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Possible Duplicate:
Powering a breadboard with USB

I was wondering if it is possible to cut open a usb cable, and use its "sub" wires to get its 5v power and gnd.

I am currently doing a digital circuit school project, and would like to power my bread board (protoboard) using the vcc and gnd of the usb. Is this possible?

I have read that the usb cable has atleast 4 wires, red for +5v and brown for gnd.

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ibaguio
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2 Answers2

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Yes, you can do that, but wouldn't it be neater to leave the cable as is and use a USB socket or plug? Then you just connect the 2 power pins to your circuit.

enter image description here

This plug allows you to enter a common USB cable, and go on from there, connecting wires to just the +5V and ground. Then you're no longer constrained by the limited length of USB cables.

If you do want to cut the cable then usually red is +5 V and black (not brown) is ground, but measure the voltage on the wires to be sure. Make sure that the data wires D+ and D- (usually green and white) are left open.

Note that USB 2 only allows you 100 mA, and a 10 µF capacitance.

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stevenvh
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  • how do i connect the power pins? – ibaguio Sep 14 '12 at 12:34
  • Saw the link post. Thanks for this answer. next time im going to use the socket instead of cutting the usb cable. – ibaguio Sep 14 '12 at 12:39
  • @ibaguio - "next time..." Does that mean you cut the cable already? Otherwise the cable plug on the bottom right of [this document](http://www.keyelco.com/pdfs/NPB-2010-USBproducts.pdf) is a neat solution: You don't have to cut the cable, but use the cable plug to further with the power connections only. – stevenvh Sep 14 '12 at 12:48
  • thanks for correcting. I had the usb3 number stuck in my head, I guess. – Scott Seidman Sep 14 '12 at 16:13
  • @Scott - Correct, for USB 3 it's 150 mA. I specified 2, because that's still much more common. I've deleted the comment, since it's corrected in your answer. – stevenvh Sep 14 '12 at 16:17
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    It's unlikely that an amateur connector assembly job would be anywhere near the reliability/durability of a factory overmold. Just get a cheap cable and cut it. – Chris Stratton Sep 15 '12 at 03:46
  • @stevenvh yes. apparently i was very curious and cut the cable. Not a problem though, I have a lot of cables running around the house. thanks for the help – ibaguio Sep 15 '12 at 07:27
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Yes, but as stevenh says, using some sort of breakout board with a socket on it is a much better idea for many reasons. Lastly, you MUST use no more current than the USB standard allows-- that's 100mA without negotiation for more through USB protocols that you have no intention of using. If you draw more current, chances are the host computer will cut off the connection, and your power!

Also, development might be a little less risky if until you knew things were right you sucked power off of a USB hub. They're a bit less expensive to replace than motherboards following catastrophic failure!

Martin Thompson
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Scott Seidman
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