LED resistors

[Also Wik]

Okay, about putting resistors before LEDs - What is the point, aside from adjusting the brightness of the LED? I'm sure it has something to do with exploding LEDs, though, no?

The reason I ask is I have this really old light (I'm not entirely sure its even an LED, its in a wierd casing and I cant open it without destroying the casing), and the brightness is perfect  when tested directly to a 9v batt. I did, however try to put a resistor in front of it (3k9 i believe), and the derned thing wouldn't light! What's up with that?

[RaceDriver205]

The reason for that is that the resistor has decreased the current through the bulb so it no longer lights. Bulbs have there own resistance depending on their wattage.
LEDs have a specified maxium current. If you put more current through an LED than it is rated for it will burn out / not last as long.
To work out the resistance required you use the following equation:
R = (Vbat-Vled) / I
Where Vbat is the battery voltage, Vled is the LEDs forward voltage (which will be provided - gen. between 2-4V), and I is the LEDs maximum current.

[Nasse]

Then there is that current limiting function. An LED needs ***ALWAYS*** a resistor in series, because the LED works with smoke, and if you not use the resistor the smoke comes out.

That "always" means always when the voltage supply has enough "oomph" to destroy the led, which does suck current uncontrolled by its nature, if nothing does not prevent it happening. There are ways to do it without current limiting resistor, but the limiting is done some other way.

[twabelljr]

because the LED works with smoke, and if you not use the resistor the smoke comes out.

[RaceDriver205]

lol, thats the translation of what I said into engrish.
But yeah all LEDs work with smoke, its true.

[amz-fx]

Okay, about putting resistors before LEDs - What is the point, aside from adjusting the brightness of the LED? I'm sure it has something to do with exploding LEDs, though, no?

Once  the voltage on an LED exceeds the Vf threshold, the LED begins to conduct (and lights up)....  but the equivalent resistance is very low... this means it tries to draw lots of current, and that large current surge will burn out the LED.  The resistor limits the current that can flow through the LED circuit to a safe level.

My online LED/resistor calculator:  http://www.muzique.com/schem/led.htm

The voltage specification on an LED does not indicate the voltage at which it should be operated...  you can use any LED at almost any voltage with the right dropping resistor...  I have used LEDs on the 120v AC line!

regards, Jack