Originally Posted by mandrix
Yeah they do but the fan controller was closer.
You got a 12v LED?
The single LED's I buy I solder a 470K resistor in line and hook up to either 12v or a fan controller.
I'd bet anything that's a 470 Ohm, (yellow purple brown gold) not K ohm (yellow purple yellow gold) resistor.
LED's are unique devices, sort of like a check valve,. . . until you apply at least the rated forward voltage, no current flows.
Once you exceed the forward voltage spec, current rushes thru without any internal limiting and unless you can precisely control the voltage right at the Vf threshold, the LED fries.
Resistors are placed in series to limit the current thru the led to stay below a safe limit.
To figure how much resistor to use with an led, look up the forward voltage for the led, red are usually the lowest at about 1.7V, green and white are the highest at about
3.4V & 3.9V respectively, and subtract that from the maximum supply voltage . . . 12V if planning on using a fan controller.
Take that result, and divide it by the maximum specified current for that led . . . Usually between 20 and 30 milliamps . . . 0.020 to 0.030 amps.
That result is the minimum resistance you need . . . so choose the next highest standard value. In that general range, they run: 270, 330, 390, 470, 560, 680, 820 ohms.
Most 5mm size leds can normally work fine at 25ma as the max. . . though they don't seem to be much brighter than they do at 20ma
White led with a Vf of 3.9 and with 25ma of current as the target:
12 - 3.9 = 8.1V
8.1V / 0.025A = 324 ohms
The nearest, going higher, standard value is 330 ohms
For a red led, Vf = 1.7V target current 25ma:
12 - 1.7 = 10.3V
10.3V / .025A = 412 ohms
Next standard value is 470, so you'll have a bit less than the target of 25ma.
You can put leds in parallel, as long as they are the same make and color, using half the resistance for 2 leds as for one, 1/3 for 3 etc.
The problem is, that if you break a wire to one, the other gets all the current and fries.
If you put them in series, then you need the cumulative forward voltage of all of them before they come on at all.
You could get maybe 5 or 6 red ones in series with a small value resistor, but 4 green or white ones wouldn't even come on at 12V.
Best practice is to run one resistor with each led.
If you find 12V leds, they have a resistor built in, so its max brightness is whatever that resistor limits the current to.