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Could I connect 2 or more USB ports together to light several leds? Answered

I'm planning to make a modified version of the led music box that is hopefully powered via USB. I'm looking to use between 10 and 15 leds. Is it possible to get this kind of power from USB ports?

6 Replies

verenceBest Answer (author)2011-03-27

Well, yes and no.

A standard USB2.0 port _CAN_ deliver up to 5 unit loads of 100mA each, so you could get 500mA out of it.

_BUT_, according to the spec, it only has to source one unit load (i.e. 100mA) unless it gets told otherwise by the connected device. So a well designed device will use a little power and tell the USB controller how much more it needs to function. The controller may or may not grant this extra power depending on its internal power budget. That's why Y cables for hard drives sometimes don't work. They might not get 2 x 500mA but only 500mA + 100mA. This is depending on the USB host (and its own power state - a USB hub can deliver more power if it has an own power supply compared to the state if it is powered by USB itself).

That said, some USB hosts (including PCs and laptops) provide the full 500mA anytime without being asked and some even more (IIRC, there are special ports to charge IPhones and stuff that can deliver 900mA)

Never use a Y cable (or any other means) to combine 5V out of different power supplys (e.g. from a PC and a powered USB hub), you might fry one of the supplies.

If the device should be mobile, either go very low power and cope with the 500mA of one port or use a wall wart power supply. Some (the ones to charge mobiles via a USB-like plug) even deliver clean 5V DC.

If you want to use the device only at your own PC, go with lemonie's advice and connect to the PS's power supply connectors. LOT of power. In that case, install a fuse, you wouldn't want to fry the PSU.

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Ovrlrd (author)verence2011-03-27

Interesting. Thanks for the info, but in fear of frying something important, I think I'll just use an ac/dc wall adapter.

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steveastrouk (author)verence2011-03-27

+1. Its alll about how you can negotiate the current with the computer.

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lemonie (author)2011-03-27

You can get half an amp out of a USB port, it should be plenty for low-powered LEDs.
If you need more current, open the case and tap a spare MOLEX connector.


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Ovrlrd (author)lemonie2011-03-27

Thanks for the help. I'm going to need these LEDs to be bright, though. I'm making a version of this:


But I'm going to use several LEDs and separate the left and right audio. Then I'm going to hang it to the back of my monitor. Based on how nice the music-led-box that I made looks. It should look fantastic behind a monitor for movies, music, games etc. I'll post a video and some pictures when I'm finished.

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AnotherBrian (author)2011-03-27

10 to 15 LEDs should be well under the 500mA you can get from a USB port unless you are trying to really illuminate something and they are not just indicators. You can connect multiple ports together for more current by just not connecting the data (green, white) wires of the extra plugs.

I have a 2.5" hard drive enclosure that is powered by 2 USP ports with a Y shaped cable. The only issue is that you need to plug in the 'power only' plug in first.

You could wire your device so that one USB plug (power+data) would run the microcontroller or whatever logic you need. The other USB port (power only) could just power the LEDs if they need that much current.

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