led moon lights

[sudsy]

Hi all, question anybody who purchased their resistors, leds, at radioshack could you give me some model numbers. i have a bunch of different transformers laying around in different voltages so i don't need one of them i just don't know how to figure out what resistor i should buy. Thanks Mike

[aehicks2003]

100 ohm

[thereefgeek]

Simple formula: r=v/i
r=resistor you're trying to find. v=left over voltage. i=current you're LEDs are using.
If you have a 12volt supply for example, take the voltage minus what the LED takes to run (RS LEDs are 3.7v @ 20mA or .02 Amps) 12volts-3.7volts=8.3volts left over. 8.3volts divided by .02 amps = 415 ohms. Therefore a 415 ohm resistor would work, BUT the power supplies produce 12 volts @ 200-300 mA. If you test the actual output with no load, it'll run anywhere from 14- 17volts, so up the resistance a bit. RS has 480 ohm resistors 5 for a buck. It would be safer to use the 480 ohm resistor because the load on the power supply is so small.
BTW, depending on the mA rating of the power supply, you could run up to 15 LEDs on a 300mA adaptor if you run them parallel. If you run them in series (like the old X-mas lights where when one burns out they all go out), you still use the same formula, but you add all the volts together until you're close to, but less than the rated output. 3 LEDs @ 3.7 volts = 11.1 volts. 0.9 volts left over from 12 divided by 20mA (.02A) = 45 ohm resistor.
Hope this helps. You also might want to do a search on this forum for "moonlight". There's a good link to this dude named Fabio at Kaotica dot something, but there's a ton if info here as well.

[intranet]

I am sorry if this link is inappropriate as it is hosted on a commercial site, I apologize if it is. I only found it while searching for info on wiring LEDs.

http://www.theledlight.com/ledcircuits.html

There is info on there about using both series and parallel , the formula thereefgeek mentioned and some very useful diagrams

[RobChuck]

Here's the link that thereefgeek mentioned at the end of his post: http://www.kaotica.com/frag/diy/moonlight/. There is a link on this site to another site that has a calculator for determining the resistance needed for wiring LEDs.

[dbradica]

I've just purchased some from LSDiodes.com and they are great and cheap. Purchase your power supply first and then use their LED Calculator to find out what resistory you need. It all depends on how many lights you hook up and what your power source is for it. 5mm blues for $0.45 each. Use the values of 3.6V Forward Volts and 20ma. The lights are fantastic and don't need to be overpowered.