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# LED Driver help (urgent)

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#### rostidesign

##### New Member
I need help powering a very powerful LED on batteries. Can anyone help me clearer understand mA and how that affects mAh rating on batteries?

Also if anyone can advise me on what's the best direction to go into with my build.

- I need to power a 20w LED or 50w led
- on batteries. I picked out 3.7v lithium rechargables- **broken link removed**
- 50w led (2600 mA draw 24-26 v)
- 20w led (1500 mA draw 14-16 v)

My big question is there a way to power either led with like a homemade driver that will supply constant current of 22 or 20v for 50 w led or 14v for 20w led? If so how long would it last on those batteries? Could someone help me design a driver? Maybe with some extra functions like 4 step voltage control so ie: 22v 16v 12v and 8v on a switch?

If anyone can help me out that would be really great. I want to finish this project for a school related project and need a bunch of help. Got an idea going but need the electronics advice!

Thanks!

The Lithium battery cells have a max allowed discharge current of 5.2A. At 3.7V then they can supply 5.2A x 3.7V= 19.2W for maybe 5 or 10 minutes. I use them to power my electric RC model airplane and they get hot.

If you make a voltage stepup constant current converter that is powered from two battery cells in series then the battery charge will last more than twice as long.

LEDs control their own voltage. You just need to make the current constant.

You could parallel the batteries or series the batteries.
Parallel 3.7 volts @ 10.4A = 20 minutes
Series=7.4 volts @ 5.2A , 20 minutes.
Series=11.1 volts @ 5.2A, 30 minutes.
A boost up PWM will work slightly better if the input voltage is closer to the LED voltage.

Ok im getting it a bit now. SO if I'm planning on using 6 of those batteries wouldn't that provide the led with the power that it needs? @ 22 v how long will it last on the charge?

Thanks!

Lithium battery cells are 3V each when near the end of a charge. Then the total for 6 of them is only 18V and the LED will not light.

You need 24V to 26V for the 50W LED plus more voltage for the constant current circuit. Maybe 28V. Then you need 10 battery cells. They will be 42V when fully charged. Their average voltage is 3.7V each so the entire battery will be 37V. 50W/25V= 2A. A battery charge will last less than 2 hours.

24-26 volts being the peak. If you have used LED's before, you will know that they operate at lower voltages. not as bright but at 18 volts they should work fine. How about running 20w @ around 14v?
That would be fine I was just wondering if 50 is feasible. 1.5 - 2hr run time is what Im aiming for. And with a controllable switch you could decrease voltage and add flexibility. So that you could use maximum output or use less output depending on need etc.

Would 20w led work? Would it last longer?

24-26 volts being the peak. If you have used LED's before, you will know that they operate at lower voltages. not as bright but at 18 volts they should work fine.
I disagree.
A Luxeon SuperFlux LED is very bright at 2.5V but does not light at 1.8V.
This powerful LED has many LED chips inside and will be the same.

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I disagree.
A Luxeon SuperFlux LED is very bright at 2.5V but does not light at 1.8V.
This powerful LED has many LED chips inside and will be the same.

that depends on LED type. A 3.7V ave GaN LED that was mentioned will NOT light bright at 2.5V, whereas the archaic 2.2V GaAs one that you posted the curve for would.

Dan

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I need help powering a very powerful LED on batteries. Can anyone help me clearer understand mA and how that affects mAh rating on batteries?

Also if anyone can advise me on what's the best direction to go into with my build.

- I need to power a 20w LED or 50w led
- on batteries. I picked out 3.7v lithium rechargables- **broken link removed**
- 50w led (2600 mA draw 24-26 v)
- 20w led (1500 mA draw 14-16 v)

My big question is there a way to power either led with like a homemade driver that will supply constant current of 22 or 20v for 50 w led or 14v for 20w led? If so how long would it last on those batteries? Could someone help me design a driver? Maybe with some extra functions like 4 step voltage control so ie: 22v 16v 12v and 8v on a switch?

If anyone can help me out that would be really great. I want to finish this project for a school related project and need a bunch of help. Got an idea going but need the electronics advice!

Thanks!

The answer to your question is the product... using a switching regulator your can expect ave battery voltage (V) times capacity (mAHrs) times 0.8 (80% efficient conversion) to be equal to LED voltage times current times time.

A voltage switch is useless as LEDs are current driven devices.

I would recommend 10 of those batteries in series using a HX9910 (I think it is) buck mode LED driver for an hour if you insist on spending \$160 on batteries.

If I were you I would use 20 **broken link removed** LiFePO4s for less than half the price and virtually impervious to thermal runaway (the rather spectacular tendency for lithium batteries to burn violently...the reason they can not be shipped by air now - a whole palette went up in smoke, luckily before it was loaded onto the plane!)

Dan

Why are these Deal extreme LiPo battery cells so low in voltage? Other Lithium cells are 4.2V max.

Of all battery types lithium polymers have the widest range of cell and chemistry configurations, if you don't understand the construction of the specific cell you do what the maker tells you period. If you want to know why contact the maker.

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Of all battery types lithium polymers have the widest range of cell and chemistry configurations, if you don't understand the construction of the specific cell you do what the maker tells you period. If you want to know why contact the maker.
But the Deal Extreme sells cheap surplus No-Name-Brand garbage without any datasheets!

Why are these Deal extreme LiPo battery cells so low in voltage? Other Lithium cells are 4.2V max.
Different chemistry you are thinking of lithium polymer as opposed to lithium iron phosphate (LiFePo4). Nice thing about them is they are robust... you CAN float charge them, i think it was 3V to 80% though you still need cell balancing in series packs

Dan

I've seen Lithium Iron Phosphate cells when I tried to look up high capacit lithium packs. DOn't know jack about the chemistry =)

ya, i cant afford the weight of 20 batteries for a flashlight.

if i settle for 20 w led. are there any 1500mA drivers out there?
i only found this one as highest to my knowledge: **broken link removed**

i think there might be one that does 1000 maybe 1500?

if anyone know that would really solve my problems.
also i want to reduce amount of batteries in use so i need higher voltage i think. less batteries better so that it can be portable.

what do you guys think?

thats the led im leaning towards.

are there batteries that can be rechargable and with higher voltage than what i showed?

how long would they last with this led in use?

could anyone help me build a driver?

thanks for anyhelp available!

Try the National web site, there are many circuits for power leds.
I build mine for a 50W led from 3 lithium celds using the LM3478.

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