LED question...

[dutchcedar]

Since Darv fried an LED and I'm gonna use a couple in one of the Cape Bullseye reservoirs... it occurred to me that I might need some help, 'cause I know zippity squat about electronics.

Can I use tailed 3.3V LED's, like are used for a power light at the front of the case, plugged into a fan speed controller that's set up for three pin fans and varies the voltage from 7-12V, without adding resistors?

[Darv]

Since Darv fried an LED

I didn't fry it, turns out it was something inside the rad :lol:

If it was me I would just use a resistor, less hassle. There's an LED resistor calculator on the Bit-Tech site and theres a few others around.

[Squelch]

Hi Dutch,
The short answer is yes it is possible.
The long answer is only with a bit of extra electronics to compensate for varying voltage from fan controller.

Is the fan controller output a requirement or is it just convenient?

[dutchcedar]

Is the fan controller output a requirement or is it just convenient?Well, there ya go... :lol: I'm putting a fan controller with four knobs inside the case, near the mainboard, but am only running two fans... so of course, I wanna do "something" with the other two... which led to the idea of plugging a couple LED's into them.

So Darv... its not as simple as just adding a 3.3V resistor to one of the wires, eh?

I wish Starbuck was my next door neighbor... :rant: :lol:

One of these days I'll do something simple, I promise... <_<

Right...

[Starbuck3733T]

add a 470 ohm resistor in series with either leg of the LED. you'll be able to adjust it from full brightness (~20mA @ 3.3V) on down.

[dutchcedar]

add a 470 ohm resistor in series with either leg of the LED. you'll be able to adjust it from full brightness (~20mA @ 3.3V) on down.Cool... big thanks!!!

I'm assuming by "in series" you mean solder the resistor into either wire of each LED, cause they're each gonna be on their own...

B)

[Squelch]

And a cool panel dimmer is born :)

[Starbuck3733T]

Cool... big thanks!!!

I'm assuming by "in series" you mean solder the resistor into either wire of each LED, cause they're each gonna be on their own...

B)

Correct...

+V ----ZzZzZz----|>|---GND where ZzZzZz is the resistor.

[BigBen2k]

I'm sure it's been covered before, but I just thought I'd bring it up; maybe it'll serve someone else.

Hooking up LEDs to a supply:

First, take a look at this sample spec sheet, for a T-1 (3mm) blue LED:
http://wbta.us/bigben2k/LED-59.pdf

Note the amperage (DC forward current) of 20 mA (milliamperes), which is typical of most LEDs. Also note the "forward voltage@20 mA", typically 3.5 for a blue LED (other colors are usually 1.4v, but the new colors (white, aqua,...) are usually 3.5v).

Then note this equation:

V = R * I

V: voltage (volt)
R: resistance (ohm)
I: amperage (amp)


If you use a 5 volt supply, and the LED is going to drop 3.5volt, then the resistor that you're going to use, needs to drop 1.5v.

so...

v=1.5 volt
I=0.020 amps (20 mA)

and you calculate R to be... 75 Ohm.

A 75 Ohm resistor happens to be standard, otherwise you'd pick the next value *up*, (to stay on the safe side), which would be 82 Ohm.


FYI; Starbuck3733T ran the same calculation using a 12 volt supply, and a 3.3 v drop, resulting in an 8.7v drop at the resistor. With I=20 mA, you get: 435 Ohm which is not standard, and the next value up is 470.

B)

Other LED info:
T1 are 3mm
T 1 3/4 are 5mm
The long pin connects to the positive of the supply and no, it doesn't matter where the resistor is hooked up, as long as it's in series with the LED.

[Starbuck3733T]

Nailed it. Since dutch was figuring on running these off fan PWM, I may have (unwisely) assumed that the supply was +12V.