How to control current to CREE?

[lostsoul01]

Hello!

I have a basic question... I'd like to build a light using a Cree star, powered from AC. I understand the current has to be controlled and can not exceed the rated amps.

I have many of these AC adapters, such as one for my cell phone that has a USB plug on the end. It says the output is 5V 550ma. Say I were to hook one of these up to a Cree star, what would happen? Would it only take the ~3.3V it needs and 550ma max, or would I fry it? If I found one that matched the voltage and was under 1A, would that work, assuming my Cree could handle the A?

Or do I need something like a buckpuck regardless? It just seems like that would provide an extra step and even less efficiency.

Thanks for your help with my basic question! I'm just trying to get started on this and it's been a while since I've excercised my engineering knowledge!

 

[LukeA]

The cree would only drop the voltage it needed.

 

[balou]

550mA means that it can deliver that current... it doesn't specify what happens if you draw more current.

Several possibilities:
-just limiting it. Then it'll work great for your application
-shutdown
-a fuse burns
-no fuse installed, something else may start burning...

A driver would be an option, but there is a simpler solution: a resistor.
You have constant, high-enough voltage, and you don't care to much about power efficiency (the wall plug already is quite inefficient).

Here's a LED resistor calculator: http://led.linear1.org/1led.wiz

 

[lostsoul01]

Thanks for the responses. That does answer my question, that the wall plug is capable of that current, but doesn't limit it.

I had avoided the resistor route because of the power waste, I did not realize the wall plugs were that inefficient. I think that's the part that bothers me so much about doing this, the power waste from the AC to DC conversion.

Thanks again for the help!

 

[likeguymontag]

I did not realize the wall plugs were that inefficient. I think that's the part that bothers me so much about doing this, the power waste from the AC to DC conversion.

There are AC/DC converters and then there are AC/DC converters. The heavy "wall wart" adapters use (I think) linear regulators, and are fairly inefficient. The ones that are lightweight and smaller (for a certain power rating) are probably switched-mode power supplies, and in theory those can be pretty efficient.

 

[blasterman]

The heavy "wall wart" adapters use (I think) linear regulators, and are fairly inefficient.

And flicker like crazy. Nothing like a 60hz Cree strobe light....

 

[LukeA]

There are AC/DC converters and then there are AC/DC converters. The heavy "wall wart" adapters use (I think) linear regulators, and are fairly inefficient. The ones that are lightweight and smaller (for a certain power rating) are probably switched-mode power supplies, and in theory those can be pretty efficient.

My Radioshack adaptors are AC/DC (the LEDs I run from them don't flicker) but my DX adapter is just a linear regulator (output reads 60Hz on my DMM).

Another thing to differentiate the two types is the weight. Inductors for AC/DC regulation are usually heavy. The DX brick is small and light.

Switching power supplies are different beasts IIRC.

 

[yellow]

I had avoided the resistor route because of the power waste,

:thinking:
but then the power is unimited, while a "real" DC-source (Battery) is heavily restricted
(+ also has the "problem" of ineffectiveness with AC-DC conversion, when getting charged)

when You get Your light power from the wall plug -> no need to be in the high 90s effective

 

[lostsoul01]

It does look like I can find some wall plugs that are in the 80% efficiency range. Then, if I put them through a 90% buckpuck I am really getting a 72% efficient device.

While that does beat some of the AC kits I've seen on DealExtreme for a 1W LED driver (one user said 25%), that still seems like a lot of waste to me. That turns a 100lm/W LED into a device that gets only roughly 72 lumens per actual used watt.

That's why I asked if the buckpuck was necessary, or if the wall plug controlled the current on its own. It's just another 10% power loss. And I assumed the resistor was just a power waste too, since it uses power.

It just seems at this point in time the best method is directly using DC, like with a flashlight. When LEDs are 200lm/W+ then they too will be a no-brainer, including the AC conversion loss. I just want to get the same amazement out of people with LED regular lighting that I do with my flashlight. To me, that's what gets people to move forward instead of just saying "why not just get a $2 CFL?"

Thanks again for the replies!

 

[LukeA]

"why not just get a $2 CFL?"

Low lifecycle cost. 72 lm/W is still better than that CFL, meaning that after about 20000 hours even the $100 Cree LR6 fixture is less expensive to operate than CFLs.

 

[likeguymontag]

It just seems at this point in time the best method is directly using DC, like with a flashlight. When LEDs are 200lm/W+ then they too will be a no-brainer, including the AC conversion loss.

Eventually efficient ballasts for LEDs will become common and cheap. There's no reason I know of why you couldn't design a 90% efficient ballast/driver to power LEDs from AC, and they're probably available today, for a price, if you look in the right places. I think you're just dealing with the perils of being on the cutting edge.