USB are ground and -5v isolated? - Page 1
Forum section: electronics / beginners

author: cowboy303
author_rank: Regular Contributor
timestamp: December 18, 2012, 05:54:23 pm
content: Hey guys I'm trying to fix a pair of USB head phones, and there's a .1 ohm short in the -5v wire to gound. Is that OK or do you think that's my problem?There's a smd reistor between the 2 but It does not look burnt.Oh and the short is on the board not in the cable.Thanks

author: mariush
author_rank: Super Contributor
timestamp: December 18, 2012, 06:45:30 pm
content: There's no -5v on the usb port.It's 5v , transmit, receive, ground... and usb 3 has an additional pair of wires for transmit and recive, but still uses only 5v.

author: cowboy303
author_rank: Regular Contributor
timestamp: December 18, 2012, 07:21:25 pm
content: OK sorry, the board says:Wire color, what the board says.Red, 5vWhite, D-Green, D+Black, -5vNon insulated, GThe black wire and the non insulated wire have a short on the board.and when I plug the head phones into my computer I get this pop up that says "overload on hub port" or something like that.Thanks for the help.I really have no clue about USB so this hole thing is a learning experience.

author: Thor-Arne
author_rank: Unknown
timestamp: December 18, 2012, 07:58:49 pm
content: My guess is that-5Vis theGND.Was these headphones working before?If not, check that the pinout on the USB connector is correct.Then move on to measure between the 5V and GND to see if there is a shortcircuit in the cable.Check outWikipediafor a USB description.

author: PhynixViking
author_rank: Regular Contributor
timestamp: December 18, 2012, 08:21:34 pm
content: Knowing from doing computers for so long, and I've had a great number of "really kick ass hubs" that have broken *sad face* anyway!!!99% of the time, Ground (outer metal of plug) IS the "-5v" (same thing) I have had some rare cases that it was not the same thing, but ended up going to the same place on the circuit anyway (and had a diode or transistor or some kinda of separator between them.)In short, either way, its safe to assume that the GND (outer metal) and the "-5v" black cable are the same thing.Maybe that resistance you're getting is like I said a diode or something... eh... probably just ignore it, lolPost a pic of the USB head phone circuit? Maybe I or someone can point you in a good direction to check something.Either way! Good luck! ^__^

author: cowboy303
author_rank: Regular Contributor
timestamp: December 18, 2012, 08:35:29 pm
content: Quote from: Thor-Arne on December 18, 2012, 07:58:49 pmI think your right, I bet the non insulated wire is the plugs case and the -5v is the ground pin the USB connector. I just check and that smd resistor that went between G and -5v is not a resistor it's a inductor (says L2 right next to it).The head phones used to work and we bought them from radio shack.I checked for shorts on on the board and cable and all I found was D+ and 5v had a 100K resistanceThe cables is good, no bad connections or shortsNow all this testing is with my multimeter so I'm not giving it a full 5 volts.I'll check it with 5 volts and will be right back.

author: cowboy303
author_rank: Regular Contributor
timestamp: December 18, 2012, 08:39:05 pm
content: Quote from: PhynixViking on December 18, 2012, 08:21:34 pmThank you,I don't have a picture but maybe I can get one (it's really small and my camra isn't that good)

author: cowboy303
author_rank: Regular Contributor
timestamp: December 19, 2012, 12:42:55 am
content: Thanks guys I think I found the problem I'm going to test them out when I get a chance and see.

author: G7PSK
author_rank: Super Contributor
timestamp: December 19, 2012, 04:38:28 pm
content: There is no -5 volt on a usb socket it's the 5 volt negative side, very often the screen is also connected to the negative as well.The pin outs are here.http://pinouts.ru/Slots/USB_pinout.shtml

author: c4757p
author_rank: Super Contributor
timestamp: December 19, 2012, 09:48:12 pm
content: Trapforyoung players causedbyyoung players. I've seen many diagrams done up by newbies who use something like that: there is a 5V supply, so they label its positive and negative terminals "+5V" and "-5V" (i.e., positiveof5V and negativeof5V), rather than "+5V" and "0V" (i.e.positive 5Vrelative to0V).Don't do this.

author: 8086
author_rank: Super Contributor
timestamp: December 20, 2012, 12:57:06 am
content: -5v is actually ground, 0v, whatever you want to call it.Shield is usually connected to 0v/groundThe small resistance could simply be the resistance of the solder/wire/track or it could simply be measurement error. They are probably directly connected to each other, or via some component as someone said before.Don't worry about a "short" between these, it's normal.

author: cowboy303
author_rank: Regular Contributor
timestamp: December 20, 2012, 02:58:10 pm
content: Thanks everybody!I found the problem, the wires had worn and were touching each other but when I tookit apart the weren't touching any more, hence I didn't find a short.So I took them all off and, cut the cable a little and resoldered everthing back on, doing it in a way that thee wires are now not touching.8086Yes the small resistance was a little smd inductor (says L2 right next to it).c4757pYeah I would say so.OK