Combining USB power

[MattP]

Hi all, I'm trying to power several devices from one USB hub, without using external power. As a USB port can only supply 500ma, I've only got two options really...

Option 1): I could jack the power from the PSU's 5v output. This works in practice (I've tried it), but I'm a little concerned as to whether it'll damage anything, or result in more noise on the USB channels resulting in errors and stuff.

Option 2): My motherboard has USB3 ports, that aren't used. Could I combine the outputs of these together, by simply connecting them both to my USB hub? Only the power lines, of course, not the data lines. I've seen portable hard drives do something similar (have two USB plugs for extra power), but is it entirely safe?

Thanks for any help

 

[davelectronic]

Hi MattP.
I would be more comfortable with the first idea, using the psu's 5 volts rail straight from the supply, as for interference i think it depends on what your going to use the tapped power for ?

 

[MattP]

I'm planning to have a laptop optical drive, two card readers, and a wireless remote receiver, all connected to one PC usb port via the use of a 4 port USB hub. So it's going to be fairly power hungry. The optical drive draws a max of 1.5A, so the whole thing is probably going to be around 2A of power. That's about 10 watts, right? A 450W PSU shouldn't have any trouble with that... but the USB ports on the motherboard would probably struggle a bit. Hmm.

 

[davelectronic]

I cant say 100% that wont cause interference straight from the 450 watt atx psu, or if usb via a pc could cope with it, if you only need power and not your pc's data lines, then why not a seperate 5 volts supply, a modded atx could supply all that and more from a modded atx + 5 volts rail, but if you need data access as well then thats not an option. You mention connection of two usb lines, but am not sure how thats configurated.

 

[MattP]

Some portable hard drives have an extra USB plug so that more power is available for the drive to use, like this: http://www.ppctechs.com/images/items/USB-YCABLE01.jpg

That seems to say to me that it's fine to combine USB power together, but I don't know if there's any electronics involved or anything, inside the plug itself. It looks like it's a straight through connection, with nothing between, but I can't really know for sure from pictures.

I wonder if it's possible to connect more than two USBs together. In my case, I could use four, as that would provide enough power for the things I'm trying to power. Theoretically, of course.

I really want to do the ATX 5v molex thing, but I'm really skeptical about whether it's actually doing a good job. USB devices seem to not operate perfectly, despite all the connections being perfect. It could be the length of the data cable though, as it's around 5M.

 

[davelectronic]

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.

 

[Resqueline]

I'm confident that the existing two-USB connections are simply paralleled. On MB's there are small PTC-fuses protecting the outputs.
I don't know if the power lines in USB-stacks (2-4 connectors high) on MB's are individually powered (& individually fused) or if they're simply paralleled with one fuse.
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.

 

[davelectronic]

Ive powered all sorts from a modded atx, charged phone iPods my daughters the voltage is very stable, and current is plenty, i cant see any thing wrong using the atx 5 volt rail via a molex connection, current limit if you want to be extra safe.

There is no difference i can see in a modded atx for power with in limits, and the atx in your pc.
PS, Both ways are short circuit and over load protected, ive shorted a modded atx and over loaded one, they just shut down.

 

[Resqueline]

There's no problem in fab'ing an external 5V power connector (though it won't fix the long thin cable issue) but you better make sure to current limit (fuse) it.
It's all fine & dandy until you happen to get a dead short. Sure, it's no problem for the PSU, but the rest of the computer might not be so happy about it.
One thing is the needlessness for a booted computer to take a dump just like that, but reverse currents flowing through any on-board regulators/IC's could damage them.

 

[davelectronic]

What about the use of a metal oxide varistor to protect the pc against or from any transients or spikes.

 

[Resqueline]

Any inputs or outputs are channels that can lead electrostatic discharges or the like into the 'puter.
A varistor won't be able to clamp the voltage to a safe level however. Anything above 7V is prone to damage 5V logic.
A transient absorber diode might be able to work within those constraints, but I've not checked their datasheets for this.
For the power supply however the relatively large (& plentiful) decoupling capacitors should damp any pulses quite well.
Ferrite cores/coils are perhaps more useful components to reduce damaging dV/dI influence (as well as rf interference).

 

[davelectronic]

I did look up other types of suppression devices and thats the second one that came up was the transient voltage suppression diode , but i did not go that far into it, the diode looked favorable but i expect as you say as per component spec and the data for one.

 

[Resqueline]

Ok, since we're to exhaust the subject I did look up a transient diode.
Just checking one brand - made for 5.0V - it drops 6.4-7V @ 10mA and 9.2V @ 43.5A.
So, in essence they seem completely useless for protecting TTL logic (and varistors are worse).

 

[davelectronic]

Yep scrap that idea, but there must be a way to stop any unwanted transients or spikes reaching the pc, it must be possible to build a circuit that allows the 5.0 volts to the gear the OP wants to run, biased on power down to not return to the pc, why not just tap the + 5.0 volts rail from the psu, and stay clear of the pc's usb ports, he did say no data lines where needed, i cant see any issues with that, a couple of amps from a molex connector, as the gear is also IT equipment i doubt interference would be a problem.


In the instance of any interference surely that could be dealt with when or if it ever arose. I might put together an interface board for my pc's atx and have a dabble, power up some stuff while running the computer, nothing thats likely to cause interference such as unsuppressed motors etc, but something like mmmm got to find something, i will interface the + 12 volts rail and the + 5.0 volts rail, but direct from the psu, not the systems usb.

 

[(*steve*)]

I just happened to be reading this earlier today. Perhaps it provides a few possibilities.

 

[davelectronic]

That looks promising, ive saved a copy of it on my ex hdd, read up on it tomorrow, uk time very late.

 

[Resqueline]

As is obvious from the first page of this thread the OP has two options:
Either pull power from four USB-connectors and make his own heavy-gauge USB cable, or connect a separate 5V supply to the gear at the end of a normal USB cable.

As for making an output straight from the internal power supply I suggest making a filter like in the attached diagram.
L1 is a common-mode choke, L2 is a separate ordinary choke, capacitors may be as big as you want, PTC to suit.
The idea is that a fast dead short on the output is moderated & delayed sufficiently so that the PTC trips before pulling the PSU down.
Depending on circumstances you may or may not want a choke (like L2) in the negative line too, and you might even want to ditch L1.
L1 will actually protect against MB damage or locking up due to transients (like electrostatic discharges) by smoothing their risetime.

 

[poor mystic]

I don't see any need to connect the on-board 5V. The USB devices can be driven by their own supplies, with just the data and earth lines used in the USB cable.

 

[(*steve*)]

I don't see any need to connect the on-board 5V. The USB devices can be driven by their own supplies, with just the data and earth lines used in the USB cable.

Yeahbut...

Hi all, I'm trying to power several devices from one USB hub, without using external power.

 

[poor mystic]

D'uh
will that do?