LEDs and 120vac

[Hinokagutsuchi]

For a project I've got now, I need to wire up a few blue LEDs, approximately 10 to 15. However, these aren't going to be connected to a computer, so what would be the best way to power them? What sort of life expectancy could be had from a few AA batteries?

What I have now are 7 5mm 4k ~5k bright blue LEDs[2.8v-3.5v]. I'm not sure what size resistors I have either; As soon as I find my meter I'll know. They're going to be used as accent lighting in a bar I'm building. I've got a few elements I want illuminated. I could do batteries, provided I have a really lengthy lifespan with them. I'd rather use ac power, but would that be overkill for 10-15 leds?

 

[Jomanscool2]

Find a "broken" computer somewhere, rip out the powersupply, and go crazy. It's even better if you toss it all in a nice little enclosure, or open the powersupply and trim off all the leads except for one molex or something.

Also, its nice to have a switch, although you can live with just unplugging it.

Oh, and jump the green and black leads together and toss something like a fan on it as well so it doesn't kerspolde.

Note: I am not responsible for you injuring/killing yourself.

 

[apt403]

Yeah, a psu would work, just power 'em off the 3.3v rail.

You could pick up a charger for a cell phone or whatever at a thrift store for a buck or two, just hack off the end with the barrel plug. One with an output of ~3v shouldnt be that hard to find, or you could just use a resistor to drop the voltage to the appropriate level. It would be alot smaller then an old psu.

 

[TMan1876]

Buy an AC/DC power adapter. You can usually find them in anywhere from 3V to 24V pretty easily. Maybe try a 9V ac/dc convertor so you can a couple led's in series with a resistor. Try to find one that can do at least 400 mA so you won't have to worry about drawing too much current.

 

[Mohonri]

Yeah, a psu would work, just power 'em off the 3.3v rail.

Please don't do this. You'll end up with thermal runaway and burn out your LEDs rather quickly. You need a current-limiting resistor.

I second the "use an old PSU" suggestion. There is a way to use LEDs directly off AC using a capacitor and a diode, but the LED will flicker at 60Hz. Using a PSU, the LEDs will be a nice, solid color. Use the 12V line(s), as you'll be able to build a more efficient system using it. You'll find this LED resistor calculator handy.

You'll probably find that the seven LEDs you have won't be enough to provide sufficient lighting, unless you have a really dimly-lit bar. You can get tons of LEDs for pretty cheap on eBay, though.

(Incidentally, you can pretty much hook up as many LEDs to a computer as you have time and patience to solder. If you use 3 LEDs per string off the 12V line, use 20mA per string, you can hook up 150 LEDs per amp of 12V capacity. The cheapest, nastiest Rosewill PSU at newegg--$11.99 + shipping--has a 12V rail capacity of 15A, which means that you could hook up over 2000 LEDs just to the 12V rail. If you use the 5V rail as well, you can get another 50LEDs per amp of 5V rail capacity, which on this particular PSU (30A on the 5V rail) gets you another 1500 LEDs. And yes, some day I want to light a room purely with LEDs )

 

[Jomanscool2]

Please don't do this. You'll end up with thermal runaway and burn out your LEDs rather quickly. You need a current-limiting resistor.

I second the "use an old PSU" suggestion. There is a way to use LEDs directly off AC using a capacitor and a diode, but the LED will flicker at 60Hz. Using a PSU, the LEDs will be a nice, solid color. Use the 12V line(s), as you'll be able to build a more efficient system using it. You'll find this LED resistor calculator handy.

You'll probably find that the seven LEDs you have won't be enough to provide sufficient lighting, unless you have a really dimly-lit bar. You can get tons of LEDs for pretty cheap on eBay, though.

(Incidentally, you can pretty much hook up as many LEDs to a computer as you have time and patience to solder. If you use 3 LEDs per string off the 12V line, use 20mA per string, you can hook up 150 LEDs per amp of 12V capacity. The cheapest, nastiest Rosewill PSU at newegg--$11.99 + shipping--has a 12V rail capacity of 15A, which means that you could hook up over 2000 LEDs just to the 12V rail. If you use the 5V rail as well, you can get another 50LEDs per amp of 5V rail capacity, which on this particular PSU (30A on the 5V rail) gets you another 1500 LEDs. And yes, some day I want to light a room purely with LEDs )

Unless something else comes out, every room will eventually be lit with just LED's. I want them to start making like light dome celings in every room with tons of LEDS that diffuse into the clear plastic or whatever to provide an even light.

However, back to the power them, I would do the old PSU idea. It is probably the cheapest and you can do other things on it too. However, you can get a small DC wall-wart, but I would get a 12V wall wart that can handle at least 250mA.