From: Arno on
John Weks <john(a)gtuo.com> wrote:
> 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.

I am talking about one for external overvoltage. That is advanced
functionality and not necessarily there.

>> 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.

So? I have serveral ASUS boards and one Gigabyte board with that
and nothing else.

>> 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.

A diode in the USB powered device does not help, it would need to be
in the Y-cable. And it would need to be a high-current low loss
Shottky type to limit voltage loss to a level that the attached
HDD still spins up.

> The USB ports arent electrically paralleled.

Are you sure? This whole construction is unspecified (and disallowed)
by the USB standard. Hence every vendor can do what they see fit.
And if both ports are driven from the same 5V line, there actually
is no need for any circuitry, you can just put the two power lines
together.

>> 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.

Oh? There is just ONE power input in an USB powered device, not two.
How would that work?

> The multiple USB power sources arent just electrically in parallel.

Again, how do you know that? I think you are deep in the real
of wishful thinking here...

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

> Nope.

Yes.

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

> Plenty of laptops cant do that.

Laptops typically do not have fused USB ports. They have proper
current control circuitry.

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

> Its completely safe in fact.

No. External overvoltage circuit need to be present for it to
be save.

>> 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.

I think you just failed EE 101. Your undertsnading on how
USB power circuitry works is clouded by limited knowledge,
wishful thinking and plain misunderstanding basic DC
circuitry design.

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: Arno on
Mike Tomlinson <mike(a)jasper.org.uk> wrote:
> 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.

That would be pretty safe and in the worst case destroy only
the hub, not the USB port on the PC.

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: Mike S. on

In article <nh0qcKIoul0LFwhI(a)jasper.org.uk>,
Mike Tomlinson <mike(a)jasper.org.uk> wrote:
>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.

Yep. Sounds like a logical compromise.


From: Mike Tomlinson on
In article <83g67vFpanU1(a)mid.individual.net>, Arno <me(a)privacy.net>
writes

>Again, how do you know that? I think you are deep in the real
>of wishful thinking here...

You're talking to another Woddles morph. Never engage in a battle of
wits with an unarmed man.

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


From: John Weks on
Arno wrote:
> Mike Tomlinson <mike(a)jasper.org.uk> wrote:
>> 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.
>
> That would be pretty safe and in the worst case destroy only
> the hub, not the USB port on the PC.

No reason why the hub would be destroyed.