Resistor for LED

[hoodoo]

G'day Madbeaners, Matt here.
I was hoping that someone could answer a question in regards to the resistor attached to the LED.
It seems most builds call for a 4k7 resistor, what i would like to know is, what is the lowest value resistor that you can use, to make the led brighter, without damage to the led.
Thanks, Matt.

[GermanCdn]

Generally 2.2k is the lowest value that works, it will get you retina burning bright on the ultra brights.

[oldhousescott]

Figure 40mA for the max LED current and the forward V drop at 2v, so:

(9-2) / 0.04 = 175 ohms minimum

But as stated previously, anything less than 1k is likely to induce blindness (not really, but you get the drift). 4k7 will yield about 1.5mA which is plenty for most LEDs.

[jkokura]

I played with some LEDs and a trim pots one day. I had yellow ones running on 120R, but a blue needed at least 170ish before blowing. Up to about 15K they were plenty bright enough.

Jacob

[hoodoo]

Thanks for the advice all. The reason i asked, was that i've hot glued an LED into a jewel lens and it dulled it down some, so i'll take the advice on board and will post the build in the next few days,
All the best,
Matt.