Best power source for LED array? - Overclock.net

**Boris4ka** · 05-27-09

I'm making a bike light out of 50 LED's with the following specs:

- Forward Voltage 3.2V MIN, 3.4V MAX (20mA)
- Reverse current: 10u (VR=5V)
- Power dissipation: 60mW
- Luminous Intension 14000 mcd MIN, 16000 mcd MAX
- Peak Forward Current: 100mA
- Recommend Forward Current 20mA
- Electrostatic Discharge: 2000V

Should I use 8 AA batteries or one 3000NiMH 7.2V battery pack? If I were to use the AA setup, how much run time would I get? What about the battery pack?

Also would I need a 3ohm resistor for 6v? And no resistor for 7.2v? I'm pretty sure I'm doing it wrong.

Ionimplant · 05-27-09

Quote:

Originally Posted by Boris4ka

I'm making a bike light out of 50 LED's with the following specs:

- Forward Voltage 3.2V MIN, 3.4V MAX (20mA)
- Reverse current: 10u (VR=5V)
- Power dissipation: 60mW
- Luminous Intension 14000 mcd MIN, 16000 mcd MAX
- Peak Forward Current: 100mA
- Recommend Forward Current 20mA
- Electrostatic Discharge: 2000V

Should I use 8 AA batteries or one 3000NiMH 7.2V battery pack? If I were to use the AA setup, how much run time would I get? What about the battery pack?

Also would I need a 3ohm resistor for 6v? And no resistor for 7.2v? I'm pretty sure I'm doing it wrong.

Are you building this from scratch or is this a modular LED light module?
If it is not a module, are you using any electronics to drive the LEDs (boost LED driver)?
You need to give me more information...plz

If you use the Energizer L91 (lithium ion) AA battery, you would get about 2000 minutes at 100mA and 6,000 min at 20mA (the graph I have is low res, so these numbers are approximate).

For the 3000NiMH I you would get about the same but it would depend on your circuit configuration.

Tell me more and I can help.

**Boris4ka** · 05-27-09

Quote:

Originally Posted by Ionimplant

Are you building this from scratch or is this a modular LED light module?
If it is not a module, are you using any electronics to drive the LEDs (boost LED driver)?
You need to give me more information...plz

If you use the Energizer L91 (lithium ion) AA battery, you would get about 2000 minutes at 100mA and 6,000 min at 20mA (the graph I have is low res, so these numbers are approximate).

For the 3000NiMH I you would get about the same but it would depend on your circuit configuration.

Tell me more and I can help.

It's from scratch. The specs I gave are for 1 LED. I will be using 50 LED's. No drivers, just wired straight to the batteries.

I don't want to buy special AA's, I want to use normal cheap ones.

As for how I'm going to wire the circuit, that all depends on the battery I use.

keith27 · 05-27-09

If you got quality rechargeable batteries that are rated aroudn 2.2Ah you'd out perform that battery pack.

Now with that said what do you mean by 3 ohm resistor?

EntropyTTU · 05-27-09

Don't use a 3 Ohm resistor...that would be 2Amps (attempted anyway, because it wouldn't be able to sustain it).

For the 7.2V battery pack:
Use a 200Ohm resistor, that will get you 20mA through the LED.

Based on a circuit of ONE led, and resistor:
Your total consumption will be about: 80mWatts through the resistor, and 60mWatts through the led (according to led specs).
Total = 140mWatts.

Multiply that by 50 = 7Watts total for your circuit. You will want to do this in parallel, AND NOT SERIES, therefore you will need 50 200Ohm resistors also.

Were it me, I would push 30-40mAmps through the LED's to make them even brighter, the specs say they can handle it, thats just my opinion.

**Boris4ka** · 05-27-09

Quote:

Originally Posted by EntropyTTU

Don't use a 3 Ohm resistor...that would be 2Amps (attempted anyway, because it wouldn't be able to sustain it).

For the 7.2V battery pack:
Use a 200Ohm resistor, that will get you 20mA through the LED.

