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I want to power a small single LED from a USB cable.

I found this LED on eBay. Would it be appropriate?

I’d like to find an LED that I can hard wire to a USB cable, without other components in the circuit. I want to be able easily & cheaply replicate it.

I have no electrical knowledge, so please be gentle with me (or at least be brutal in plain language!)

  • 1W / 3W 5V Lumen Lamp High Power LED SMD Different Colors Chip Lamp - Beads COB
  • Specification:
  • Power: 1W,3W
  • Forward Voltage: 3.0-3.4V
  • Reverse Voltage: 5V
  • Forward Current: 700mA ,300mA
  • Luminous Flux: 220-240LM
  • Working Temperature: -20C to 60C
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  • \$\begingroup\$ I suggest that you learn a bit of theory on how leds work, what the specifications mean and how to limit their current. For your application you need an led with a forward voltage of 5V and a forward current of <= the current your usb port can provide. \$\endgroup\$
    – Panthera
    Nov 27, 2022 at 15:51
  • \$\begingroup\$ Such a LED that takes 5V in may not exist. And even if it did exist, pulling more than 100mA from USB without properly communicating how much current you intend to draw is not allowed by USB standard and the devices you would plug that LED into may fail to work with it or fail to power other USB devices because your LED does not care about the rules of USB devices. \$\endgroup\$
    – Justme
    Nov 27, 2022 at 16:22
  • \$\begingroup\$ You can get 5 V LEDs with built-in resistors that can be connected directly to 5 V, without additional components. Just google the italics. They tend to be < 15 mA, though. \$\endgroup\$
    – ocrdu
    Nov 27, 2022 at 16:32

2 Answers 2

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That LED is not appropriate. A bare LED will require an external circuit to control the current. From the description, the LED you have selected is a bare LED.

If you connect it straight to the 5V from the USB port on your computer, one of two things will happen:

  • The LED will burn out.
  • The 5V supply from the computer will be damaged.

If you are really lucky, the USB port on your computer will limit the current to something safe for the LED and neither will be damaged. That is pure luck, though. It depends on how the mother board designer chose to protect the 5V supply. Some don't bother, and just let the USB port draw as much current as it wants. Some use a fuse to cut off current if the circuit draws too much. Some use a positive temperature coefficient resistor or polyfuse as a resettable fuse to cut off the current. Some don't bother to protect it, they just have a really anemic 5V supply connected to USB.

You can't tell from the outside. If you try it out, you stand a good chance of damaging the motherboard.

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You need a resistor to limit the forward current. The USB supply will try to put out 5 V all the time, but your LED only wants a 3 - 3.4V drop. This will result in overcurrent and blowing out the LED quickly. You need a resistor in series to drop about 2V and to limit the current to <300mA.

Since you want lets say 250mA current, and a 2V drop, start with V=IR or R = V/I 2V/0.25A = 8 Ohms. Also P=V^2/R, so 4/8 = 1/2W.

So connect an 8 Ohm 1/2W resistor in series with the LED to and connect that to the USB supply and you should get what you want.

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  • \$\begingroup\$ Might work but dangerous. What if the device can't output additional 250mA and you plug 250mA LED without negotiating you need 250mA. \$\endgroup\$
    – Justme
    Nov 27, 2022 at 16:54
  • \$\begingroup\$ I agree it may not work with some supplies if they wait for negotiation to put out any current. I can't see how this could be dangerous. \$\endgroup\$
    – sk3ptic
    Nov 27, 2022 at 17:41
  • \$\begingroup\$ Well, imagine having already near maximum load with devices on a hub, and then you plug in your 250mA load without warning, so it may blow a fuse or trip a current limiter and shut down the whole hub, disconnecting any devices like mass storage devices unsafely. Of course it won't be dangerous to humans, just dangerous to break hardware or lose data. \$\endgroup\$
    – Justme
    Nov 27, 2022 at 18:34
  • \$\begingroup\$ OP had a very simple question asked by someone who knows nothing about electronics. It was a simple learning opportunity. OP never asked if it was a good idea, he asked how to do it. \$\endgroup\$
    – sk3ptic
    Nov 29, 2022 at 14:15
  • \$\begingroup\$ Like you said, the OP may not understand why your suggestion is not a good idea and why it should not be done as per your suggestion. \$\endgroup\$
    – Justme
    Nov 29, 2022 at 14:24

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