Discussion in 'LEDs and Optoelectronics' started by ngwanhoe, Apr 20, 2011.

1. ### ngwanhoe

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Apr 20, 2011

I have DIY a lighting system with LED. Anything wrong with my work?

Using 4 X 1.5V Alkeline batteries connected 3 in parellel and 3 in series = 3V, attached to a step up circuit from 3V to 12V which then connected to 6 X K2
3W Leds input current 4.5V - 5V each 3in parellel and 3 in series.

2. ### Resqueline

2,848
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Jul 31, 2009
Yes???
Have you done it?
If so did it work or not?
Have you only planned to do it?
What kind of step-up converter is that?
Do you envision any resistors with those LED's?
Their typical forward voltage is only 3.85V at 1.5A, why 4.5-5V?
What current do you intend to run them at, and have you provided heatsinking?
The battery pack contains max 12Wh and the LED's can draw 18W ( = 40 theoretical minutes). How long run time do you need?

3. ### ngwanhoe

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Apr 20, 2011
Thanks Resqueline.

-Yes I have done it.
-It works.
-Stepup converter LM2577s (Refer to picture attached)
-No need resistors as the stepup current is constant, I think.
-The specs of the Leds is :- Name - K2 3W (made in Taiwan) , Using 4.5V to 5V to power. The Leds, I think is similar to Luxeon K2 Leds.
-I am going to run them at 13V to be safe. Have tried on 15V but just for 2mins.
-Wanted to run 1 - 2 hrs.

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

25,418
2,788
Jan 21, 2010
OK, here are the problems:

1) LEDs run from a current source, not a voltage source. We have a sticky thread here which explains it.

2) The power available from the AA batteries is limited and this will probably prevent excess current. If you connect it to a lower impedance source of power, you may damage either the DC-DC converter or the LEDs.

3) If the DC-DC converter has a minimum voltage of 3V then the batteries may well get below this point before you have completely exhausted them. This means you will not get the most life out of them.

I would advise you to measure the current drawn by these diodes. This is what they are actually rated for. Also ensure you have adequate heatsinking for the LEDs to prevent them from exceeding their maximum temperature.

5. ### ngwanhoe

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Apr 20, 2011
Thanks Steve.

You sudgest that I should up the initial power source? Say maybe 3 AA or 4AA batteries in series which 4.5V or 6V?

How / method to test the current drawn of these diodes which I have already connected them 3 in series and parellel in 2 rows? I have a digital test meter.

6. ### Resqueline

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Jul 31, 2009
You'll need (at least) 12 AA batteries if you're to have a chance at powering 18W for (anywhere near) 2 hours. You want 36Wh and each cell contains less than 3Wh.

I would doubt such LED spec's (unless they're rejects). What probably saved your LED's is the low voltage and low number of batteries you used for the test.

Before you up the number of batteries you should set you meter for 2 Amps (or more) and insert it as shown in the diagram. Note the current and remove the meter.
Reconnect the original wires, and with you meter set for 20V (remember to move the test plug) measure the voltage across the LED's, and also across the batteries.

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7. ### ngwanhoe

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Apr 20, 2011
Thanks Resqueline.

This sounds like its not going to work that well.

What I am trying to do is to power a high power Led lights with minimal amount of batteries for at lest 2hrs.

Do you have any other ideas or simply driver design which can step up voltage and boost current? Ps. I dont know how to read circuit drawing

8. ### Resqueline

2,848
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Jul 31, 2009
To test your setup you can insert two resistors instead as shown (and measure their voltage). These will also enable you to use that inverter to drive the LED's safely.
This wiring diagram is as picture-identical to the real setup as you can get - without having an actual photograph.

The laws of physics limits what you can achive. There are some alternatives to explore though.:
1: Use a different battery chemistry: Lithium and Zinc-air are examples. Notice the different Wh=Watt-hours = J=Joules (MJ=MegaJoules) per kilogram and per litre.
2: Use more efficient LED's. Cheap/poor ones give only 30-40 Lumens per Watt, better ones give 60-70 Lm/W, recent LED's give 90-100 Lm/W - or even more.
3: Use a dedicated LED driver with the highest efficiency you can find. LED drivers are switchmode inverters like your booster but have a constant-current output instead.

You also need to define if you need/want primary batteries, or if you can use secondary batteries, and if size or weight is important to keep down.

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