it makes economic sense (hid) or puts out more light for the price (luxeon).
As a side note pretty much all LED lighting fixtures purchased for 12v lighting systems have voltage dropping resistors built in. Even though it is a waste of energy as some of the remaining energy not used in lighting gets converted to heat its still much more efficient than incandescent lighting. I've put about 150 miles on my xr-75 since my last charge and my power meter, another small inefficiency, is still reading as full at 13v on my 5ah SLA. I run my lights all the time for safety and legal reasons, none of this riding was done at night with the halogen. I'm hoping that my future daytime running headlight will be bright enough to use at night in the city and I'll only have to resort to the halogen for pitch blackness in the sticks and fast night riding.

When I update my electrical system I'll be sure to take some pictures and do a "how to" as I think this is an ESSENTIAL part of a motored bike. Everybody should be running lights all the time or else you're invisible and you get no respect on the road or RUN OVER.

Good thread, lets kick this problem in the arse.
I'm planning on buying two cheapo LED flashlights from harbor freight and retrofitting them into some classic style schwinn lights. Run the whole works off of a battery. I actually need some sort of headlight to legally ride through one of my neighboring towns.

ETA these links:

This one is only $5.89 and 90,000mcd
This one might be the easiest to retrofit. $5.99
This one comes with a rechargeable battery (wall charger or hand crank!). 12-28 hours run time. $19.99
1 Watt Luxeon (super bright) for $19.99 ($14 on sale now)
3 Watt Luxeon (super brighter) for $19.99
15 LED flashlight. $12.99

I'm leaning towards the Luxeons... but that one with the hand crank and rechargeable battery seems like it might be the easiest to adapt. Although the little $5.99 12v plug in is probably the perfect size to just slap into a headlight.

So many choices!
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An update to the LED discussion. I was poking around in Sam's Club the other day and saw a 2 pack of 3 watt Luxeon K2 LED flashlights for $28 batteries included. They are advertised to output 150 lumen's and have an intense beam with pretty good floodlighting. These things are incredibly bright!!! They run off of 3 AAA batteries at 20hrs a whack and are supposed to have a bulb life of 50,000 hours. These things were a steal. They light up the whole road at LEAST 75-100 feet away in complete darkness. Using the both of them at once would be wicked bright.
I'm seriously considering ditching the halogen lamp I have on the bike since these are definitely a brighter and more focused beam and both of them combined would use 6watts rather than the 55 that my halogen uses cranked all the way up. My 5ah SLA would run these suckers for a whole day!
Check them out they could even be easily mounted with a couple hose clamps looped together around the handlebars and then the flashlight to be fully adjustable as well as easily removable.
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