Yes, a shorted 5v rail on the Raduino would prevent the Nano from powering up from the USB port:
? ??
I assume the Nano clones are similar.
Not obvious what's going on there.? Here's more than you want to know:
Power from the USB port at J3 comes in through fuse F1 to node VUSB and then through schottky diode D1 to drive the 5v rail.
That 5v rail goes to J2 pin 4, which is the same as U1 pin 12 of the Raduino schematic, which is tied to the output of the LM7805 regulator.
So if the LM7805 is working, it powers the Nano through J2 pin 4,? and diode D1 prevents that power from going back up into the USB port.
If the USB port is powered, power can go through D1 to the Nano's 5v rail.
The FT232RL USB interface chip on the Nano (clones use the cheaper CH340 chip) includes a 5v to 3.3v linear regulator to power internal logic,
that 3.3v is available from IC1 pin 17 to power other stuff.? On the Nano, that 3.3v node only goes to J2 pin 14, which is U1 pin 2
on the Raduino, and powers the Si5351 chip on the Raduino.? So no need to include a 3.3v regulator on the Raduino.
If the 5v rail on the Raduino is shorted to ground, then the output of D1 on the Nano is shorted to ground and no logic
on the Nano gets any 5v for power.? However much current the USB host cares to feed us is pouring through F1 and D1 to ground,
this might fry F1 and/or D1, though probably nothing else.
Incidentally, if the Raduino is hooked up to an unpowered Bitx while USB power is supplied,
that 5v can go backwards through the LM7805 and power the Bitx (maybe enough to make the receiver sort of work) from the USB port.
I'd recommend a 1n4007 (or any 1n400* diode) between the Bitx's 12v rail and the LM7805 to prevent this, though have heard of no ill effects yet.
In addition to that 1n4007, perhaps add a 25 ohm 1/2 watt resistor in series to further drop that 12v so the LM7805 doesn't get so hot.
And add caps where the 12v leaves the Bitx and where it enters the Raduino to avoid tuning clicks from the LCD from getting into the receiver.
?
Jerry, KE7ER
toggle quoted message
Show quoted text
On Fri, Mar 2, 2018 at 10:00 am, Michael Hagen wrote:
If the LCD backlight is bright on USB (you got near 5V) then look for an open regulator.
If the regulator was shorted, it would kill the USB port on your computer?? They parallel up on Raduino?
Someone else this week had a bad 7805, hope that is not a trend?
