I read a few posts here about resistor in led applications. The thing is, the resistor was always use with a power source delivering a higher forward voltage than the led could tolerate(ex: a 9volt battery hook up to a 3w 3v led).But what about a power source that the voltage can be adjusted to fit the led? My science courses are long gone behind me so I want to make sure my project doesn't explode on my face ;).
Right now I'm building a led lamp for a photography project using a 400w 36V 11A Single Output Switching power supply AC to DC SMPS which has a voltage adjuster so I can go from 32.4 to 39.6V. I'm using 168 3w Leds for my lamp, with a voltage rating of 3.2 to 3.4V and 700mA of current. I hooked up 12 leds in series so I can put 36v to them and not pushing them too hard. I have 14 series of 12 leds in parallels. The leds are soldered to a copper board that I cleaned with acid with the proper circuit to have 14 series of 12 leds in parallels.
My question is do I need to include resistors for safety let say a led in a series dies so it doesn't blow up the rest of the led in that series? Is it possible to put a resistor at the end of each series or I need to put a resistor after every led?
I took some measurements:
-The lamp is drawing approximately 4A at the power supply which I find odd since it should draw more power if the led are supposed to draw 700mA
-Power supply is pushing 36V precisely
-Measurements at the led give me 3v as it should
Maybe I'm wrong here but normally to calculate a resistor resistance you have to do R= V/I so 36V-(3V*12)= 0/.7A =0R
But the thing that is bugging me is the power supply is rated for 11A which is more of what the leds should take. Why my concern for a resistor somewhere in my lamp design.
Thanks for the help! I hope my explanations are clear enough!