I am making an LED lamp using an LED strip. The LED strip I am using is a 240 LED/m SMD2835 strip, which operates at DC12V, and is supposed to use 18W of power per meter.
My lamp is constructed as seen on the diagram: I cut it into 20cm strips, installed them on an aluminum heatsink and wired in parallel. I have tested each of the 20cm strips separately, and they draw around 0.3A of current at 12V, which is very close to the specification.
However when I wire everything together, the completed circuit draws only around 1.4A of current, which is 16.8W, while it's supposed to be around 28.8W (18W/m * 1.6m). I also noticed that each individual 20cm strip is not as bright, compared to when powered individually. I also measured voltage in the beginning and in the end of the strip - and it is much lower than 12V!
First I thought the problem is with my power supply - and I have tested it with different ones, ranging from cheap 12V 3A wall plugs, to bigger 60W bricks, but unfortunately - same result. Finally I tested it with an adjustable voltage power supply, and I noticed that when I set output voltage to around 14V, then voltage across the strip becomes around 12V and it draws around 2.5A of current, which makes it much closer to how much power it is supposed to draw according to my design (and it is much brighter, too - like when I powered 20cm strips individually).
I am very confused... Can you explain this behavior? Should I buy a power supply with a slightly higher output voltage, so that it drops down to 12V when connected to the strip? If so, how do I calculate how much output I need? Thank you.