Make your own LED lighting

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kwackers
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Postby kwackers » 19 Oct 2008, 6:26pm

If you're using a 12v battery and your LED's only need 6v then (assuming your regulator is a bog standard linear one) half the power your using is lost in the regulator.

Can you not stick another LED in series - current stays the same but voltage required lifts to around 9v, half the power wasted suddenly appears as light. Increase in light output of 50% for no cost!

And your regulator runs cooler - smaller heatsink etc.

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lauriematt
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Postby lauriematt » 19 Oct 2008, 8:03pm

i am only using 12v because the website gave an example

because the current controller has a lot of 'play' in it you can over-power a component so to speak

it gave an example of using a 6v component, saying you could use anything from 7v battery up to 19v battery......from this range the website recommended using a 12v battery


....i could get away with using a 9v battery with the same current output. would it matter if i used 9 or 12v???
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EdinburghFixed
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Postby EdinburghFixed » 19 Oct 2008, 8:47pm

The forward voltage of the LEDs I used, plus the drop of the current supply came to 6.9V - the battery I use is rated 7.4V but reads 8.5V when charged, and I get the 'right' amount of power from it (dividing mAh capacity by 700mA is close to the real run time).

kwackers
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Postby kwackers » 19 Oct 2008, 9:08pm

lauriematt wrote:i am only using 12v because the website gave an example

because the current controller has a lot of 'play' in it you can over-power a component so to speak

it gave an example of using a 6v component, saying you could use anything from 7v battery up to 19v battery......from this range the website recommended using a 12v battery


....i could get away with using a 9v battery with the same current output. would it matter if i used 9 or 12v???



Batteries are rated in ampere hours, if you strung cells in series to get 12v and say they lasted 3 hours at 700mA, then putting less in series to get 9V they'd last exactly the same amount of time.
But they'd be cheaper and smaller!

The only important bit is.
Does the forward voltage drop of the LEDS (6V??) plus the voltage drop across the regulator (say 1.2v) add up to less than the batteries when they're in a reasonably discharged state.

If they don't then the lights will go out before the batteries are discharged and your run time won't be as long.

However don't assume using a 12v battery will allow you to run it down to 7.2v (above example), in doing so you may seriously harm the battery.

The manufacturer of the batteries you're using should be able to provide you with the discharged voltage, the LED's should have their voltage drop and ditto the manufacturer of your regulator, some simple maths should tell you whether you can get away with 9v, but if you can then personally I would.

GeoffL
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Postby GeoffL » 22 Oct 2008, 8:47am

This is my first dabble into LED technology, so apologies if my questions are banal!

If you offer an appropriate DC voltage to an emitter, do you actually need a current controller? I'm trying to get my head around creating a light set based on the Cree XR-E Q5 and I've found a constant current controller with only a 0.8v drop, but I want to use an R/C car NiMH power pack and that 0.8v means I'd effectively need one more cell than standard packs have.

The spec for the Cree XR-E Q5 gives V max as 4v, V typ as 3.5v, with currents of 1000mA @ 3.7v and 350mA at 3.5v. Light output is given as 107 lm min and 228 lm max. AIUI, LiMH cells give 1.2 or just under for almost all of their discharge cycle, so if I power a Q5 emitter with three cells instinct tells me I'll be presenting 3.6v to the emitter for most of the cycle tapering to 3.3v at the end of useful power. That should give (deduced from info by Ay-Up and EdinburghFixed) 160 lm or thereabouts for most of the cycle. If this works, I should be able to power 2 Q5s in series from a standard stick or saddle pack, but am I missing something here?

TIA,

Geoff

kwackers
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Postby kwackers » 22 Oct 2008, 10:05am

GeoffL wrote:This is my first dabble into LED technology, so apologies if my questions are banal!

If you offer an appropriate DC voltage to an emitter, do you actually need a current controller? I'm trying to get my head around creating a light set based on the Cree XR-E Q5 and I've found a constant current controller with only a 0.8v drop, but I want to use an R/C car NiMH power pack and that 0.8v means I'd effectively need one more cell than standard packs have.

