Need help figuring for resistors.......

Hey guys, Would someone be kind enough to help me find a resistor for the following specs? I keep getting different values with the online calculators. Thanks

Source voltage will be 12v running one led
LED voltage is 3.0 to 3.4



Specifications:
Size (mm) : 3mm
Lens Color : Water Clear
Reverse Current (uA) : <=30
Life Rating : 100,000 Hours
Viewing Angle : 20 ~ 25 Degree
Absolute Maximum Ratings (Ta=25°C)
Max Power Dissipation : 80mw
Max Continuous Forward Current : 30mA
Max Peak Forward Current : 75mA
Reverse Voltage : 5~6V
Lead Soldering Temperature : 240°C (<5Sec)
Operating Temperature Range : -25°C ~ +85°C
Preservative Temperature Range : -30°C ~ +100°C

How long is a piece of string?

R = U/I = (12V-3.2V)/0.02A = 440 ohm

Closest standard resistor is 470 ohm.

How did you come up with that? Everything else is saying upper 100's.........????

scott011422 said:

How did you come up with that? Everything else is saying upper 100's.........????

Click to expand...

MSI showed you how:

R = U/I = (12V-3.2V)/0.02A = 440 ohm

Try a variation driving it a bit harder:

R = (12 V - 3.4 V) / 0.03 A = 287 ohms

Nearest standard value 300 ohms.

Power rating:

W = I²R = 0.03 A * 0.03 A * 300 ohms = 0.27 watts

Use a 1/2 W resistor.

[Edit: But 300 ohms is driving it to the limit. For long life I would stick with the 470 ohm value. You are unlikely to notice much difference in brightness and the LED will love you for it.]

Great, Thats what I was looking for. Just as a question, Why are all the automatic calculators so far off? Most are showing me at roughly 180ohm .5 watt resistor?

scott011422 said:

Just as a question, Why are all the automatic calculators so far off? Most are showing me at roughly 180ohm .5 watt resistor?

Click to expand...

It really depends on what numbers you are entering.

For instance, I just picked this one at random: http://led.linear1.org/1led.wiz

I entered:

Source voltage: 12 V
Diode forward voltage: 3.2 V
Diode forward current : 20 mA

And it said:

"The wizard recommends a 1/2W or greater 470 ohm resistor. The color code for 470 ohms is yellow violet brown."

Note: you have not included the forward voltage in your list of specifications, though this and the forward current are the two most important numbers. Somewhere around 3.0 to 3.5 V is typical for a white LED (but you don't mention the color either). For a red LED the forward voltage could be as little as 1.9 V.

I see, It would really help if I were to pay attention. I couldn't find a recomended continuous current so I took the max and ran a little less. Looking back, I see the recomended current is 30. Using 30 and not 50 I get your numbers.