Running Arduino off internal USB

Hi All. Haven't seen the answer anywhere else, so I hope I'm not posting an oft repeated stupid question.

I want to build a binary clock into a PC drive bay. I also want it powered via USB.

I intend to use this parts kit. and some strip board.

I will be using the circuit from the playground shift out example, with 4 LEDs for hours and 6 for minutes, running from two shift registers. Perhaps 3 if I get carried away,and add seconds. And acouple of buttons to set the time. Everything works fine when my Duemilanove is powered from USB.

I want the Arduino and clock powered on all the time, which the USB connections can handle as I want. I think... And I have a few spare internal sockets on my motherboard.

Problem. USB provides 5 volts. But after going through a 7805, it will be way too low to power anything. I tried just now with the same regulator, and only got 2.something with no load.

Would it be safe to bypass the regulator, and power the Arduino and all directly from the 5 volt USB connection,with a capacitor to smooth out any bumps? And would the USB supply be smooth enough for the job? Especially during boot?

Or would this be a really really stupid idea that may damage my PC and my Arduino?

Any advice would be greatly appreciated.

You have also 12V in your PC and Arduino will accept it (OK the 7805 will be a bit warmer). See specs

USB provides 5 volts. But after going through a 7805, it will be way too low to power anything.

Not really sure why you would need to go through another regulator. If you are connecting to an Internal USB header, it adheres to the same specifications as the ports on the outside of your computer. So it is just as "safe" to use an internal USB as an external USB.

Along those lines, there really isn't a need for additional filtering.

Sadly, the 12volt power goes off when the computer is turned off. The USB power on the other hand, stays on unless the PC is unplugged or the PSU is switched off.
My aim is to have a clock that stays on all the time. Worst case, I can run a cable in from a wall wart, and power it that way, but the USB idea seems so much more elegant.

Hi James. Sorry. I didn't explain very well. What I'm trying to do is run a strip board Arduino, a pair of shift registers and 10 LEDs from the always on USB power of my PC.

The only regulator would be the one built into the Arduino. The problem with that, is the voltage loss from the regulator. On further advice, I'm kind of going off the idea, and will most likely use a wall wart plugged into an expansion slot, and power it independently of the PC. Much safer I think.

Thanks Richard. This is what I wanted to know. I'll take your advice, and make it independent of the PC entirely. I have a nice 7 volt wart that should fit the bill. I've finally got the code running well, and I already have an RTC chip and matching crystal, so it shouldn't be too hard to modify it to use that instead of relying on the Arduino clock.

Accuracy isn't that big a deal with a binary clock, but not needing to reset it would be a definite plus.

Looks like I have a nice busy Christmas ahead of me. :)