Based on a circuit of ONE led, and resistor:
Your total consumption will be about: 80mWatts through the resistor, and 60mWatts through the led (according to led specs).
Total = 140mWatts.

Multiply that by 50 = 7Watts total for your circuit. You will want to do this in parallel, AND NOT SERIES, therefore you will need 50 200Ohm resistors also.

Were it me, I would push 30-40mAmps through the LED's to make them even brighter, the specs say they can handle it, thats just my opinion.

I thought instead of using a 200 Ohm resistor on each LED you could wire one resistor thats 200 Ohm / 50 = 4 Ohm.

Wiring a resistor to each LED is my last resort.

Also now I'm confused about the voltage. Is a resistor to drop voltage or mA?

Sorry, I always get confused by this stuff.

EntropyTTU · 05-27-09

Quote:

Originally Posted by Boris4ka

I thought instead of using a 200 Ohm resistor on each LED you could wire one resistor thats 200 Ohm / 50 = 4 Ohm.

Wiring a resistor to each LED is my last resort.

Resistance does not divide among loads, like that, unless I am not understanding completely what you are getting at? Are you planning on running them in parallel, after the 200 Ohm resistor?

Quote:

Also now I'm confused about the voltage. Is a resistor to drop voltage or mA?

Resistors drop voltage, all loads do. They consume, or limit the current, and it is dissipated through heat. This is where the 80mW I calculated came from.

Here is a good circuit to get you started:

Except you will be using 7.2V, and 200 Ohms, for 20mA, and you will have 50, instead of 5.

Let me know if you have any other questions.

**Boris4ka** · 05-27-09

Ok so you're saying that I need to:

Use the 7.2v 3000nimh battery
Wire one 200ohm resistor to each LED
Wire all the LED's in series

But since I have 7.2v, I can use two parallel LED's in series, no? 3.4v x2 = 6.8v, 6.8v < 7.2v.
If this is true, does that mean I use a different resistor, and only 25 of them?

I am saying a 2x25 array (2 parallel, 25 series). Makes sense? Or am I wrong again? Seems it would be easier to do this way if it's possible.

Ionimplant · 05-27-09

Entropy's ckt will work just fine.

However, at 20mA per leg you will be wasting 80mW per leg. The LED itself dissipates 64mW so this is pretty inefficient. Yes, it is very simple to build but if you are going to use cheap batteries you will buy a lot of them.

I have a gimme cap on which I installed three LEDs that were ballasted just as E illustrated. I used three AAs and it works just fine. I don't really care about how many batteries I burn. But if you are agonizing over Lithium AA vs Alkaline, it might matter to you.

Are you going to hand wire 100 components?

Your LEDs will dissipate about 3 watts.

If I were dead set on doing this myself, I would fab a little PC board for 50 bucks and make a much neater build.

Ionimplant · 05-27-09

Quote:

Originally Posted by Boris4ka

Ok so you're saying that I need to:

Use the 7.2v 3000nimh battery
Wire one 200ohm resistor to each LED
Wire all the LED's in series

But since I have 7.2v, I can use two parallel LED's in series, no? 3.4v x2 = 6.8v, 6.8v < 7.2v.
If this is true, does that mean I use a different resistor, and only 25 of them?

I am saying a 2x25 array (2 parallel, 25 series). Makes sense? Or am I wrong again? Seems it would be easier to do this way if it's possible.

You have to ballast the LEDs!! They are not perfectly matched. You never NEVER put two pn junctions in parallel!

If you do put two in series, you would use 40 ohms!

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Best power source for LED array?

05-27-09	#4 (permalink)
keith27 New to Overclock.net Join Date: Nov 2007 Posts: 1,670 Rep: 146 Unique Rep: 109 Trader Rating: 1	If you got quality rechargeable batteries that are rated aroudn 2.2Ah you'd out perform that battery pack. Now with that said what do you mean by 3 ohm resistor?

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