what kills a LED voltage / amperage

Hi all

can someone please clarify:

question 1.
sometimes I use led's in car/bike applications and always use a 1 ohm resistor in series else the led fries , now i know these leds are 3.7 volt but the cars system is +-12 volt.
so what does the resistor do , reduce the voltage or reduce the amperage ?

Question2
Im planning to build a high power (5W) flashlight. I have ordered the 5w led and a driver board (buck) that has a input of voltage of 3 - 18 v , im also planning to use 3 x 18650's in series (11.1v) , now this driver board limits the amperage to 1000mA but the specs on the led are 6-7 v forward voltage and 1000mA , I would like to know how the driver board knows / inputs the correct voltage for the specific led ?

----> i want to do the same but using a 3w led ( 3w, 3.7 volt, 750 mA)

thanks

It limits the current - it's excessive current that kills an LED, but excessive current comes from excessive voltage as well (assuming simple resistor limiting).

LEDs have a resistance which varies dependent on the applied voltage. In your case at 6V the diode will let 1A flow, thus has a resistance of 0.166ohms.

However if you increase the voltage then the resistance of your LED decreases and thus more current flows - eventualy the LED will be destroyed. Importantly the resistance is not a linear function like in a basic resistor.

I suggest you buy a cheep LED and gradualy increase the voltage applied to it say 1V to 4V, and measure the current flowing. Plot your results on a graph, even better plot V/I against V, and you'll see exactly what i'm saying.

Your power supply btw is set up as a current source so it will provide whatever voltage needed to drive a current of 1A, if you leave it open circuit the output will be at its maximum voltage.

thanks
here is the driver im planning to use
**broken link removed**

IN STOCK In Stock: ships in 2 to 4 days (5 to 8 during new year season) Worldwide Free Shipping on Everything Worldwide Free Shipping
- 1*Cree 5W regulated power output
- Output current: 1A with P4 emitter; 1.2A with Q5 emitter
- 16.8mm diameter base, 5.5mm overall depth
- Voltage: 3V-18V
- Fully regulated circuit design
Specifications Return to top
Item: 18V 5W Cree Circuit Board for Flashlights (16.8mm*5.5mm)
Dimensions: 0.67 in x 0.67 in x 0.2 in
Weight: 0.07 oz
********************************************************
with this led:

**broken link removed**

DC Forward Voltage (VF): 6.00~ 7.00Vdc
DC Forward Current (IF): 1000 mA
Reverse Voltage (VR): 5V
Reverse Current (Ii): 5uA
Temperature Co-Efficient: 0.04nm / Degree C
Viewing Angle: 140 Degree
intensity Luminous (Iv): 350 ~ 360 LM
*******************************************************************

from what i can understand is that the driver will drive the led at 6-7 volts and 1000mA
correct ?????

Because that is the requirments of a typical 5watt LED

BTW, the thing that often kills ICs/LEDs/etc. is excessive heat. This occurs when the product of the current through a part of the device times the voltage across that part exceeds a reasonable value. Note that it's also not a simple relationship between power and failure. For example, a high power level can result in a conductor in the device dissipating too much power and catastrophically failing (e.g., through melting). A lower power level won't cause this melting, but can result in a different failure mode (say, something like electromigration). I haven't worked on IC reliability for a number of decades, but even back when I was, there were many different failure modes, many of which were thermally activated (i.e., there was an Arrhenius exponential relationship for the time to failure as a function of temperature).