LEDs are diodes that emit light, (Light Emitting Diode). White LEDS actually emit ultra-violet light but are doped with phosphorus that is excited by the ultraviolet light to produce a white light.
The voltage required to pass current through the LED is called the forward voltage drop.
Most White LEDs have a forward voltage drop between 3.2 and 3.7 volts. Even LEDs from the same batch can have a variance in the forward voltage drop. If two early LEDs were put in parallel, only the one with the ever so slightly lower forward voltage drop would light. Today most LEDs have a very small resistance added to them to allow operation in parallel.
If the voltage is less than the forward drop no current passes and the LED is dark. At a voltage just above the forward voltage drop, current flow and the LED lights. A LED has very little resistance once current starts to flow. As the voltages rises, more current is passed, at a voltage only a little higher than the forward voltage enough current passes to burn the LED out.
LEDs are rated for output at a current, higher output LEDs tolerate more current and usually have a heat sink to dissipate the heat.
Its important to limit the current to the LED for it to last, The easiest way is a series resistor. Constance current sources give better results when a constant light output is required. There are also switching power supplies and controllers that pulse higher currents to the LED, but over time the average current still stays within the maximun current rating.
For motorcycles its important to consider the lowest voltage that will be available. This means with all loads on and stoped at idle, I recommend using 11 volts.
3 LEDs in series has a forward voltage drop between 9.6 and 11.1 volts. Three LEDs in series is the most I would consider using with a 12 volt battery system, however, even 3 LEDs in series may go dark in idle on and XS11.
I prefer putting one series resistor for each set of series LEDs. This allows lower wattage resistors and distributes heat better. It also insures that the LEDs emit to their potential
The value of the resistor is determined by the maximum bike voltage, less the series LED total forward voltage drop divided by the recommended forward current. The Cree LED has a max 1000ma (1 amp current), 1 would recommend 800ma (.8 Amps)
Resistor for 3 LED in series (13.9-(3.7x3))/.8 = 3.5 ohms (3.6ohms 2 watts is a standard value)
Resistor wattage = Current x Voltage across resistor = .8A x 2.1 = 1.68 watts
Resistor for 2 LEDs in series 13.9-(3.7x2)/.8 = 8.125 ohms (6.2 and 6.8 omhs at 7 and 10 watts are standard values)
Resistor wattage = Current x Voltage across resistor = .8A x 6.5 = 5.2 watts
Also many LEDS are not tolerant to reverse voltage.
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Yes I did and speaking from experience I still believe you my be disappointed in the performance with the hardware that's readily available at a reasonable price.Maybe another year or so.But don't be discouraged.This is a good idea.TerryLeave a comment:
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Quest for Sunlight
81xsproject,
I took a look at things again and I can't help but feel that steering you away from building the Constant current power supply might disappoint you, depending on how attached you are to that idea. I mean the finnish guy used one for his project and it worked well so it would seem to be the right thing to do. Why vary from something that worked for him, right? What follows is my line of reasoning for his use of the circuit and some insights about your application.
He starts out with the leds of course but the end package looks like this:
I have a flashlight like this and know that it uses four alkaline D cells at 1.5 volts for a total voltage of 6 volts. At his point the guy could have done one of two things to use this power supply for the leds and make it work without the voltage regulator circuit.
1) Wire the eight 3.7 volt leds in parallel with each other with an inline resistor matched for 6 volts. This would result in safe operation and max lumens but with a side effect: as the leds drew current from the batteries the overall voltage would decrease over time. The leds would be bright when the batteries were new but get dimmer as the batteries were drained of power. Not a bad option for those of us used to the way regular flashlights operate.
2.) Wire the 8 leds in 4 sets of two in parallel with each other: 2 times 3.7 volts=7.4 volts. But since the battery supply voltage is 6 volts the leds would be underpowered: 6/7.4 volts=81%. That's losing almost 20% power/lumens at the very start. Not the best option if you're striving for the most light from the parts you have. This arrangement would also get dimmer over time as the batteries were drained. Dim and getting dimmer? I can see why he didn't choose this option.
He chose this arrangement for the leds:
Two sets of 4 leds with each led set wired in series. That's 3.7Volts X 4= 14.8 volts. That's way more voltage than the standard flashlight package of 6 D cells can deliver. So to achieve this higher voltage needed by the 4 leds the guy used two 8 cell holders of AA sized batteries wired in series: 1.2 volts times 8= 9.6 volts per holder times two holders for 19.2 volts. (The guy gets major points for bailing on the alkalines and choosing rechargable Nimh's...)
Here's where he's painted himself into a corner where the only way out of his predicament is to go for the step down voltage regualtor . There is no configuration of series/parallel of the 8 leds which can match them to the available power supply. He can use an inline resistor but that gives him the dimmer over time effect as the batteries drain. In a very strange way I suspect that he already had access to the reduced voltage regulator and therefore integrated it....Not built it and installed it specifically to solve his dilemma.
As options go this ain't too bad. The leds start out and stay at max lumens while the batteries drain down. Well, at least to that point where the supply voltage drops below the 14.8 volts and the step down is no longer needed. On the down side if the voltage regulator fails then it either burns out all the leds or simply stops providing power. Both cases mean darkness in the land of the midnight sun and walking the bike back. BTW the failure rate of an active device like the regulator and the semi-passive leds are separated by a very large margin: Regulator fails way more often.
