Resistance calculation- please help

Hey guys,
I'm rigging up 6 blue 3 mm Nichia NSPB320BS LEDS to my computer case, and I need to know what resistors to buy.

Here are the LED specs: (triangles represent the LEDs, all leds are uniform in characteristics)
- 3.6V each
- 20mA each

They run off a 12V DC lead on my power supply. 3 LEDs per node (in series), two nodes in parallel. The two resistors (represented by squiggle lines) are assumed to be of the same resistance value.


Thanks.

Hmmm, I thought the folks here would be all over this one, but perhaps I'll be the first to post the answer

My favorite LED calculator is at http://linear1.org/ckts/led.php. I've been doing exactly what you describe above and it works very well.

3.6v times 3 leds in series at 20ma gives you a forward voltage of 10.8v at 20ma. Plug that into the calculator and you get a resister value of 60 ohms. The nearest resister value is 68 ohms. You only lose 21mw in the resister since the supply voltage and desired voltage are very close to each other.

You will still want to measure the amperage being drawn by the LED's by placing your meter between the resister and the led's. A good multimeter to measure that can be had for around $40. It's definitely worth it. You may find that your batch of LED's has a higher or lower forward voltage and may need more or less resistance to get them to draw the proper current. You can also experiment with overdriving them a little bit if you can keep them cool. The ones I've experimented with dont product significantly more light when slightly overdriven, but they do produce considerably more heat which will shorten their lives if you can't get rid of it.

great. thanks for the help.

One thing you would want to do is put the resistors in front of the LED's so they don't short out. The way you have your drawing (not sure what it's called) you have 12 volts heading to the LED's. For further reference on the battery symbol, the small line represents the negative terminal so in the one you have showing the electricity is flowing counterclockwise.

that depends if you're using positive or electron flow...
I was taught that it doesn't matter which side it goes on, the sum of the voltage drops across a series loop equals the input voltage. And I am aware that the small line represents the negative terminal. thanks for the heads up.