Cutting usb cable for power

A while back I took spliced a USB cable and was able to get +/- cables from it and use it as a way to feed power from a battery pack to an Arduino project.

Yesterday I spliced an apple iPhone charging cable and I can only get a voltage from the red cable and the threaded metal jacket surrounding the cables. Why is that?

How do I know which cables to tear apart and which ones not to?

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Why is that?

Apparently, they are using the shield as ground. It's normal for the shield to be grounded (if there is a shield) but sometimes there is an additional ground.

How do I know which cables to tear apart and which ones not to?

You don't. They aren't sold to be torn apart... But, there has to be +5V and ground at the "far end".

You take them apart, and see if they're any good :-P

Luckily, they're cheap as dirt (I bought 9 pretty green braided flat micro USB cables for $7.50 this week - though I haven't chopped any) so it's not going to break the bank. You can also, for only a small premium, get cables that just end in the 4 wires, or in a barrel jack (ebay) - like 1.50-2.50 a pop.

I would prefer generic cables over fancy brand name ones for chopping up (and in general - I'm cheap). The branded ones are more likely to do something weird (like your apple ones did).

As it happens, I have a bunch of USB A male to USB A male cables. Like, over 100 of the damned things - I can't think of any use other than chopping them up to get USB power cables for projects (you get 2 for 1). I need to sell them or something...

The shield is the ground. Or use a proper usb lead instead of Apple Shite

Standard USB is red/black for power, green/white for data, shield for shielding. Getting rid of the black wire allows a much thicker red wire for high current.

The original limit for USB is 0.5A maximum, but these days phone-chargers routinely used 2A which is 4 times the original design current and thus 16 times the dissipation in the cable (I-squared-R losses) if you don't upgrade the wire thicknesses.

For just powering, get a USB lead without data wires, then the red wire can be a decent thickness.

Cheap long USB cables are not satisfactory for high current 5V power, rather large voltage drops are seen.

Hey @Marciokoko, maybe you just want to get yourself a little usb/pcb adapter to attach it to your Arduino? Also available for other sockets.

Best