Hey all, I'm looking to do a very simple LED wiring project. What I have to work with right now is basically what I can pick up with radioshack, and I want to make a bright, reliable, string of blue LED's. The reason why I want to do this is because I watched a video on metacafe about a project they did involving lighting up a bottle of bawls with LED's and crushed glass. It looks amazing imo, and I think it would be an awesome gift for a friend of mine who is into both bawls, modding, and bright blue lights. For those who haven't seen it, here's a link So I put together a list of supplies they recommended (5000mcd blue leds, 12v 500ma DC adapter, and a 220 ohm resistor.) and I headed over to the local radio shack. I'm afraid I was let down. Looks like they have the adapter and resistor I need, but the LED's the guide recommends aren't here at the store. These are the ones that radio shack has available. I understand that if I use different LED's then obviously my resistor needs to change, or else they may overheat and burn out, or they wont produce the right amount of light. So I read through a few LED faqs on the boards here, and I also found the LED resistor calculator, and here is where my question lies: My inputs for the tool are as follows: 12v voltage source 4.6v diode forward voltage 10 diode forward current (mA) 4 LED's in my array. The tool tells me I need two 330 ohm resistors. I got the values for these LED's from the FAQ here. It says that if I use the "radio shack" LED's I should design for 4.6v and 10mA. Am I doing everything correct here? Any suggestions? Is there a better place to get LEDs? I would need to pick them up locally by Sunday. Are these numbers too conservative? Are these LED's even capable of the light output that's shown in the guide? Thanks in advance to anyone who takes the time, I would really really appreciate it, as I think this would be an amazing gift to my (lady) friend!
if you were to use those LED's (4 of them) you could quite happily run them all in series off of a 12v supply with no resistor at all (as long as they are in series mind). Not sure where your getting 10mA from but thats lower than I've ever seen led's those ones you linked to at radioshack are 20mA @ 3.7v (quite typical values tbh) 4 of those in series would total 14.8v you would lose a very minimal amount of light by running them off a 12v supply, you probably wouldnt even notice it.
http://www.jab-tech.com/4000MCD-5mm-Blue-leds-10-pack-pr-2390.html Jab-tech charges a bit more for these LEDs than other modding suppliers, but they ship LEDs for free. If you're in the mood to order additional supplies, Performance-Pcs have somewhat brigher LEDs for cheaper, but usually charge hefty shipping charges. http://www.performance-pcs.com/catalog/index.php?main_page=product_info&cPath=235&products_id=2556
Wouldn't the amperage from the power supply fry them though? It supplies 500mA of current, and these are only rated for 20mA each..
thats the wonderful thing about current, the stated current on the power supply is a maximum that the supply can ever provide at any given time. The LED's only draw as much as they need (20mA). This means the supply will provide 20mA to an LED when it wants it. Another way to think of it is that current is drawn from the supply, it is NOT "pushed" out to the LED, the LED will only draw what it needs from the supply. You only have issues if you somehow manage to exceed the 500mA max (my calculations say that would be some 25 LED's) hope this helps, im not great at explaining things, they make sense in my head dragon2309
So voltage is really my only concern here then, correct? If I were to put, say 2 LED's on this line, then I'd be hosed. But with 4, I'm fine? What if I used 8? Would that make the LED's dim? (Since the voltage just isn't there?)
Yes as long as you're not using an obscene amount of LED's, then yes, voltage is your main concern. 2 LED's roughly totalling 7.4v (2 x 3.7) then yes, you would almost certainly fry them both if you connected it up to a 12v source, but thats where resistors come in... Say you went for 2 on each strand from the supply, you would need... let's see.... 12v - 7.4v = 4.6 / 0.02 = 230ohm resistor... i doubt you would get that exact value, and its fairly safe to vary a bit, the nearest standard value is 220ohms, which would do the job fine... 8 LED's, yeh, you would lose theoretically half of the brightness, kind of a no go if you want bright lights... 4 LED's is your best option here, its simple because a string of 4 can run off 12v happily and not lose too much brightness at all.
Ok thank you! One more question: Do I need to be concerned with the orientation of the LED's? If so, how do I know which way is the correct way (without frying everything lol)
hehe, yes, the longer leg is positive, you wont fry it id its round the wrong way, it simply wont light up, and neither will any other LED's in the circuit.
