led driver/resistor question

[shortstack]

i always though that for a 1 watt or bigger led (ie led 3 watt star) well i was one this led calculator site (http://led.linear1.org/1led.wiz) that shows you what resistor to use and i typed in the info for a 3 watt led (3.7v 700 mA and 12 volt input for led) and it gave me the resistor "5.9 Watt capable or greater 12 ohm resistor" now these resistors are less then a dollar a peice, and it doesn't say that these will have a problem (ie get to hot or too much load) so why do we have to use a led driver, that is over $5 when we could but the resistor for .50 cents??? someone please clarify this for me. thanks

although on the bottom of the page it says

"
* This calculator rounds the resistance up to the next standard resistor value. You should actually be able to buy a 5% resistor with the value returned by the calculator.
** Power calculations assume use of the standard value current-limiting resistor shown above. Resistor power ratings are chosen based on operating within 60% of the rated value."

 

[yellow]

simply put:
a led runs on a certain current, when supply voltage is at a certain value,
if supply voltage lowers, current lowers in a higher ratio (thats how led work)
means a slight decrease in voltage --> high decrease in brightness.

now with that resistor:
You have a certain battery setup. At the very beginning the voltag is very high, so Your resistor has to be a large value, that the led does not get too much current (not to ).
Immediately the voltage of the battery starts to lower, so does the current (as typed: in a higher ratio).
So just after a few mins, Your current through the led (= output) is way under possibility.

a good regulation driver keeps the current to the led at the level chosen

see here:
red line is the output.
typical behaviour regulation: keeps level stable, for shorter time (draws higher current)
typical behavior direct-drive (= resistor): immedately dropping light, at lower outputs starting to fade away. longer running while consuming less current
ignore the names, pics chosen just for showing general behaviour.
pics are stolen from flashlightreviews

Your resistored light behaves like down and left

 

[Oznog]

Resistors are also inefficient. That causes 2 problems. One, if you're running on batteries or doing this as a "green" initiative then the consumption is a problem. Two, it's heat- a lot of heat. 5.9W is twice what the LED produces. Now the nice thing is a resistor doesn't NEED to stay nearly as cool as the LED so no need for a fancy heatsink. But the heat does need to be rejected somehow, the resistor cannot be left insulated inside the enclosure with its temp climbing indefinitely. And it can't be allowed to heat up the LED's heatsink because that raises LED die temp.

 

[shortstack]

so basically of i try to use a resistor the led wont last long?

 

[Oznog]

If you size the resistor so that the LED does not exceed max specified current at the maximum batt voltage and min Vf, then the LED is in no danger to the LED provided it is sunk properly. BUT, it will dim as batt voltage drops.

If the resistor was sized to the max current of the LED for the mid-range of batt voltages, it may be in danger of degrading due to high currents when the batt voltage is at its highest. It will still dim as the batt voltage drops.

If you have a large ratio between batt voltage and LED voltage, then the LED will be considerably less sensitive to variations in the source voltage. That's like putting 1x 3v LED in series on a 12v system vs 3x 3v LEDs in series. BUT, the efficiency is entirely due to the ratio of Vout/Vin. 3v Vout to LEDs on a 12v system is 25% efficient/75% heat burned in the resistor. Which makes for very poor batt life and a lot of heat that can melt a plastic component, burn you, or heat up the LED heatsink (if thermally connected or nearby). Now if I put 3x 3V LEDs in series efficiency goes up to 75% @12v BUT it is now poorly ballasted and sees wide intensity variations from 14.2v (engine running/batt charging) to 10.5v (batt is dead). It's 3.47x brighter at 14.2v in fact. Not very good! If I size that resistor so it gives the max current the LED can take at 14.2v then it's gonna be a lot dimmer with the engine off and really dim if we wanna use up all the batt's capacity.

On a 12v system, I could make the single 3v LED run nearly 4x longer off the same battery by utilizing a buck converter, AND it will not fade at all as the batt voltage decreases.