The spec for the Cree XR-E Q5 gives V max as 4v, V typ as 3.5v, with currents of 1000mA @ 3.7v and 350mA at 3.5v. Light output is given as 107 lm min and 228 lm max. AIUI, LiMH cells give 1.2 or just under for almost all of their discharge cycle, so if I power a Q5 emitter with three cells instinct tells me I'll be presenting 3.6v to the emitter for most of the cycle tapering to 3.3v at the end of useful power. That should give (deduced from info by Ay-Up and EdinburghFixed) 160 lm or thereabouts for most of the cycle. If this works, I should be able to power 2 Q5s in series from a standard stick or saddle pack, but am I missing something here?

TIA,

Geoff



LED's aren't linear devices, the forward current compared to their forward voltage doesn't have a linear relationship. So the current through the LED might be 1A at 3.7v, but it could (for example) be 4A at 3.8v!

You need to consider the battery pack as being in series with a resistor who's value changes. The voltage of the batteries doesn't reduce as they discharge - what happens is the resistance of the series resistor increases which causes the voltage drop across it to increase dependent on the current drawn.

The whole thing is fairly complex and difficult to predict, if you have the curves for the LED's and for the batteries, you could make a prediction of the outcome. Without it's just a guess.

Only very cheap light sources connect a LED directly to a (usually very small) battery, and with good reason.

If you fancy experimenting and don't mind losing the LED, give it a go. Worth bearing in mind though if you force too much current through it the damage usually causes them to become inefficient and so the light output drops rather than they just stop.

The likely outcomes are, initial current too high, LED damaged (either immediately or over time). Or brightness falls off very quickly and so your effective run time isn't what you were hoping for.

I'd personally stick an extra cell on and put the current limiter in. You'll get consistent results and probably be much happier.

GeoffL
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Postby GeoffL » 22 Oct 2008, 11:10am

Thanks, kwackers.

I understood that LEDs are not linear (350mA @ 3.5v and 1A @ 3.7v from the Q5 specs shows this).

Since the Q5 is rated at 4v max I guessed that 3 NiMH cells (i.e. nominal 3.6v) aren't going to overdrive them but I take what you say on board. The issue is that 7.2v power packs are cheap and readily available. 3300mAh stick packs can be had for just over fifteen quid, which would give a theoretical burn time of 3 hours or more at 1A and 8 to 9 hours at 350mA, which isn't a bad burn time considering the output is approx 160 lumen per emitter at the lower current!

That said, I've just had a quick search around and found some 3300mAh 8.4v stick packs for under thirty quid. I've even found a mains charger for under a tenner that can "do" 300mA at that voltage, making overnight top-ups possible while touring.

I guess that you're correct, and it makes sense to limit the current even though it makes the component cost much higher.

Thanks again,

Geoff.

Edited to add: I've just found this, 4300mAh NiMH 8.4v stick pack for £30.40 a pop. At a guess this probably weighs less than 400g and would give a theoretical burn time of 12 hours at 350mA. HTH.

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lauriematt
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Postby lauriematt » 22 Oct 2008, 2:27pm

you can find some good battery packs on ebay
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insanityideas
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Postby insanityideas » 23 Oct 2008, 12:09am

lauriematt wrote:you can find some good battery packs on ebay


And with a constant current controller you can plug in various different sizes of pack without risk of damage (within reason as higher voltages generate more heat)... so you get room to experiment as battery technology changes.

As someone who has nuked a few LED's I now use a 10Watt resistor to test my circuits, it works pretty much the same as an LED for the purposes of testing and calibrating a constant current controller.

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EdinburghFixed
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Postby EdinburghFixed » 23 Oct 2008, 11:12am

I tested mine by holding it all together on the arm of the sofa!

After I regained my vision, I was quite pleased with the results... :oops:

This light business is ridiculously addictive. I've already started toying with the idea of reinforcing my front Ay-Up (which is a relatively wide beam) with a super laser spot pointing further ahead. Something well-behaved enough not to go in drivers' eyes!

Must resist...

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Graham
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Postby Graham » 26 Oct 2008, 8:30pm

OK, I've done a bit of research, based on the previous posts, and I still don't quite understand !!

Startpoint is to build a new LED front light using my existing Lumicycle NiMh 13.2V battery pack.
Hopefully I can use the Seoul P4 LEDs, recommended by "insanityIdeas".

Do I need a 350mA driver or a higher output ??

Despite a scientific education, all this electrical stuff has faded from my erm . . .
. . . memory!

