Re: [Paparazzi-devel] Reply to "Problems with USB Programmer (David Conger)"

[David Conger]

Hi Dave,

With magnification I could not see anything.

Good came from this. I now know that R20 damage or air-gap (I found  
several boards from the assembly company where R20 floated leaving a  
gap) will cause the Tiny2.11 not to be seen as a USB device. I had 4  
Tiny2.11 set aside with this issue I have no just repaired (cool).
NOTE: So if you have a board that won't come up as a USB device check  
R20 for damage or disconnect.

Question: Could over-voltage on the USB connection break the 1.5k R20  
resistor? Could a very high over-voltage damage T1? I'm looking in the  
area of R20 and see that T1 (BC807) is right there. I was finding 5v  
on that 3.3v rail by testing the voltage at the middle pin2 on IRV and  
IRH.

This is not pointing blame I will replace the whole assembly but would  
like to continue to understand the real source of the  5v. It seems  
there must be a short allowing 5v onto the 3.3v rail somewhere unseen  
or as you mention a "ground loop"...funny, to me "ground loop" is what  
I do when trying to take off not something to do with electronics. :)

Thank you everyone for the help. I'm going to just replace the board  
with instructions for this customer to measure all voltages at the USB  
connector very carefully. I remember someone recently posting about  
very high voltages. See here: http://lists.gnu.org/archive/html/paparazzi-devel/2009-05/msg00052.html

Best Regards,
David Conger

On Jul 19, 2009, at 12:49 PM, David Carlson wrote:

> David,
>
> Most likely problem is that the 5V bus is shorted to the 3.3V bus.   
> I'd recommend grabbing a magnifying glass and see if you can find  
> any obvious shorts.
>
> Another possibility is a ground loop.  Make sure the 3.3V regulator  
> is properly grounded.
>
> Besides those, I would be out of ideas.
>
> Buzz
>
> David Conger wrote:
> > Hello Chris,
> > Thank you for the information. I am unable to check the PCB track  
> > unless there is a very easy way to manually check.
> > I tried replacing the 3.3v regulator but still I get 5v at the 3.3v  
> > end.
> > I'm wondering if maybe 5v comes backwards to the 3.3v end of the  
> > 3.3v regulator somehow via short?
> > -David
> > On Jul 19, 2009, at 9:14 AM, chris wrote:
> >
> > > David the pcb tracks are damaged or just the 3.3 volt regulator?
> > > The only way to burn the 3.3v regulator is by heat caused by  
> > > excessive current draw since the input voltage is fixed by the 5v  
> > > regulator i guess.
> > > Another possible cause is the accidental connection of the 3.3v  
> > > rail with the 5v rail.
> > > Can you check for a broken pcb track from excessive current?
> > > Chris
> > >
> > >
> > >
> > >
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