Power FireWire using USB ports?

Do you have a question? Post it now! No Registration Necessary.  Now with pictures!

Threaded View
Here's a crazy notion; someone with more than a basic ignorant
understanding of electronics and hardware can straighten me out, but
for the heck of it:

On PC's that can't power firewire devices (such as laptops, even with
PCMCIA cards I'm told), would it be possible to cut up a couple USB
cables, plug them in to 1 or 2 or 3 or so USB ports and, using the
power supplied from there, perhaps joining the voltages in series, a
transformer here or there, I don't know, and finally splicing together
with the end of a FireWire cable, create a FireWire socket that has the
normal 4 pins of FireWire and the 2 powered pins deriving adequate
power from the USB port(s)?

I'm guessing that perhaps USB and FireWire voltages are all variable
and guest-circuit-controlled and not that simple, but it was an idea.
:-) Maybe a little outboard card with some simple circuitry could
accomplish it, or maybe the USB ports just don't have enough juice...

The point? The potentially small market of us who want total
portability with our FireWire powerable devices but have non-powered
firewire ports...



Re: Power FireWire using USB ports?

Quoted text here. Click to load it

I think USB has a 5V wire on it. The per port limit is 500mA.

Firewire on computers might use 12V and is allowed to use
higher voltages than that. Current is at least 1A and maybe
more. (I'd look it up, but I believe the IEEE1394 spec is one
that you have to pay for, to own.)

Even if you used a switching converter to convert the 5V to
12V or higher, to be able to provide decent levels of
current would require ganging too many USB ports together.

Be aware that, for some Firewire enclosures that are
bus powered, the enclosure uses a trick. It stores
energy in a capacitor, until it has enough to provide
the spinup current for the disk drive. You are much better
off with an enclosure that provides its own power, via a
wall wart, as no tricks are required to get things
spinning. No surprises on a cold morning.

I think the quickest way for you to get the portability you
desire, is with flash memory devices. With nothing to spin,
the current required can be quite a bit less. Max size
is still not the greatest, but they are geting there.

"Samsung's Solid State Disk (SSD) based on NAND Flash Memory"
http://www.mobilemag.com/content/100/102/C4048 /


Re: Power FireWire using USB ports?

On 18 Apr 2006 22:38:05 -0700, "casey"

Quoted text here. Click to load it

As Paul mentioned, USB is 5V and firewire 12V.  You can't
wire them in series, they use a common ground.  USB has
lower current too so the only real solution would be a
pass-through adapter that takes the firewire from
non-powered port, has a power input jack to which you
plugged in the power feed from an aux power source (like a
wall wart PSU or a battery pack), then output that dongle
with a powered firewire port to which the device plugs in.

Given some experience crafting similiar things the dongle
would be trivial to make but there is still that need for
the alternate power source.  More elaborate would be to tap
into the power on host computer for the 12V, either directly
from it's battery pack (which is usually over 12V so a
switching buck regulator would typically be used to attain
the 12V) or if it's onboard power board supported enough
additional 12V current, a tap could be taken off of it, but
it is doubtful that the onboard power board would have much
margin for higher powered firewire devices if it supplies
12V at all.

Re: Power FireWire using USB ports?

Kinda what I expected. :-)

Thanks for the info guys.

Site Timeline