But here at this point the guy was at a decision node where he could have broken out of the box and eliminated the need for the regulator. If he would have added 2 more leds for a grand total of 10 he would have been golden. 5 leds wired in series times 3.7 volts = 18.5 volts. The supply he used provided 19.2 volts. 18.5/19.2 volts =96% power and lumens. That's quite respectable with that safety margin as well. No regulator needed and he could have also made the two sets switchable/selectable for safe in town riding and bright as the sun when pedaling in the forest. Still would have the dimmer over time effect but the only way he could avoid that was to include the regulator somehow.
The XS1100 power supply is a battery with a regulated charging system which according to the manual, keeps the voltage at 14.5 volts at above 2000 RPM. It will be 14.5 volts after 1 hour of running or 100 hours of running. The dimmer over time factor which justified the finnish guy's use of the step down voltage regulator isn't present on our bikes unless we have a serious problem with the bikes charging system. The matching of the leds to the available 14.5 volts can be done with passive devices like resistors or by a convenient electrical wiring of the leds themselves. Short version: he needed it and you don't.
4 leds times 3.7 volts each equals 14.8 volts. The 14.5 volt supply the bike delivers provides 14.5/14.8 or 97% of the power the leds need to make those lumens safely. IMO the addition of another separate regulator could be used to match the power/voltage needs to the two sets of 6 leds you proposed. I would consider the regulator a bottleneck in the circuit where if the regulator went belly up then none of the leds would work.
Still, you've got the free choice to make the mod the way you want it. I, like the finnish dude live in an area where the sun disappears for so long that some people actually are affected by the lack of sunlight. The only improvement to your mod would be for you to add those full spectrum leds:
Wouldn't that be nice....sigh.Leave a comment:
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Sorry I wasn't paying attention,like larry said in his second post after coffee kicked in.Use enough leds so you don't have to worry about resisters or voltage regulators.Resisters waist power and so do passive regulators.A switching regulator would waist less power but is not necessary with the right number of leds.You may need to add some heat sinks and venting inside your head light assembly as well.I don't think you'll gain a lot of illumination per amp with the parts we have available right now,but I like your idea.Keep us posted.
TerryLeave a comment:
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I already found perfect lens to take the light down to around 10 degree beams.Check out this site if you want to know what Cree chips are capable of, in particular the X21 (welding with batteries).
I have a P7 hand held and an H7 headlamp I use for hunting. The P7 fits to a scope mount and is by far the best torch I've ever owned. I think the stumbling block for a headlight application will be the lens required for focussing. Led LED's are plenty bright enough but are useless without a well designed reflector and lens setup. Not saying it wont work but it'll require some clever engineering.Leave a comment:
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OOps!
I re-read the original post again. (Coffee finally kickin' in..)
Consider keeping the total number of LED's as 12 but breaking them up electrically into groups of four to keep them happy...
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Magic number?
I agree with Randy about using a resistor to drop the voltage. Just make sure it is the correct wattage. The reason a separate constant current driver is unnecessary is because the bike's rectifier already regulates the supply voltage.
Just a thought, but if you used 8 of the LEDs instead of just 6, with each set of 4 of the LEDs wired in series, you wouldn't need the resistor (which wastes energy in the form of heat...) in the first place.
3.7 times 4= 14.8 which is .4 volts over the 14.4 max voltage the rectifier will likely ever put the supply voltage. Sorta a nice safety margin between the max voltage of the LEDs and the supply.
Then of course the two different sets of 4 could be switched to operate independently or together for that High/Low beam effect.
Keep us posted.
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B. Walker 5 that is
an amazing light system. I gotta go thru that site slowly and learn a lot. I have some LED projects in mind, an this will be a big help. Thanks.
Ole JackLeave a comment:
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LED Lights
Check out this site if you want to know what Cree chips are capable of, in particular the X21 (welding with batteries).
I have a P7 hand held and an H7 headlamp I use for hunting. The P7 fits to a scope mount and is by far the best torch I've ever owned. I think the stumbling block for a headlight application will be the lens required for focussing. Led LED's are plenty bright enough but are useless without a well designed reflector and lens setup. Not saying it wont work but it'll require some clever engineering.Leave a comment:
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As Ivan stated, a Radio Shack Voltage reg. is all you should need. They usually use resistors to adjust the voltage. You will still need resistors in line with the LED's to keep the Amps below the fail rate of the LED's. Remember, the brighter the LED, the lower the life!Leave a comment:
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You don't need anything complicated, just the right resistor value to drop the 12v to the working voltage of the led.
calculator here:http://led.linear1.org/1led.wizLeave a comment:
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You talking constant current or constant voltage?
Solid state voltage regs are cheap anddd can be found at most electronics supply houses, even some radio shack's have them.
Interesting using LEDs for headlights. Didn't know they were that high output, but they keep getting brighter all the time.Leave a comment:
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Electrical Wizard/Guru/Engineers help needed
So, I have begun the planning process to build an LED headlights. There will be two lights similar to THIS with 6 of THESE LEDs in the pic each for a total of 12. Overkill...? Maybe, but that is my personality. Like all my projects, it will be as much DIY as possible. In that spirit, I would like to build a constant current driver circuit for the lights. This is also where I need help. I don't know much about electronics therefore don't know how to design the circuit. I have been reading through websites like Insructables but haven't found one where anyone is powering anything from a 12v automotive power source.
If anyone has the know-how in this area, your help would be much appreciated.
Did you explore the link in my first post? That guy's 8x 3W Luxeon light would practically blind you.
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