As a continuation of this question: http://tinyclr.com/forum/1/2763/ , I’m wondering if it’s safe to power my FEZ via the 5V pin while it’s connected to the computer over USB?
No it is not.
What board do you use?
For example if you look at FEZ Mini pin-out:
http://www.tinyclr.com/downloads/Mini/Broch_FEZ_Mini.pdf
5v on mini is “output” pin when powered from USB.
I can’t see why not. Its the same as powering the FEZ via the power adapter… and the connecting to a computer via the USB, I’m sure there is a diode in between the rails…
Cheers Ian
Sorry I didn’t reply earlier, wanted to check the Panda schematics and put it online before giving false answers. This will have to wait till tomorrow.
I have done it without smoke on panda and domino. Just thinking, maybe this ~could increase amps over limits - yes/no? The LM1117-3.3 regulator can take 4.75-10V on 5VIN, but would guess you have to concider anything else in the way and trace widths and copper ounce. As Fez folklore, what is the copper ounce used anyway?
Thanks Gus. Better the right answer than a guess 
Confused.
So on Mini when it is powered via USB 5v pin is not an output pin? If it is and there is a diode, what is the purpose of supplying power to the pin?
Panda II.
Seem to have a flurry of 5v questions. Not sure if talking about same thing. But looks to me the 5V is IN/Out also. I see a diode protecting the USB only. So question is, if you plug another 5V into 5V what happens? The 5V bus volts is still 5V right? You just have potentially more amps available. But as the 3v reg will only draw so much amps, this would still be same as it no addition supply - no?
Right. The situation is, that I need 12V and 5V power in my system (12V to the stepper motor, 5V to the FEZ). These two voltage sources need to have a common ground for the stepper driver (EasyDriver) to work. Because of this, I’m using a converted ATX power supply to provide those two voltages. The computer’s USB port, while perfectly capable of powering the FEZ, does not share a common ground with the ATX power supply that is providing the 12V. During development, however, it would be mighty convenient if I could safely have the USB connected while powering the board and motor in this manner.
Don’t see the diagram of the Panda II. I have done that before with panda and domino. Also. IIRC, the gnds just need to be *connected, but not required to be same gnd source. So you should be able to do it with the usb powering the logic/fez. I am almost sure I did that same thing with both the easystepper and also the pololu version.
Fritzed the same kinda deal with another motor driver here.
[url]http://staceyw1.wordpress.com/2011/03/14/fritzingyour-virtual-breadboard/[/url]
The fez powered by 5v usb. 12v rail powers HighV side. Connected gnds. Fez 3v powers the logic rail.
[edit] Note the gnds are all connected on most of these boards. Maybe not the easystepper. In case one can not see connected gnds here. Also, toss out any of those tiny SF “molded” jumper wires - They are junk. Get the premium ones with the normal sqr header connectors. I had a bad gnd wire that took me a day to figure out what the heck was going on.
On the Panda1 it is safe, I do it all the time. Have not seen the schematic for the Panda2, but I must assume they also use a diode.
The Diode between the USB 5V and the Panda 5V prevents you from forcing power into the USB port. Also, as the diode has a 0.6V voltage drop, it gives 4.4V on the 5V rail when powered by USB.
If you then power the board from 5V too then the "OR"ing function of the diode selectes the higher voltage and thus powers your board purely from the external 5V supply.
My robot can run from battery, hooked up to the 5V In/Out pin, but even if the bot is switched off the panda will still run when I plug it into USB. My Bot then shows that it is using 0mA from the battery(because the battery is switched off). But, still connected to USB, if I switch the bot on then it reports that 200mA to 300mA is being drawn from the battery. This does not change when I plug the USB in/out…
Also, if your 5V supply connected to the 5V in/out falls below 4.4V then the Panda will seamlessly switch back to powering from USB…
So here you go http://www.ghielectronics.com/downloads/FEZ_Panda_II/FEZ_Panda_II_sch.pdf
There is a protection diode on 5V regulator. You will be fine connecting 5V source to the 5V pin
Perfect, thanks for all the help!
Just popped smoke from my Mini 
Supply ~8V on power pin, without USB plugged in; popped smoke. USB + Power had been supplied just prior to that though…
Is there seriously no diode preventing this? I’ve never seen a piece of programmable hardware that can’t be powered independently and plugged into its designed programmer at the same time…
What pin did you put power on? 5v in?
Yes; the one labeled
5V out USB-powered
5-9V in
looked like one of the 10uF SMT caps was cooked so i replaced that; power applied very briefly after replacement (via USB) yielded a very very hot processor within moments. Nothing else on the board looks burnt but my only other thought is the 3.3v vreg.
Heh, I’m more pissed at myself because I designed the power distribution block for this project specifically so i could remove power from the Mini easily when i wanted to drop code because i was worried about this. Then I told myself “Meh - who would a design a board that cant be programmed while its externally powered”. Oops 
I’ve never seen anything like this on a USB programmable board… theres always a layer of protection (ie: at least a diode) to act as a latch and prevent this from happening…
Did you apply 8V AC to the 5V rail?
What in the world runs at 8V AC? :o
No; ~8.4V max from a LiPo battery.
From the diagram, it looks like there is a diode.