Prev: BIOS not detecting hard drive
Next: Recomeneded 2.5" external enclosure?
From: Mike S. on 22 Apr 2010 10:43

I've always been leery of storage devices that claim to be 100% USB
powered. Most of them having moving parts usually come with a "Y" cable
that draws power from USB ports in the expectation of getting 1 amp total,
rather than the expense (and possible inconvenience) of providing a
separate plug-in power supply.

Sometimes I'm in a situation where there is only one free USB port. Other
times, I'm still distrustful - like the portable DVD burner whose
manufacturer's drive specs tell me that it may very well draw more than 1
ampere at a time.

For that reason I've taken to connecting a wall-wart that provides 5V USB
power to the dummy plug on the other end of that "Y" connector. But then,
I'm worried again. Does the back current from an external PS pose any danger
to the PC's USB ports? Then again, what about the back current from one
USB port to the next, when connected to both limbs of that "Y"? Anyone
with knowledge of motherboard design care to comment (besides saying that
I worry too much...)



From: Rod Speed on 22 Apr 2010 13:41
Mike S. wrote:

> I've always been leery of storage devices that claim to be 100% USB powered.

It works fine for the lower powered devices like mice etc.

> Most of them having moving parts usually come with a "Y" cable
> that draws power from USB ports in the expectation of getting
> 1 amp total, rather than the expense (and possible inconvenience)
> of providing a separate plug-in power supply.

Only the high power devices. You dont get that with mice etc.

> Sometimes I'm in a situation where there is only one free USB port.

You can always add a powered hub.

> Other times, I'm still distrustful - like the portable DVD
> burner whose manufacturer's drive specs tell me that
> it may very well draw more than 1 ampere at a time.

Plenty of powered hubs handle that fine.

> For that reason I've taken to connecting a wall-wart that provides 5V
> USB power to the dummy plug on the other end of that "Y" connector.

Makes more sense to get one with a USB socket.

> But then, I'm worried again. Does the back current from
> an external PS pose any danger to the PC's USB ports?

Not if its properly designed.

> Then again, what about the back current from one USB port
> to the next, when connected to both limbs of that "Y"?

Not if the device being powered is properly designed.

> Anyone with knowledge of motherboard design care
> to comment (besides saying that I worry too much...)

They're just in parallel at the motherboard end for the power
lines, usually with some form of over current protection.


From: Arno on 23 Apr 2010 10:03
Mike S. <retsuhcs(a)xinap.moc> wrote:

> I've always been leery of storage devices that claim to be 100% USB
> powered. Most of them having moving parts usually come with a "Y" cable
> that draws power from USB ports in the expectation of getting 1 amp total,
> rather than the expense (and possible inconvenience) of providing a
> separate plug-in power supply.

> Sometimes I'm in a situation where there is only one free USB port. Other
> times, I'm still distrustful - like the portable DVD burner whose
> manufacturer's drive specs tell me that it may very well draw more than 1
> ampere at a time.

> For that reason I've taken to connecting a wall-wart that provides 5V USB
> power to the dummy plug on the other end of that "Y" connector. But then,
> I'm worried again. Does the back current from an external PS pose any danger
> to the PC's USB ports? Then again, what about the back current from one
> USB port to the next, when connected to both limbs of that "Y"? Anyone
> with knowledge of motherboard design care to comment (besides saying that
> I worry too much...)

If it is a current controlles port, I think there may
be some protection circuitry. However, many mainboards only
have 2A fuses in the USB power lines per port pair. A
"wall-wart" 5V PSU could blow these (very hard to replace)
if it had a bit higher voltage that the computers's PSU.

Incidentially, the external PSU will only supply power in the
first place, if it has higher volatge. This means that for
fused USB ports it is entriely useless. These would also be
able to supply something like 1.5A per port-pair anyways.
For current controlled ports, the situation is a bit better,
but still problematic. You may well damage hardware there as
well.

Bottom line: These Y-Cables are only ever safe to use if
the used USB ports are on the same 5V supply. I would strongly
advise you toi stop using the external PSU in addition.

Arno
--
Arno Wagner, Dr. sc. techn., Dipl. Inform., CISSP -- Email: arno(a)wagner.name
GnuPG: ID: 1E25338F FP: 0C30 5782 9D93 F785 E79C 0296 797F 6B50 1E25 338F
----
Cuddly UI's are the manifestation of wishful thinking. -- Dylan Evans
From: John Weks on 23 Apr 2010 14:20
Arno wrote:
> Mike S. <retsuhcs(a)xinap.moc> wrote:
>
>> I've always been leery of storage devices that claim to be 100% USB
>> powered. Most of them having moving parts usually come with a "Y"
>> cable that draws power from USB ports in the expectation of getting
>> 1 amp total, rather than the expense (and possible inconvenience) of
>> providing a separate plug-in power supply.
>
>> Sometimes I'm in a situation where there is only one free USB port.
>> Other times, I'm still distrustful - like the portable DVD burner
>> whose manufacturer's drive specs tell me that it may very well draw
>> more than 1 ampere at a time.
>
>> For that reason I've taken to connecting a wall-wart that provides
>> 5V USB power to the dummy plug on the other end of that "Y"
>> connector. But then, I'm worried again. Does the back current from
>> an external PS pose any danger to the PC's USB ports? Then again,
>> what about the back current from one
>> USB port to the next, when connected to both limbs of that "Y"?
>> Anyone
>> with knowledge of motherboard design care to comment (besides saying
>> that
>> I worry too much...)

> If it is a current controlles port, I think there may be some protection circuitry.

There normally is.

> However, many mainboards only have 2A
> fuses in the USB power lines per port pair.

It isnt that many now that have nothing but that.

> A "wall-wart" 5V PSU could blow these (very hard to replace)
> if it had a bit higher voltage that the computers's PSU.

Nope, there should be a diode at least in the USB powered device that stops that from happening.

The USB ports arent electrically paralleled.

> Incidentially, the external PSU will only supply
> power in the first place, if it has higher volatge.

Thats wrong too if the USB powered device is designed properly.

The multiple USB power sources arent just electrically in parallel.

> This means that for fused USB ports it is entriely useless.

Nope.

> These would also be able to supply something like 1.5A per port-pair anyways.

Plenty of laptops cant do that.

> For current controlled ports, the situation is a bit better,

Its completely safe in fact.

> but still problematic. You may well damage hardware there as well.

Nope, thats the whole point of the current control.

> Bottom line: These Y-Cables are only ever safe to use if the
> used USB ports are on the same 5V supply. I would strongly
> advise you toi stop using the external PSU in addition.

More fool you.


From: Mike Tomlinson on 23 Apr 2010 22:47
In article <hqpnbb$qk3$1(a)reader1.panix.com>, Mike S.
<retsuhcs(a)xinap.moc> writes

>For that reason I've taken to connecting a wall-wart that provides 5V USB
>power to the dummy plug on the other end of that "Y" connector.

It's not something I would advise doing.

How about buying a powered USB hub, plugging both connectors of your Y
cable into that, then plugging the hub into the PC? That way, the wall
wart for the hub is powering the external drive, not the PC's PSU.

--
(\__/)
(='.'=) Bunny says Windows 7 is Vi$ta reloaded.
(")_(") http://imgs.xkcd.com/comics/windows_7.png


 |  Next  |  Last
Pages: 1 2 3
Prev: BIOS not detecting hard drive
Next: Recomeneded 2.5" external enclosure?