Cheers

kwackers
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Postby kwackers » 26 Oct 2008, 8:58pm

Graham wrote:OK, I've done a bit of research, based on the previous posts, and I still don't quite understand !!

Startpoint is to build a new LED front light using my existing Lumicycle NiMh 13.2V battery pack.
Hopefully I can use the Seoul P4 LEDs, recommended by "insanityIdeas".

Do I need a 350mA driver or a higher output ??

Despite a scientific education, all this electrical stuff has faded from my erm . . .
. . . memory!

Cheers



Looking at the led data sheet. Each led drops 3.25v - so you want to put as many in series as your supply can deal with, in this case I'd suggest 3.

The total forward voltage drop is then 3 x 3.25 = 9.75v

That leaves 13.2 - 9.75 = 3.45, allowing for drop across your regulator and voltage drop as your batteries discharge.

The max forward current is 1000mA - each led producing 240lm each, at 350mA they produce 100lm.

So you need a constant current source that produces somewhere in the range or 350mA to 1000mA. Even better if you can switch it to give different brightness levels.

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Postby insanityideas » 27 Oct 2008, 12:00am

I think my addiction to LED lighting (as suffered byEdinburgh Fixed) has gone to a new level. I just purchased a small microcontroller thingy (won't bore you with the more technical explanation). That can run software code that I write myself. Primarily I did this as a lighting controller for a LED lamp I plan to build for in home use (RGB colour mixing using a remote control).

But

I discovered it comes in a version soooo small that it would fit in my bike lights, enabling such things as a more wacky flash pattern, onboard battery voltage monitoring (and feedback through flashing/dimming the lights).

I think I would probably be more tempted to make a POV (persistance of vision) spoke light that makes pretty patterns in the wheel.

I think maybe I need to get a life.... although with my life as it is I don't expect to do anything useful with my new toy for at least a year!

Anyway just some random waffle for you.

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Postby insanityideas » 27 Oct 2008, 12:07am

Had an interesting dialog with a car driver about bike lights on my way home from work...

He had been following me, coming to a 3 lane set of traffic lights on red. I waited in the ASL, he patiently waited behind in the lane next to me. After a while he slowly rolled forward into the ASL wound down the window and asked me "is it legal to have flashing lights on your bike", to which I replied that it had been for some time, he then informed me that "I could really freak someone out with my lights", I informed him that it wasn't at the correct frequency to induce photosensitive epilepsy, at which point the lights turned green and our conversation ended. He failed to pull away from the lights quickly enough and a Bristolian car driver tooted him. I didn't have time to point out that he was the only one breaking the law by being in the ASL (probably for the best).

So far I have had two positive comments from cyclists stopped at ALS's asking where I got my lights... and one car driver concerned about the fact they flash.

Nobody has complained about brightness. Although I must assume that all other flashing bike lights are not bright enough for the car driver to have seen them, given his surprise at this new fangled flashing bike light phenomenon. Either that or the £4 flashing BS approved LED on my pannier bag caught his attention!!

I love commuting by bike, you get to talk to people.

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Graham
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Postby Graham » 27 Oct 2008, 10:47am

kwackers wrote:Looking at the led data sheet for Seoul P4 LEDs, each led drops 3.25v - so you want to put as many in series as your supply can deal with, in this case I'd suggest 3.

The total forward voltage drop is then 3 x 3.25 = 9.75v

That leaves 13.2 - 9.75 = 3.45, allowing for drop across your regulator and voltage drop as your batteries discharge.

The max forward current is 1000mA - each led producing 240lm each, at 350mA they produce 100lm.

So you need a constant current source that produces somewhere in the range or 350mA to 1000mA. Even better if you can switch it to give different brightness levels.

Thanks Kwackers. That info provides a breakthrough in my understanding. This could be fun. :wink:

insanityideas wrote:I just purchased a small microcontroller thingy (won't bore you with the more technical explanation). That can run software code that I write myself. Primarily I did this as a lighting controller for a LED lamp I plan to build for in home use (RGB colour mixing using a remote control).
. . . . it comes in a version soooo small that it would fit in my bike lights, enabling such things as a more wacky flash pattern, onboard battery voltage monitoring (and feedback through flashing/dimming the lights).

Please keep us posted on this. . . . sounds like even more fun.