Driving LED with low voltage

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Been some time since I last did electronics, so I'm a bit rusty.

I'm building a bike light for night riding and have a 700lumen LED with Vf = 11V @ 900mA

For power I'm using NiMH batteries, but don't want to use more than 8, which will give me 9.6V

Don't suppose there's some way to run the led from just the 8 batteries?

Also, what is the most efficient way to drive a LED?

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That is a serious lamp with a 10W LED!

There are special chips for driving high power LEDs. Effectively they must step up the voltage, and limit the current appropriately. i.e. if they match Vf and Iaverage, then no power is lost in any series resistors.

Since they adjust the output as necessary, they will continue to work as the battery terminal voltage dies.

If you are determined to construct your own control system, you do a Buck step-up converter. You monitor the current to adjust the 'step-up'.

So you can use any convenient Vcc to start off with. The Buck can produce an output.

If this is too daunting, go for 10 cells and lose some power in the 1V voltage drop.

David.

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The power of Led is 10W.If a boost converter is used without any power losses 10W will draw from battery also,so less is battery voltage output,higher the current drawing from the battery.
My opinion is that it would be better to have higher voltage with more battery cells,up to 20V or more and to have a buck mode for step down conversion.Without the power losses of the conversion(which are negligible) 900mA will flow from the battery pack.

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Thanks. I'd rather build the whole thing than buy a LED driver. Will have a look at the options.

Don't really want to have too much cells, as they add to weight and need to fit in a water bottle.

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Don't underestimate the mechanical and heat issues. High-power LEDs do generate quite a lot of heat, which needs to be dissipated. One of the design challenges of AC LED lamps is that the electronics are close to the LED and get hot.

If you have a look at the LED bulbs you can buy they are all in all-metal housings for this reason. I fear your (plastic) water-bottle housing will melt !

Concerning the number of cells: Assuming 18650 Li-Ion cells (2600mAh) you get about 9Wh from a cell. With your 10W LED this is less than 1 hour run-time and there are no losses calculated. I think you can count on half an hour per cell in a real life situation, so you might want something like four cells anyway, just to get enough run-time.

Edit: Just reread and saw you were using NiMh: A NiMh AA cell has around 2000mAh at 1.2V or 2.4Wh. The eight cells mentioned give towards two hours run-time, again assuming no losses.

Markus

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Quote:
what is the most efficient way to drive a LED
That is with short high current pulses, the best way must be in your datasheet!
If you can live with some thing like 6W you should hardly see any difference and the batt. will last longer.
About heat from AC LED's some of those that is out now hardly get's warm and have no metal "heat sink" at all.

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markus_b wrote:
Don't underestimate the mechanical and heat issues. High-power LEDs do generate quite a lot of heat, which needs to be dissipated. One of the design challenges of AC LED lamps is that the electronics are close to the LED and get hot.

If you have a look at the LED bulbs you can buy they are all in all-metal housings for this reason. I fear your (plastic) water-bottle housing will melt !

Concerning the number of cells: Assuming 18650 Li-Ion cells (2600mAh) you get about 9Wh from a cell. With your 10W LED this is less than 1 hour run-time and there are no losses calculated. I think you can count on half an hour per cell in a real life situation, so you might want something like four cells anyway, just to get enough run-time.

Edit: Just reread and saw you were using NiMh: A NiMh AA cell has around 2000mAh at 1.2V or 2.4Wh. The eight cells mentioned give towards two hours run-time, again assuming no losses.

I have a heatsink for the lights and the light sits on the handlebar where the wind will help with cooling of the heatsink.

The waterbottle is only for the batteries and I'll use NiMH with capasities of 2700mAh or higher.

The run-time is somewhat of a problem, but for most of a ride I'll only need about 200-300 lumen, so not full power.

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I see. So I assume you'll add a PWM brightness controller.

I'd use enough cells that you can run the light in full brightness with just some current limiting. You get most out of your batteries that way as no power is lost in a (lossy) boost circuit. 10 cells for 12V might just be right.

Markus

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Was thinking of PWM for brightness yes or some sort of variable current source.

Will have to play around

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I built a high power LED driver circuit for use in a flashlight I built. The input voltage is 4.5V, and it's driving 700mA into a series string of four 3.4V LEDS in one package. The total light output is 430 lumens. It makes for one heck of a flashlight :) The circuit is based around a LM3410 LED driver chip. All the components are surface mounted, and fit onto a 1 inch diameter circuit board.

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I've been driving around with some of these for years: http://www.reelight.com/

It might inspire you to fix the battery problem.

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mmm...
Have been toying around with the idea, but 10watt?

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Another approach could be many common white leds of 3,5V Vforward,driving from an mcu,lighting only one each time.To see them lighting all of them a speed of
80HZ x number of leds must be used.
I have done this for front light using 5 leds,a Tiny12(works perfect in 3V) and two coin cells of 3V connected in parallel.
For rear 15 red leds a tiny2313 and two NIMH 1200mAH connected in series.
The hole idea is instead of one led hungry for AH,is to use many multiplexed,lighting in a fast rate only one each time.The human eye will see all of them lighting together.

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The idea of the lights is not so that people can see me, it's for me to see the trails when I'm flying down singletrack at 20mph(33km/h) on my mountainbike in complete darkness.

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Have you considered a 7Ah 12v SLA batt in the bottle holder

/Bingo

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I have considered it and would maybe use it for training, but they are too heavy for racing.

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"Dare to be naïve." - Buckminster Fuller

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Expensive, but that would be perfect.
8 hours of riding at full capasity

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If your going with a battery voltage higher than the LED voltage you might consider the LM3404...

I've used it recently for a 9W LED desk lamp...
It's a constant current buck mode...

The schematic and pics are here:
http://krazatchu.ca/?page_id=242

Michael

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So it seems that energy per weight is the important factor here...

You might look at lithium thionyl chloride batteries...
While not rechargeable they are supposed to have the highest energy density...
Or you might consider a fuel cell...

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Quote:
I have considered it and would maybe use it for training, but they are too heavy for racing.

Night bike racing? Great idea. Can't you move the race?

No RSTDISBL, no fun!

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Just came across a bike lamp, like you want to build it:

http://www.dealextreme.com/details.dx/sku.44459

- 900 Lumen
- 3h run-time
- Price: $80

Markus

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Brutte wrote:
Quote:
I have considered it and would maybe use it for training, but they are too heavy for racing.

Night bike racing? Great idea. Can't you move the race?

Night racing is awesome. Also want to do some 24h races.