Hello!
I’m making a studio light of a led strip. I’ve bought a 8.5m of a led strip 11w/m 12v and cut it into 34 pieces 25cm each. I’ve stick it to a piece of aluminum. I’ve soldered each strip to a thick 3mm copper wire via short 1mm copper wires. I’ve decided to check if it works properly and measure the current connecting it to a 120w power source. But my multimeter shows just 4.5A maximum. It is about 50w instead of 93,5w. Why could it happen? I’ve tried my power source with a 25w 1.5. Ohm resistor and it shows 7A maximum.
12v into a 1R5 resistor should give 8A (12 / 1.5), not 7A. I suggest that means your supply is less than 12v, of the resisitor is more than 1R5, or there is extra resistnace in the circuit, or your meter is reading low.
LED strips are sensitive the the applied voltage, a small change in voltage produces a big change in current. If the supply is a little below 12v that might account for your problem, as would a fairly small additional resistnace in series with it (from the wires or connections for example).
How many LEDs, or how many per meter.
And what is the value of the resistor on the strip.
Leo..
Aka a link to the strip 
Tofer:
Hello!
I’m making a studio light of a led strip. I’ve bought a 8.5m of a led strip 11w/m 12v and cut it into 34 pieces 25cm each. I’ve stick it to a piece of aluminum. I’ve soldered each strip to a thick 3mm copper wire via short 1mm copper wires. I’ve decided to check if it works properly and measure the current connecting it to a 120w power source. But my multimeter shows just 4.5A maximum. It is about 50w instead of 93,5w. Why could it happen? I’ve tried my power source with a 25w 1.5. Ohm resistor and it shows 7A maximum.
I've measured the voltage. it is 12V with the strip connected and 11.98 with the resistor.
Wawa:
How many LEDs, or how many per meter.
And what is the value of the resistor on the strip.
Leo..
It has 60 leds/m and SMD 750 - 75 Ohms on board.
septillion:
Aka a link to the strip
And here is the link: ecola p2lv11esb led strip - Google Search
But it's all in Russian.
See the photo in the first post.
One section of the strip has three LEDs and one resistor in series.
White LEDs have a Vf (working voltage) of about 3.3volt, so three in series is about 10volt.
Meaning 2volt drops across the resistor if powered by 12volt.
That's 2/75= ~27mA if the resistor is 75ohms, and about 100.027= 0.27watt per three LEDs.
Making your 8.5meter strip 8.520*0.27= 45.9watt.
Educated guess is that that LED size has an absolute max rating of 60mA.
You could increase supply to 14volt, to almost to double current (power).
Not sure how long the LEDs will last with the increased current.
Leo..