Suggestions for powering my project?

[pedalinbob]

I apologize for the question, but I have searched this forum and still feel a bit shaky on what to do. In the past, I have built LEDs which run from transformers, but I only used low-power LEDs and only one at a time.

I am building a display cabinet for my wife. It will have five 1'x2' shelves, and I just ordered 5 SSCs (1 LED/shelf).

I have been tinkering with an SSC, and determined that 100 milliamps per LED should provide plenty of light, so the question becomes: how should I power this?

I have serveral wall-warts, ranging from Nokia cell phone chargers (3.7v with 340 milliamps) to a 12v model (cannot recall the amp rating).
So, I am looking at 0.5 amp of total current at 3.5v. I looked at several power supplies/transformers/adapters and a case of beer (totally kidding!), and am still not sure what would be best to use (it looks like there are a bunch of different types, which is incredibly confusing!). I went here:

http://www.allelectronics.com/make-a-store/category/815/Wall-Transformers/1.html

I thought I should be able to use a 6v 800ma model and resistors, or the 5.5v 300ma regulated model with resistors...and I believe I would have to run the LEDs in parallel.
(the regulated part sounds good, but I'm a bit confused about how it would react to limiting resistors).

I have also read some threads suggesting using higher voltage and running the LEDs in series. Apparently, I would need a 17.5v+ transformer (24v?), and resistor back the current.
I intend to run the lights for just a few hours after sundown each night, so ultimate efficiency isn't a huge concern, but I still like the power-saving feature of LEDs.

Does anyone see a simple solution? I was really hoping for a transformer, wires, LEDs and a switch...;o)

Many thanks,
Bob

 

[Jumi]

Hi
Vf of Seoul At 100mA is less than 3 volts, so you don't need 17.5 volts when driving 5 is series.
I would try one of those 12VDC and 200mA transformers and measure the current with 5 leds and small resistor in series. At small currents those transformers can give higher voltage than nominal 12V and 200mA.

If you get lucky you don't need any resistor or only a very small one and efficiency of system is high.

Juha

 

[brickbat]

I can see two good ways to do this:

A

Put the LEDs in series,
Use a small resistor to limit the current.
and (the hard one) find a regulated 15V supply.


or B

Put the LEDs in series
Use a larger resistor to limit the current.
Use an unregulated 18-24V supply.


Resistor-free designs are OK for a flashlight, but in a house, where the consequences of overdrive and failure are higher, I'd want to know with more certainty what the drive current will be. Not only that, but small variations in the mains voltage will cause large variations in LED current without regulation. In option A, the regulated supply takes care of the line voltage variation. In option B, the drop across the resistor, being larger will stabilize things. Option A is more efficient, while option B is lower cost.

BTW, you may know this, but most wall warts, like the ones you linked to, are not regulated.

 

[pedalinbob]

Just a follow-up:

I purchased this: http://www.dealextreme.com/details.dx/sku.13690 and it works beautifully. I didn't want to drive the LED's at 320ma, but I can always dim the lights by placing a filter lens or something.

I figure that even if it is terribly inefficient, it still has pretty low power draw.

Oh, and when I tested it, it remained pretty cool.

 

[bshanahan14rulz]

I am thinking about getting that same regulator, since it has the highest output voltage. How's your cabinet lighting holding up?

 

[blasterman]

When you say 'SSC' are you talking about a P7? Seems a terrible waste of lumens, but if it's what you got....

If you have 5 LEDs, and want approx 100mA sent to each one, then why not get a dedicated 350mA LED power supply and run them 3x2+resistor?

 

[bshanahan14rulz]

october of last year. I was just curious how the driver was holding up.

 

[pedalinbob]

Sorry for the delay, I am so busy these days I rarely even get to lurk!

The drive worked fine, BUT created a ton of EMI, which was so potent that my AM radio would click and pop while coming up the driveway!

I removed it and ordered the Xitanium. Works just as well with zero interference.

Bob

 

[bshanahan14rulz]

hmm, I guess I'll pick up a xitanium one of these days... cell phone already gets horrible service inside.