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The product concerned here is : 5M Led Strip LED Strip of 5 Metre with 30 LEDs per Metre.

The data sheet for the LED used : SMD5050

And I know(Not checked physically, just read) that TV USB are capable of providing 5V 500mA Output.

Considering only the RED Color LED what I understand is I can connect 2 LEDs in series since Voltage Drop(Max is 2.4) with a 10-30 Ohm resistor.

I can connect 25 such strips in series parallel to each other as Forward current of LEDs are 20mA.

That makes a Total of 50 LEDs only from the TV USB Power if I only intend to use it for RED colour.

How can they connect 150 LEDs to a TV USB Power. Am I missing something or Are they other ways to connect LEDs? Or do they put current limiting resistor of value thrice than needed to accomodate all the LEDs? Or is my calculation wrong?

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  • \$\begingroup\$ Divide a second into very tiny slices. Use a fraction of each slice to energize an inductor. Use the rest of the slice to dump the voltage from the inductor into the LEDs. \$\endgroup\$ – Ignacio Vazquez-Abrams Feb 17 '18 at 7:37
  • \$\begingroup\$ They just light the LEDs with less power than 2.4V * 20mA continuous. Also, they probably use PWM to enhance brightness (taking advantage of persistence of vision) and probably use a boost constant current driver, rather than a series-parallel combination with resistors, which wastes power. \$\endgroup\$ – dim Feb 17 '18 at 7:44
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    \$\begingroup\$ @dim Persistence of vision and gaining free luminous efficiency from PWMing is a myth. Please stop spreading it. You need to go for lab environment black and duty cycles so short that you overcome the threshold of cone and rod excitement energy before this comes into play, i.e. no real life usability. \$\endgroup\$ – winny Feb 17 '18 at 8:21
  • \$\begingroup\$ I don't see where the manufacturer claims what you are claiming about plugging imto a TV and working on 500ma. It kinda sounds like you dreamed it up. \$\endgroup\$ – Harper - Reinstate Monica Feb 17 '18 at 14:44
  • \$\begingroup\$ @Harper Not on PC or else I would have attached pics, but till then, see the Item/description or details. There is guide showing you how to connect the LED strip to the TV USB \$\endgroup\$ – MaNyYaCk Feb 17 '18 at 17:55
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...as Forward current of LEDs are 20mA.

Many LEDs have a maximum current specification around 20mA. But turn down the usage (perhaps via PWM) or increase the resistance and they will consume less current (and not be as bright).

That said, I wouldn't trust that there's anything in this product to keep a long length of it from drawing more than 500mA.

The spec sheet says 4W/m, so 5 meters would be 20W. A 5V supply would need 4A for that much power. My guess is that if you plugged in the full length and tried to turn the brightness up, you'd either damage the USB port or the voltage would sag horribly.

It would be possible to add resistance in the strip to make sure 5m wouldn't draw too much. But if they had done that, it wouldn't show the 4W/m power spec. Instead this appears to just be a standard LED strip with an interesting USB control/power device.

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    \$\begingroup\$ Also manufacturers of said LED strips lie though their noses about said power. Measure your next purchase! \$\endgroup\$ – winny Feb 17 '18 at 8:22

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