The question is not that dumb as it sounds because the answer is 8. And what I really want to know if there are different 5050 LED's or I've got some particulary bad batch or am I making a measurement misstake.
I've bought gauss 14.4 LED strip which is marked "5050-60P-12V-10D-V1.1" So 60 5050 LED's per meter. This should consume 14.4W boths according to descriptions of such strips and according to calculations on 5050 datasheet. But my power supply shows 2.0A at 12V for 2.95 meter strip. That's 8W per meter. Voltage on the other end of the strip is 11.6V. The reason seems to be that voltage drop on a single LED is 2.9V when it should be 2.3V. Voltage drop on a single LED is 2.9V. For each 3 LED's there are two 39 ohm resistors (marked 390, and measurements confirm that). And I've checked my power supply (HY 3005) with resistor load, reading seems correct: 0.17A at 4V for 23.5 ohm (two 47 in parallel).
Update:
I've made a mistake by not describing the strip properly and not giving datasheets. This is a warm white strip and it's target voltage is really 12 volts. It's for home usage and is meant to be used with 12V power supply which you can buy in the same store. That supplies can't give 14.4 volts.
Unfortunately manufacturers site is very uninformative so there is no point to linking to it. There is also no datasheet available and I don't know LED's manufacturer. So I can only use datasheets for similar LED's. 3V drop for white LED seems OK, 2.3V from unedited question was for red LED's, which is a mistake.
Conclusion:
All components in the strip seems to be fine, it's just designed to drain less power than specified at 12V. Probably that's made for extending lifetime. Or, as @TonyStewart.EEsince'75 noted, extra room is left to allow usage of the same strips in cars at 14.4V. By increasing voltage I can make it brighter and still will be within allowed current limits for LED's, but stock power supplies for that strips can be maximally adjusted to 13V.
Answer
Let me try a simple analysis with no specs
- my initial reading was you had two 39 in series = 78Ω, so I will show Rev B
- let 14.4W / 60LEDs = 0.24W /LED including 1/3rd of Pd in 78Ω
- let's assume they "cheated" ( or exaggerated marketing with no specs)
V+=14.4V ( car alternator = 14.2V nom) e.g. 14.4V*1A = 14.4W
thus 60 LEDs in 20 strings = 50mA per string
- and Vdrop on both R is 50mA*78Ω = 3.9V
- with Vf = (14.4-3.9)/3led=3.5V (cheaper quality LEDs)
Conclusion
12V will never achieve rated power for these StripLeds, you need a battery and charger or a 15V or 14.4V power supply, because they are rated for Cars but still work on 12V since the LEDs add up ~9V at low current.
These will also be running near maximum temp so suitable for semi-trailer trucks with forced wind cooling. lol.
Rev B
Analysis with updates on Rs current limit using two 39Ω in parallel=20.5Ω
- Given 60 5050 LEDs/m in parallel(P), series(S) array 20P(3S + 20.5Ω)
- your results of 2.0A at 12V for 2.95 meter = 8W/m and not 14.4W/m
Now what would we expect at 14.2V with 2.9V/LED?
- or a rise of 2.2V or almost 20% in power source to match opt. alternator voltage in cars
- do we expect a linear incremental rise in power due to series R? no but,
- a 20% rise in voltage, results in a 40% rise in power if it was linear
- we need almost 6.4/8W=80% rise to achieve 14.4W, so it is quasi-linear
- this is because the most of the voltage rise is across Rs=20.5Ω
- You measured 2.9V/Led @ 8W/m thus If= 8W/(20S * 12V)= 33.3mA /string
- thus V across 20.5Ω * 33.3mA= 0.68V
- my trick is knowing LED ESR is due to incremental V/I above 2.7-2.8V
- i.e. (2.9V-2.7V)/33mA=6.7Ω assuming your reading was accurate
now I can estimate power vs voltage for arrays in two ways, Can you?
- such as 20S(3P+Rs) using LED ESD & threshold Vth or from specs
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