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What resistor?

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killer7

Member
Joined
Apr 2, 2003
Location
North Carolina
I don't know anything about ohms and the LED calculators have me lost. What resistor would I use for a super blue 3600MCD LED from radioshack of these specs?. I would be putting the in a mouse, so the voltage would be 5.

"Wavelength: 468 nm (typ.), Type: T-1-3/4 (5mm), Lens Color: Clear, Viewing Angle: 30°, If mA: 20 (max.), Vf: 3.7 (volts typ.), Vf: 4.5 (volts max.)"
 
According to the led calculator I used.. a 50 ohm resistor would do the job.. given that you want 4 V.. if you want it to run 4.5 (which is max) then get a 25 ohm

Someone confirm this though.. as I haven't used that led calculator before..
 
They haven't said enough in the specs for this LED to be sure.
The 4.5 Vf drop could be during a short pulse condition. I would suggest trying 65 - 25 ohms and see when you hit 20mA.
If you don't have an amperemeter go for 65 ohm. You can try 50 ohms for a short time, if the blue light seems very whiteish you're definetly overvolting the LED.
 
xX~EXCELSIOR~Xx said:
just so you all can bookmark this

http://www.bit-tech.net/article/68/

Best LED calculator around IMHO ;)

Thanks, but I have my own calculator:D

Get a 82 Ohm 1/4 watt E24 (5% tolerance...gold stripe) resistor and you'll be fine no matter what.

While slightly more current won't kill the LED instantly, it does however shorten its lifespan, but as PSU's have a certain tolerance the voltage will fluctuate, also keep in mind that resistors come in different flavors (E3 --> 40%, E6 --> 20%, E12 --> 10%, E24 --> 5%, E48 --> 2%, E96 --> 1%, E192 --> 0.5%)

For your refference:
With a 82 Ohm resistor the Voltage can be 5.25V (Tolerance for most PSU's is 5% or less) w/o exceeding If Max. in any case (If Max. stands for Maximum forward current).

So in the worst case scenario the 5V line voltage would be 5.25V (5V+5%) and the 82 Ohm resistor would be 77.9 Ohm (82-5%).....Ohms law:
U(V)=IxR....5.25V-Vf(3.7V...voltage drop of the LED)/77.9=0.0198973A...or 19.8973mA.
 
killer7, I have done the exact mod you are attempting with the 2600mcd super blue LED from radioshack. the resistor i used is a 100-ohm 1/4 watt 5% tolerance resistor, as I erred on the safe side and radioshack doesn't have the greatest selection. hope this helps. oh btw, a 5-pack of these is $.99 at radioshack :D
 
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