Ok a couple of ?s about these leds LEDs. I plan on making 2 diff led kits and want to know about resistors. Now I have used the Resistor calculator but what I want to know is this. Now I plan on useing pretty much most of the leds from this site. 1. I want to make a USB led kit and I want to know if I can use a 22 or 33 ohm resistor for 2-3 leds per USB kit, using the 5v line? 2. Using the same leds I also want to make a molex 12v line led kit and want to use the same type of resistor. Also I only need to use 1 resistor for all 3 leds right? I am only useing 3 leds per kit max. I hope this makes sense. Thanks
Let's see... From the 5V line, you would be better off running each on it's own resistor... legs of circuits or single components in parallel have identical voltage, and the voltages of those led's are all different... If they are the same rated voltage, go for 3 led's to 1 resistor... Use linears calculator, and pretend it's one led rated at the sum of their currents (i.e. 20ma * 3 = 60ma) If you are running off of the 12v, wire them in series and add their voltage ratings... Use the sum of their voltage ratings as if it were 1 led and keep the current the same... (i.e. 3.5V +2.5V + 3.3V = 9.3V at 20ma) and use the calculator again... For this you need only one resistor... Again if you are using the 5V, and you have DIFFERENT rated LED's, use 1 resistor in series with 1 led, and all 3 legs in parallel Code: 3 identical on USB |----|>|----| R wire | | --^^^^-+----|>|----+--- Black wire | | |----|>|----| 3 different on USB |---^^^^--|>|---| R wire | | --------+---^^^^--|>|---+---Black Wire | | |---^^^^--|>|---| 3 of ANY on 12V (As long as combined voltages not >12V) R Wire -----^^^^----|>|---|>|---|>|-----Black wire
I'd tend to recommend the middle of those three drawings, because LEDs vary a bit even from the same batch, and because resistors are cheap when you;'re buying them by 1000 and up. Going with the first drawing may lead to slightly different brightnesses, the third drawing won't work on USB since you're working with 5V. USB can handle a 500 mA current to all devices (my recollection--I didn't look at the specs) so three parallel LEDs would draw about 60 mA, well under the limits. current will continue to add as you add LEDs. You may also find through experimenting that driving them @ 15 mA is plenty bright (LEDs don't typically get substantially brighter when driven at max current--maybe a little brighter).
