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USB Power Design Issue - Any one any ideas?!?

Hello,

I recently bought a new head unit for my car which has a USB port on
it, for the sole purpose of being able to read MP3's. So I foolishly
bought a 60Gb hard drive to go with it.

Set it all up, and it turns out the Headunit can not provide enough
power to spin the Harddrive up so......

What I want is more than just a simple 7805 regulator. Being a
harddrive I want it to be shut down properly each time I turn the head
unit off, because I dont want bad data/ head crashes onto my brand new
drive!

So, what I was thinking was some sort of switched 5v supply, switched
on by the 5v that the Head unit provides on the USB connection. The
limit being this 5V can only supply 500mA Max, so ideally I would only
want to draw around 250mA if possible.

The two options I have thought of were, have a 7805 connected
permenatly, and then use the 5V from the USB to "Switch" the regulated
5v on to the hard drive. (Thought about this method, was not too keen
on having the 7805 live all the time)

Or the second method, use the 5V from the USB to turn on the 12v to the
7805 regulator. However I'm totally stumped with this one., Thought
about a relay perhaps? Wanted to avoid this one though as I dont
possess any and thought maybe a relay would draw too much current from
the head unit?

Thanks in Advance

Ben
 
I

Ian Stirling

Jan 1, 1970
0
Hello,

I recently bought a new head unit for my car which has a USB port on
it, for the sole purpose of being able to read MP3's. So I foolishly
bought a 60Gb hard drive to go with it.

Set it all up, and it turns out the Headunit can not provide enough
power to spin the Harddrive up so......

What I want is more than just a simple 7805 regulator. Being a
harddrive I want it to be shut down properly each time I turn the head
unit off, because I dont want bad data/ head crashes onto my brand new
drive!
Or the second method, use the 5V from the USB to turn on the 12v to the
7805 regulator. However I'm totally stumped with this one., Thought
about a relay perhaps? Wanted to avoid this one though as I dont
possess any and thought maybe a relay would draw too much current from
the head unit?

It won't.
I would use a 5V relay, and you must put a 6.2V zener diode across the
coil, as the switching mechanism in the head end may not be designed for
inductive loads.

Something like 619-3007 from http://rswww.com/ .
5V coil, >1A.

On the 7805, as protection, you want before it a 1A fuse, 1 ohm 3W resistor,
then a 1n4007 diode, leading up to a 1000uF 25V capacitor, with a 100nf
capacitor across it. Then the 7805, with a similar set of capacitors on
the output.

Also, put the regulator next to the hard drive, and feed it a switched
12V line, rather than running 5V all the way to the drive.
 
R

Roger Hamlett

Jan 1, 1970
0
Ian Stirling said:
It won't.
I would use a 5V relay, and you must put a 6.2V zener diode across the
coil, as the switching mechanism in the head end may not be designed for
inductive loads.

Something like 619-3007 from http://rswww.com/ .
5V coil, >1A.

On the 7805, as protection, you want before it a 1A fuse, 1 ohm 3W
resistor,
then a 1n4007 diode, leading up to a 1000uF 25V capacitor, with a 100nf
capacitor across it. Then the 7805, with a similar set of capacitors on
the output.

Also, put the regulator next to the hard drive, and feed it a switched
12V line, rather than running 5V all the way to the drive.
Get a 12v switcher, that has a 'shutdown' input. Connect this to the USB
5v. Play it carefully, regarding trapping and suppression on all the lines
(the car, is a _nasty_ enviroment for spikes etc.).

Best Wishes
 
Roger said:
Get a 12v switcher, that has a 'shutdown' input. Connect this to the USB
5v. Play it carefully, regarding trapping and suppression on all the lines
(the car, is a _nasty_ enviroment for spikes etc.).

Best Wishes


Thank you all for the suggestions, one other idea thats hit me, is to
perhaps use the 5V to drive the base of a trransistor. Connecting the
emitter and collector to the 12v supply rail and ground, and drive the
7805 that way?!?
 
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