Firewire bus power + mobile RAID 0 drive = concern!

[rogacasa]

Hi. I'm really interested in this mobile drive, which is actually a
RAID 0 configuration, from G-Technology:

http://www.g-technology.com/Products/G-RAID-mini.cfm

I really like this idea, but I'm a little worried about running it on
FireWire bus power. I'm not worried about battery drainage -- I'm
worried about the current draw frying my FireWire 800 port. This
little device has two 7200RPM 2.5" SATA drives in it, PLUS a fan. It
can be bus powered, which I like, and that is why I am interested in
it. But doesn't 2 drives and a fan seem like a little much to be
trying to run off bus power?

The G-technology website does not say what the current draw of this
drive is.

Does anyone know the maximum allowable current draw for the Firewire
800 spec?

Thanks.
Roger Carlson

 

[David Empson]

Hi. I'm really interested in this mobile drive, which is actually a
RAID 0 configuration, from G-Technology:

http://www.g-technology.com/Products/G-RAID-mini.cfm

I really like this idea, but I'm a little worried about running it on
FireWire bus power. I'm not worried about battery drainage -- I'm
worried about the current draw frying my FireWire 800 port. This
little device has two 7200RPM 2.5" SATA drives in it, PLUS a fan. It
can be bus powered, which I like, and that is why I am interested in
it. But doesn't 2 drives and a fan seem like a little much to be
trying to run off bus power?

The G-technology website does not say what the current draw of this
drive is.

Does anyone know the maximum allowable current draw for the Firewire
800 spec?

According to <http://en.wikipedia.org/wiki/Firewire>, the maximum power
which can be carried over a Firewire cable is about 45 watts at 30
volts.

The actual wattage and voltage will vary depending on the computer or
other power sources connected to the bus, and the current would vary
accordingly.

Apple's developer notes for each Mac model specify the power they can
supply via the Firewire ports, and it is typically in the order of 6 to
8 watts per port, and I expect the voltage is typically in the order of
12 to 24V (it may be unregulated). Some models may have some
flexibility, e.g. if you have three Firewire ports you might be able to
get away with one power hungry device (in the order of 18W) as long as
you don't try to draw much current from the other two ports.

There is likely to be a current limiting device in the Mac which will
switch off the Firewire power output if the device draws too much power.

You might be able to get a rough idea of the current consumption of the
device by looking for specifications for 7200 RPM 2.5" SATA drives and
seeing what their maximum (turn-on) surge current is on the 12V rail,
combined with their 5V current, then allow some headroom for each
figure. To get a power figure, multiply the voltage and current and then
add them together.

Converting that to an estimate of power drawn from Firewire is
difficult, as it depends on the efficiency of the device's internal
power supply. Assume 50% for argument's sake, i.e. double the wattage
figure from the calculation above.

 

[rogacasa]

For the record, G-Technology makes a power adapter for this drive, so
that it can in fact run of AC power if you want it to. Personally,
I'm trying to avoid using an AC adapter, which led me to post my
original question.


Roger Carlson

 

[Eric Gisin]

Firewire has no minimum power, max is around 40W.
Typical desktop power is 1.5A @ 12V, notebooks probably less.

All ports have self-healing fuses, you can't damage them.
Mac users often plug them in backwards, blowing the transceiver.

 

[Folkert Rienstra]

According to <http://en.wikipedia.org/wiki/Firewire>, the maximum power
which can be carried over a Firewire cable is about 45 watts at 30 volts.

The actual wattage and voltage will vary depending on the computer or

other power sources connected to the bus,

And let the fireworks begin.

and the current would vary accordingly.

Apple's developer notes for each Mac model specify the power they can
supply via the Firewire ports, and it is typically in the order of 6 to
8 watts per port, and I expect the voltage is typically in the order of
12 to 24V (it may be unregulated).
Some models may have some flexibility, e.g. if you have three Firewire
ports you might be able to get away with one power hungry device (in
the order of 18W) as long as you don't try to draw much current from
the other two ports.

There is likely to be a current limiting device in the Mac which will
switch off the Firewire power output if the device draws too much power.

You might be able to get a rough idea of the current consumption of the
device by looking for specifications for 7200 RPM

2.5" SATA drives and seeing what their maximum (turn-on) surge current is

on the 12V rail,

Happy hunting.

combined with their 5V current, then allow some headroom for each figure. To
get a power figure, multiply the voltage and current and then add them together.

Yeah, right.

Converting that to an estimate of power drawn from Firewire is difficult,
as it depends on the efficiency of the device's internal power supply.

Well, it is not very likely to be an inverter, is it.

Assume 50% for argument's sake, i.e. double the wattage figure from the
calculation above.

Quite some imagination you have.