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I'm planning on having a set of blue LEDs connected in series. I'm looking at having about 30-40 LEDs in series. The power supply will be from a USB 2.0 port mounted on my television. Reading up on the specs for USB 2.0 it has a pin for 5v and ground.

If I used this as my power supply, will my LEDs light up as much as they should or would they be really dim.

The specs for the LEDs are:

  • Material : Semiconductor; Light Color: Blue
  • Head Dia.: 5mm / 0.197"; Forward Voltage: 3.2-3.4V
  • Luminous Intensity: 2000-3000MCD; Wave Length: 460-463
  • Size: 24 x 3mm / 0.9" x 0.1"(L*W); Package Content: 50 x Light Emitting Diode

And a link to amazon where I purchased is this

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    \$\begingroup\$ They won't light up at all if they're in series. \$\endgroup\$ – Ignacio Vazquez-Abrams Apr 20 '14 at 23:22
  • \$\begingroup\$ What is the forward volt drop and current required for the LEDs. Your spec is of no use. It's like saying I have a transistor that has three legs and is black. \$\endgroup\$ – Andy aka Apr 21 '14 at 0:12
  • \$\begingroup\$ @Andyaka He says Blue LEDs so VF in 3-4V range and he also says Vf 3.2-3.4V. Question may have been edited. \$\endgroup\$ – Russell McMahon Apr 21 '14 at 0:51
  • \$\begingroup\$ @russell ok thanks dude but he's still missing the current consumption. \$\endgroup\$ – Andy aka Apr 21 '14 at 9:50
  • \$\begingroup\$ @andyaka if the led specs are missing, assume the typical vf and current limits for the color. Let's not be pedantic about it. \$\endgroup\$ – Passerby Apr 21 '14 at 12:04
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They definitely won't work from USB if connected in series (even two LEDs in series won't work).

If you connect a ~100 ohm resistor in series with each LED, you may be able to run about five LED/resistor sets connected in parallel from a USB port. Without negotiation, a USB port is only guaranteed to supply up to 100 mA (although many USB ports have no current control, and may supply 500 mA or more).

According to the comments on Amazon, there is no recommended maximum current data for these LEDs, so I'm guessing 20 mA per LED would be acceptable.

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  • \$\begingroup\$ would connecting them in parallel work instead? \$\endgroup\$ – lecardo Apr 20 '14 at 23:47
  • \$\begingroup\$ I did suggest connecting LED/series resistor combinations in parallel, if you read my answer. \$\endgroup\$ – Peter Bennett Apr 21 '14 at 0:03
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40 LEDs in parallel with separate current limit 220R's @ 10mA each is half brightness but 400mA total

only if you are lucky, the TV may support this level without enumeration from a smart USB chip. You can test the TV with 12 Ohm >2W resistor before building this using a voltmeter or read TV specs.

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