Resistance to use for a LED circuit

Question

I have extracted a strip of 6 LEDs from a LED strip that has these caracteristics :

LED Quantity 300leds Input Voltage 12V Power 72W/5M Cuttable Every 3LEDS Package 5 meters/reel

I have calculated that the 300 led draws 6A (http://www.supercircuits.com/resources/tools/volts-watts-amps-converter) So that 6 LEDs should draw 0,12A (6*6/300)

So I have deduced that powering those 6 leds with 9V, I should use a 9 = 0.12*R R = 75 ohms

I chose to use a 68 ohms resistance, not having a 75 one.

The problem is that when I wire the circuit, I have half the luminosity using the resistance than when I don't use it. Best of all, if I use a 150 ohm resistance, it's almost the same effect.

What did I miss ?

Thank you for your help.

Answer 1

See those little black or white things in between the LEDs on the strip? Those are resistors. This sort of strip does not require external current limiting if it is run off a 12V supply. This allows you to run them directly off an automotive supply without external components (and subsequently turn your car into a multicolor glow stick).