Resistors and LED

bonzodog2

http://www.ebay.ie/itm/10W-LED-Warm-Day-White-High-Power-1100LM-LED-Lamp-SMD-Chips-light-bulb-for-DIY-/351051601034?pt=UK_BOI_Electrical_Components_Supplies_ET&var=&hash=item51bc4e548a

Its a 10W 9-12VDC LED chip. What resistor should I use?

milltown

LED linked is 27-30v.
You'll only get a couple of watts out of it at 12vdc.
My Ohm's law app tells me you'll get 4w with about 35ohms resistor at 12v.
10w from 12vdc would need 15ohms and, at three times the current would most likely fry it very quickly.

bonzodog2

Sorry, the ebay link was just an example, to give more detail than the image. The one I have IS 9-12v. Connecting it to a 12v 1A router PSU momentarily lights it up great, but I don't want to fry it

milltown

Then a 15ohm resistor will give 9.6W, drawing 0.8amps.
14.4ohms should give you the full 10w, in theory.

backboiler

You'll need the datasheet for the LED you have.
In particular you need two values from it. VF and IF.

Then you subtract VF from your supply voltage. This gives you the voltage that will be across your resistor when the LED is in its operational range (if the result is negative, you need a higher voltage supply!)

You then need to calculate the resistor that will limit the flow to the IF you require, so divide the "remainder" voltage found above by the current IF expressed in A (i.e. if it's milliamps, (mA) divide that number by 1000).

All that said, the original ebay link there seems to be a complete unit including any necessary current-limiting. It's unlikely that any LED would have a VF in the 30 V range, though I'm open to correction.
In this case, you just connect it directly to a supply at its stated rated voltage and off you go...

whizbang

9-12volts is the voltage that the LED will see across it when you apply the proper current.
So you can see it varies quite a bit.

If you calculate for 11v, the drop across the resistor is 1v, required current=300mA, equals 3.3 Ohms.

So now if the LED needs only 9v, the drop is 3v, resistor = 3.3 Ohms, so current now is 900mA!
- that's enough to fry a 1W led in seconds.

Also remember you must cool the led. More than 100mW requires heatsink. (3v@30mA)