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I am working on a project where I have to power a 5 meter long LED strip (https://www.sparkfun.com/products/12022) and 54 3.4V 20mA LEDs and control them all from an Arduino. My current idea was to have one 12V source for everything, direct power to the strip in parallel with a voltage regulator to 3.4 volts to power the LEDs. After that stage, everything would be controlled from the Arduino through transistors.

According to the datasheet, the strip is 300 20mA LED assemblies at 12V, bringing total power necessary to 6 amps to power it. The individual LEDs are 20mA each, and with 54 of them the power draw would be 1.08A. This would bring total consumption to 7.08A. Do I really need a +7.08A 12V power supply, or am I doing something wrong?

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  • \$\begingroup\$ What sort of regulator were you thinking of using? \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Apr 3, 2014 at 20:23
  • \$\begingroup\$ Are the 54 individual leds, individually controlled, or are you going to have them all (or a bunch) be the same color? \$\endgroup\$
    – Passerby
    Apr 3, 2014 at 22:56

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Your calculations are correct. 354 LEDs at 20mA each is 7.08 A (354 * 0.02).

The thing to be careful about with LED strips is the voltage drop. The copper thickness on strips like the 5 meter one you have tends to be 0.5 to 1.0 oz, and the voltage drop is quite noticeable. In other words, if you supply voltage to one end of the strip (12V), the opposite end will only receive 10-11V. The instructions may indicate that you should supply power at both ends of the strip, which is a good idea. Personally, in my projects, I do not run strips longer than 2 meters with that type of copper.

Edit:

Looking at the datasheet for the LED strip, it is a 5 meter strip with 60 LEDs per meter. Each "LED" is actually a 5060 package SMD RGB assembly, which means there are three LEDs per package. The datasheet specifies the maximum If at 30mA (not 20) for each color, R, G, and B. (The characteristics are derived using a test If of 20mA, but the absolute maximum value is a worst-case scenario, which you should use in calculating power supply size.)

Unless I am reading it incorrectly, this means to light the strip at 100% brightness at white (all three colors), it will need 90mA per LED. That works out to 5.4A per meter, or 27A total.

Comments on the product page suggest that the strip only requires 6A. I suggest measuring to be sure. (Strips that I have require about 20mA per color, which is what made me think to come back and add this addendum.)

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  • \$\begingroup\$ If there's something in error with this to warrant a downvote, I'd like to know what it is. \$\endgroup\$
    – JYelton
    Apr 4, 2014 at 20:44
  • \$\begingroup\$ 5050 leds, not 5060. Strips are multiple parallel segments of 3 leds in series, on top of 3 parallel color sections. ~18 to 22mA per 3 led segment, not per led. 300 leds per 5 meters = 100 segments * 20mA = 2A. 3 colors = 6 Amps per 5 meter strp at full rgb on. Not 27 Amps. Since each segment of each color has a resistor avergaing out to ~20mA at 12v, your assumption of 30mA is also wrong. \$\endgroup\$
    – Passerby
    Apr 4, 2014 at 21:57
  • \$\begingroup\$ @Passerby The product page specifically says 5060, as does the datasheet (page 1). I agree with you that with a 12V strip they are probably arranged 3 LEDs in series, which I overlooked. (A schematic would have helped, but there isn't one.) Thanks for pointing it out. \$\endgroup\$
    – JYelton
    Apr 4, 2014 at 22:54

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