Powering a 20W LED with Laptop Battery

candle_thrower

Newly Enlightened
Joined
Aug 4, 2010
Messages
1
Hi -- I'm a noob and this is my first project, so be nice about chastising me. I bought a 5pack of 20W 1500lum LEDs on ebay. Each module is about 2 sq inch rectangle with a metal mounting plate and 20 tiny 1W leds. Not knowing anything about LED driving when I bought these, I assumed I could just hook it to a battery and be done. Then my friend told me I would need a constant current circuit to avoid burning the LED out. The battery I plan to use is from my laptop.

LED Characteristics:
20 W
11 V
1.82 A

Battery Characteristics:
5000 mAh
6 cells
?? W max
11 V
?? A max

I've searched, and I can't find exactly what I'm looking, and that's why I'm posting:

(1) I see BuckPucks that are almost what I need but only go up to 1A.
(2) I see lots of modules on DealExtreme that are all made for 120V household AC.
(3) This instructable (link to circuit schematic): http://www.instructables.com/id/Pow...t-with-constant-current/step2/Specs-Function/


>>> SO... What configuration would you suggest to power this LED? Is there a pre-built power module that could help me?

Some other possible questions to answer:
1. Is there anything I'm badly misunderstanding about powering LEDs?
2. Can you share a good explanation of the reason for needing constant-current circuitry?
3. What's inside a BuckPuck or other commercial power module?
4. Is there a big problem with using the simple circuit in (3) Instructable above?
5. What's the simplest thing that could work, and the most complicated thing?
 

beizebopp

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Joined
Apr 27, 2011
Messages
1
Im having the exact same dilemma?sp

20w led and a laptop battery. I was thinking i could simply make it constant current with a resistor although its wastefull. Apparently it isnt a good way to do it.

Ive read a good way is to make a constant current circuit with a bjt transformer or just use a dc-dc constant current driver.

Hope someone can explain the best way to do it.

My LED is 20x1w Epiled chips in about an inch and a half square. Rated at 10-11V DC and 1000-2100mA

Cheers.

 

onetrickpony

Enlightened
Joined
Mar 10, 2011
Messages
262
You only need a constant current driver if your source voltage is higher than the voltage of your led, HOWEVER, you will get entirely different runtime characteristics.

Example 1: 11v battery pack (check this with a meter when it's fully charged), 11v led (look for the forward voltage rating, or vf), hook them together, preferably with a switch and your meter set to amps. Flip it on, and you should be pulling the max current the led will draw from its spec sheet.

You will see full brightness initially, then the light will dim over time until it gets down to almost moonlight, then it will continue running for a loooonnnnggg time until it finally goes out. With the fairly stable voltages seen in Lion batteries, this isn't a bad option. If you were using alkalines, not so much.

Example 2: Same battery and led, but this time with a constant current driver designed for say between 8 to 12 volts input and 1800 ma out. It will have to be capable of driving multiple leds in series. The reason for this is that your vf is much higher than a single die led, typically 4v max or thereabouts.

You will see a much more stable output, the light will be at roughly the same brightness as in example one, but it will not dim very noticeably for a much longer time. The tradeoff is that your total runtime will be much shorter, and if the driver does not go into direct drive mode at a certain low voltage threshold, it will just turn off. That means very little or no warning at all and then "lights out."

The reason for this is that the driver will "boost" the voltage in order to deliver more current without damaging the led. Once the voltage of the batteries drops below a certain point the driver will be unable to boost it enough to drive the led, and it will either shut off or run in direct drive until the batteries are dead. Be careful with the lion battery packs as well, if you over-discharge them, then can become dangerous to re-charge. I would suggest a driver designed for lions with a low voltage shut off, or if you go direct drive, either monitor voltage regularly or put in a low voltage shutoff yourself.

Other people can tell you more about how to deal with lion batteries safely, I'm a nimh guy myself.
 

Benson

Flashlight Enthusiast
Joined
Feb 15, 2009
Messages
1,145
The laptop battery pack will be 3s2p config, should read 12.6V fully charged. Internally, the cells are almost certainly capable of a 2C discharge (so 10A), but the internal charging/protection circuit likely sets a stricter limit. 2A draw should be fine, though.

You might try direct-drive, as onetrickpony says, run it just a couple seconds using a DMM to check the current doesn't much exceed 2A on a fresh battery (a slight overdrive is ok, since the current will drop off quickly as the battery drains). If you're unlucky, you might fry the LEDs this way, but probably not -- check with a partially-charged battery first if that makes you nervous.

If the direct-drive current is too high, I'd use an AMC7135-based driver -- search the forums for more info on these, I haven't time to explain in detail. You'll need a ~5V supply to power the AMC7135s.
 
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