| As I progress through
this little project ( mostly in my spare time at work ) I must remind you
that I really have no idea if this will work. A standard USB hub usually has
no issues in a mobile environment. You plug it into the PC, laptop, tablet
what have you, and then plug the peripherals, i.e. mouse, camera, flash
card, HD drive, DVD rom whatever, into that and everything works. |
| My problem is the combined
peripherals draw far in excess of the 500mA of the port on the PC. The
"Retail Plus (tm)" mini hub I have purchased has a small plug on it for
external power, as most hubs do these days. This normally plugs in from a
120VAC wall adapter, not to good in a mobile environment. (Never could
figure that one out!) I can create the 5
volts required from the 12volt mains, but problem is the thing continues
running even when the PC is off. I could put a switch on it, but I'd
probably forget, or get lazy and leave it on.
The solution is to have a circuit that senses
when the PC is on, turns on the power to the hub via a regulator. |
 |
| This way the
peripherals all get their power and are online by the time windows starts up
to be detected and loaded.
Circuit description:
The end pin on the USB from the PC has 5 volts when PC is on. This switches
the 2SC3904 transistor to ground which in turn turns on the larger 2SA1012
PNP. The 180 ohm resistor is just enough to keep the voltage up for my
application ( around 2 amps max total draw ) and could be changed slightly.
The .1 uF caps are just for RF protection, a good habit! I have LEDs on all
outputs maily for trouble shooting. A fusable link is feeding the whole
thing at the 12 volt side ( not shown ) The caps around the regulators are
absolutely necessary, and must be right at the pins of the regulators. (
Even though the designers at Cobra don't seem to know this! ) This prevents
oscillation.
My Edirol firewire audio interface can only use
4 pins on the firewire ( PC has only 4 ) so this can supply it's required 9
volts as a bonus! I have managed to stuff it all in a tiny project box
purchased from Digikey. They have some great little boxes. I'd recommend
these guys to anyone out there, especially in Canada as their shipping is
next day.
Anyway, I hope it all works. Wish me
luck!
| Well, the Edirol
power was sans problems with only a slight warming of the regulator. The
5 volts powering the hub was a different story. The amount of power on a
single USB plug is max 500 mA. It is unbelievable how much power a
portable HD or DVD draws! Now I see why they come with an extra power
cord to plug into a second USB socket, they each both surge ( when
working hard ) to nearly 1 amp! |
| The poor little regulator for 5
volts in my poor little plastic box immediately began to fry
like bacon! I was so disappointed and pondered around for the
rest of the afternoon. It had to be rebuilt, or become a
coffee warmer.
Heat is always a concern because
if it is hot at ambient room temperature, how hot will it be
in 35ºC to 40ºC equatorial temperatures?
I added in a 2N3055 transistor,
upped the reg. voltage by .5 volts with a rectifier to account
for the .5 to .6 volt drop in the 3055's B-E junction and all
is well. ( Diagram to the right ) |
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The photo to the left shows
the final outcome. Because I had heavily glued the USB sockets ( nice,
gold contacts! ) into the plastic face panel of the box, it seemed more
logical to cut a hole for the entire recessed face from the aluminum.
A higher power transistor is now driving
the regulated current at a possible 10 amp surge and is well heat sunk.
With both the DVD and hard drive as well as
a mouse plugged in, it warms up over time, but not enough to cause any
problems. I will be using an Evolution U-Control midi-USB controller
board on it fairly regularly while doing music, but only the mouse on
the face panel usually.
Still haven't resolved the external fire
wire issue. The 4pin-4pin cords are $40 here. No way I'm paying that
much to "try" it. May just get a fire wire hub instead. (OH NO!!!)
|
May 20th 2007 |
