Combining USB power

[davelectronic]

I cant view the circuit on this devise very well, but some of the MB's lay out contain what looks like similar circuits, i wonder if there also doing a similar job, from small detail ie pic size on my cheapo phone, it dont look to big an effort to build a circuit like that in line. I look better at it on the pc later, and read up on Steve's PDF file link.

 

[MattP]

I'd like to avoid the use of external power, simply because I want all of these devices to turn on and off with the PC itself.

I've tried connecting it all up, but it's not going well. With USB devices connected up to the 5m cable (using ATX power), low power devices seem to work just fine, but when I tried an xbox controller, the increase in power draw when the vibration function was on caused it to disconnect. Also, when the USB hub is connected, it's seen by the computer as normal, but any devices connected to it are "not recognised". This is really quite confusing as, really, there's not much that's different from a standard 5M extension lead. The only difference is the source of the power.

I read the theoretical usb lines from a pc should not exceed 3 meters unless power is added up stream via a portable powered hub, my dongle is on a 3 meter extension now. I think you would need to add more power at or past the 3 meter point from voltage and power drops of line distance.

I would be fine if it where me to tap the molex and try that first, but it up to you.

Some interesting info here about USB cable length: http://www.usb.org/about/faq/ans5

You're right, it seems that hubs almost reset the length of the cable, which is good as I'm going to be using one. It does seem that 5m should be okay, though, going by that article. I might trim it by 50cm, just to be on the safe side.

At 5V you can't afford to lose much voltage, and at 2A and 5m you'll need a relatively heavy gauge wiring.
Dropping 0.25V (5%) over a total of 10m of wire at 2A requires a total R = 0.25V / 2A = 0.125 ohms.
Conductor Area = 0.0175 * 10m / 0.125ohms = 1.4 square milli-metres = 15-16AWG.

This is interesting, and quite important. I'm using an HDMI cable (with the ends cut off) for all of my connections, including the 5v wires. The cable has about five sets of individually screened twisted pairs, and one is dedicated to power transport. It's not particularly thick wire, so do you think that will be an issue? I'd say it was about 26AWG, which is considerably thinner than your calculated ideal.

I do have plenty of left over wires in the HDMI cable, though, so I suppose I could simply use a couple more so that the current is spread out between them.

 

[Resqueline]

Hehe, if indeed they're #26 you'll need to use all ten conductors for the +5V alone, and then hope that all the screen grounds together are of an equal conductor area.
At 2A you lose 2.74V through those 5 metres (two ways = 10m) of #26AWG. For each doubling of the number of conductors you halve the voltage drop. Easy math.
If you need only two for signals then you might get away with using the rest (8) for power.

Then there's the alternative; there are mains "extension cords" made for those needs. I don't remember their marketing name.
They sense for example a computer power draw on a "master socket", and then turns on & off other accessories on the "slave sockets" in unison with the computer.

 

[MattP]

Wow, I had no idea that it was so significant! Hmm, I might indeed have to use external power then, as I only have a total of 6 leftover wires in the cable. I wonder if that's the cause of the devices not being recognised, after the USB hub, because the wire isn't able to deliver enough power.

Maybe what I could do is use a 5v power adapter, and then some kind of electronic switch that is turned on by current from the PC. I suppose a good way to test would be just to hook up an adapter AFTER the 5m cable, but leave the data to go through the 5m cable, to make sure that bit works okay. Will post back with my findings.

 

[davelectronic]

A powered hub is not that expensive, but saying that i dont know there max current, i doubt its as much as 2 amps, you would get a rock steady + 5 volts from a modded at psu, there quick to adapt for all types of purposes and plenty of current, if you can find a unit surpless to requirements. The problem with adaptors unless switch mode, linear units regulation might cause load and line drops, a switch mode adaptor of 3 amps plus would be ok, but unless you've got one lying around there not all that cheap.

 

[MattP]

Right, so after three weeks of being too busy to work on my project, I've finally tried powering USB devices externally.

So far I've severed the +5v and ground wires, and attaching them to a 5v adapter (at the device end: the PC's 5v USB line goes nowhere now).

Now, charging devices works fine, but I've not been able to get a device to be detected yet. I've left the insulation/ground wire disconnected, because I don't want to short anything out in the PC. I've been thinking that this might be what's wrong, and devices are purposefully not letting themselves be turned on because of the ground sheithing not being connected.

Do you think this is the case, and do you think that there's any harm in connecting them up?

I'll diagram my connections to make it a bit clearer.

Data 1 - connected right through
Data 2 - connected right through
5v - severed from the PC (so it's not actually powering anything) - 5v coming in from an external source after the sever.
Ground - severed from the PC - ground coming in from an external source after the sever.
Ground sheath - severed

 

[MattP]

Bingo! Yep, the ground sheath needs to be connected for devices to be seen.

Edit:


Okay, so I'm pretty much ready to give up on my HDMI cable idea. I just can't get the USB stuff to work properly. Some devices work fine (perhaps the ones that don't require a lot of bandwidth), and others aren't detected at all.

So what I've decided to to is dump the idea of powering externally, and just use separate wires for different jobs, tidying them up with heatshrink or something.

I've got to connect the following

Audio
USBs (two, preferably)
5V
various very low power things, like the power button and HDD led


For the 5V, I was thinking about purchasing this wire. Will it do the trick? On page 1, Resqueline concluded that the wire must be 16awg, to avoid a voltage drop.

For the USB data lines, I was thinking about some 2 core screened wire, with 10 .14mm strands in each core (does that make each core 1.4mm 15 AWG???).

For the audio I was considering the same wire as the 5v connection above. Is that overkill? The soundcard will be driving its maximum volume through it, but I'd like to get the best possible audio from it.

Thanks for any input!