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Powering a 3.5" external usb hard drive with a firewire cable

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jms1989

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hey guys. just a theory. how feesable would it be to power a 3.5" usb hard drive from a firewire port on say a laptop? I understand it puts out 30 volts but I'm not sure. would you need to use a voltage regulator to drop it down to 12v for the hard drive? I understand that laptops may not be able to supply the full 30 volts and if it can, can it support such a task? I only ask for the possibility of using a large drive in a mobile environment.
 

tpetar

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Re: Powering a 3.5" external usb hard drive with a firewire cable

For powering USB device with higher demands, use power injector for USB see circuit example :

circuit-diagram.jpg

Lots of USB cables for external HD have one additional USB connector just for power (to get power from another USB port).

With this circuit as USB power injector, you can use your HD direct on some other device such as TV, CCTV recorders, ....



Firewire port have maximum 30V and can supply max 1,5A.

For higher current demands is USB 3.0. This can offer several amperes for various devices, but connectors are different from USB 2.0.

:wink:
 

jms1989

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I wouldn't need a usb injector for a 3.5" desktop drive... I'd just need to power it from the firewire port. what would I need to drop the 30V down to 12V? I'm sure a 7812 reg would need some help in that regard. The hard drive has a seperate 12v barrel power input. laptop size drives are no problem. I have a Y adaptor for them.
 

tpetar

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Re: Powering a 3.5" external usb hard drive with a firewire cable

I wouldn't need a usb injector for a 3.5" desktop drive... I'd just need to power it from the firewire port. what would I need to drop the 30V down to 12V? I'm sure a 7812 reg would need some help in that regard. The hard drive has a seperate 12v barrel power input. laptop size drives are no problem. I have a Y adaptor for them.
What current on 12V ?

For green HD not problem, but for some older....
 

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What I know FireWire is up to 1,5A of current. I dont know what will happen if you try this, maybe melting of connector plastic parts or damage of pins will occur. Maybe disk will not start under low power. I supose FireWire have current limiter.
 
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