# Driving LEDs from var power source - Efficiency

Discussion in 'LEDs and Optoelectronics' started by kickit2, Nov 20, 2011.

1. ### kickit2

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Nov 20, 2011
Just for starters let me say that I haven't done really anything with electronics since high school and if Im missing something stupid, I appoligize.

I am working on a basic light fixture with consists of 84 leds. The idea is that this system will run off 10V via a LDO regulator allowing some flucuation in the power source. The need for that fluctuation is due to this lighting fixture being connected to a solar system - Hence the voltage could be 11v - 13.4v or so depending on the charged state of the batterys.

Here is the issue - the lighting portion of the circuit uses roughly 7.5W. Problem is that by the time you trim off 1 to 3.5v, I am disapating up to 3W in heat. Thats a 40% loss due to the need for a voltage regulator. Is there a better way to do this? Something that is in the order of 10% loss or even better. Perhaps a new way of wiring the LED's that I'm not thinking about.

Also, keep in mind that efficiency is the key here and not so much cost - but even cost has a point where it becomes prohibitive.

Shawn

2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,497
2,838
Jan 21, 2010
The answer is to use a switchmode regulator.

However you're still going to have some additional losses because your 84 LEDs will not be in one string so you will require resistors (or multiple current sources) to ensure that each string gets the same current.

It would be more efficient to run the device from a higher voltage. The losses are in several areas, but series resistors to balance the current in the LED strings and diode losses in the regulator tend to become larger with decreasing voltage.

3. ### BobK

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Jan 5, 2010
Is there a reason you want to use 84 LEDs adding up to 7.5W? High power LED's designed for lighting are actaully much more efficient than the indicator LEDs you are likely using. Most likely you would get more light out of a single 3W LED.

Bob

4. ### kickit2

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Nov 20, 2011
The main idea behind it was a more diffused light over a larger area then a single intense point of light. Although I do feel that you are probably right and I could perhaps fird 6-10 higher power lighting leds that would give the same effect more efficiently. Thanks for the suggestion..

UPDATE: Actually - after looking into this, I don't see how the high power LED's are more efficient.. Perhaps more power (luminious) for the ammount of room they take up, but using very bright white 5mm strawhat LED's I can get 20k mcd for .0825W each. The same level of light from a high power LED seems to be at least one watt? The light dispersion seems to be better on the 5mm strawhats too with 140 deg spread. So what gives here - Im I just confused here?? Physical space consumption is not a concern here, just the ammount of light generated / the power consumed.

Last edited: Nov 23, 2011
5. ### Resqueline

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Jul 31, 2009
The mCd value need to seen in context with the beam angle, as it increases as the angle is narrowed (for the same chip). Lumen can be calculated from these numbers.
It's the Lm value that determines the general illumination ability. It's probably hard to calculate the Lm value of strawhat LED's though - due to an irregular beam profile.
There's no "law" that says small LED's are not as efficient as high-power LED's. Both comes as cheap ~30lm/W, ok ~60lm/W, and good (expensive) =>90lm/W types.
The lm/W value goes down as you increase the current through a LED. There's also the question about color quality, which can be hard to combine with a high lm/w value.

6. ### BobK

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Jan 5, 2010
Okay, maybe I am wrong about the efficiency. I was just going by the fact that I get a lot more light out of a 3W LED run at 20ma then an LED that is supposed to run at 20ma. I know that the output does not scale up linearly with current, so maybe by the time you get to full output on the high power LED it is no more effiicient.

Bob