Okay, I have a bunch of bright 3v LEDs that I want to use in a mod and want to power them with my PSU. First I have a number of questions, sorry if the answers are blatantly obvious but I spent all our electronics lessons at school soldering rulers together 1) Which side of the LED is positive? Is it the one with the flat bit? 2) Will I get away using a 3v LED on a 5v line without a resistor? 3) If I use a 12v line, I should be able to connect 4 3v LEDs, and if I add more the LEDs will get dimmer. Is there a way to boost the current so I can add more without making them dimmer or will I just have to use a seperate line?
1 )If you look inside the LED, one the leads inside with look like a bowl, or a little dash ( - ) that is your negative side. So therefore, the other side is your positive. I cna never remember which the little flat bit is, and on some of teh LEDs I've got, that flat part isn't there, and to go by the leg length is a poor method, especially for used LEDs. 2)No. It will burn out. However you may be able to run 2 LEDs off of 5V without burning it out, as the voltage will be divided, and the amperage lowered. I'm sure Linear can break in here about that and explain your problems to death 3)How many LEDs do you want t power ? Puttign 4 LEDs to 12V will still require a resistor, however, powering five or six LEDs off of 12V might get you by without a resistor. Also if you put them in a parallel series formation, you can power quite a few more. Although best advice is to just go grab a little resistor and stick it in place, they cost about 10 cents, so it's not expensive.
I'm in no way an expert on this.. I'm not even a novice, but I'll try to help a little. Anyone should correct me please if I'm wrong on anything. If I remember correctly, the positive side of an LED is the longer leg of the LED. Negative being the shorter leg. A 3V LED won't work on a 5V line without a resistor. I can't remember the exact resistor rating, but you can buy them here: http://www.themodhouse.co.uk/shop/product_info.php?cPath=26&products_id=37 I'm pretty sure it's 300 Ohms for 12V, not sure about 5V. I haven't done any basic circuitry/electronics theory for about 7 years now, but if you connect the LEDs in parallel, they brightness will erm.. still be there with quite a few in.. Argh I can't remember what it is! The current get's split or something. I think someone else should go a little further into this for you Hope that helps
Welcome to Bit-Tech, Norky. 1. The positive side is usually the side with NO flat and a longer lead, but there are a few exceptions. Kargin's method is OK if you can see the guts, easier to use a multimeter with a diode test position or a quick test with 2 AA dry cells won't usually do any lasting damage. 2. You need a resistor to limit the current, value is (5-3)/0.02 or 100R for 20mA current. Check the safe current and insert above if different to 20mA (0.02). 3. You could put 4 @ 3v onto the 12v line, I'm wary as I've not come across a 3v led - usually 2.0-2.2v for red, yellow, orange, most green and 3.5-3.7v for blue or white. So check first, 4 blues on the 12v won't die, but you'll get the current (and brightness) you aim for with just 3 and a resistor. For extra, make up strings of 3+resistor and connect them all in parallel. linear's led resistor calculator is here to make life a tad easier.
