Re: Any problem to have 2 sources powering the Raduino??

Jack, W8TEE


Are you sure about this:

First off, there is no 5v regulator on the Nano board.

The specs say it can be powered by:
    1) the USB port
    2) a 6-20V DC source (pin 30)
    3) a regulated 5V source (pin 27).

Jack, W8TEE

From: Jerry Gaffke via Groups.Io <jgaffke@...>
Sent: Thursday, December 7, 2017 2:15 PM
Subject: Re: [BITX20] Any problem to have 2 sources powering the Raduino??

I'm not so sure.

First off, there is no 5v regulator on the Nano board.  
The Nano's 5v rail is wired directly to the Raduino's LM7805, no protection diode there.
The Nano's 5v rail can also be powered from the USB connector through a protection diode.
 The 3.3v Si5351 is operating off that same 5v rail by way of a 3.3v regulator hidden inside the USB chip on the Nano.

The protection diode on the Nano is an SS1P31 schottky diode, according to this schematic:
Here's the diode datasheet:
Forward voltage drop through the diode is somewhere between 0.1 and 0.4 volts, depending on how much current and how hot it is.
The LM7805 datasheet is here (one of many manufacturers), output voltage could be as low as 4.75v.:

Let's assume we have Bitx40 power on while the Nano has a USB cable in place.
If the host computer supplies a bit more than 5v to the USB port, or if the LM7805 output voltage is near the low end of their spec,
we may be powering the Nano plus the Si5351 from the USB port.
But that's no worse than the situation when we have the Raduino unplugged from the Bitx.
More typically, we'd get 5v from the LM7805 to power the Nano and Si5351, no current would be coming from the USB host.

If we power down the Bitx40 and Raj's diode is not in place, we might have 100ma of 5v going backwards
through the LM7805 into the Bitx40, and perhaps powering up the receiver.  
But an extra 100ma through the protection diode probably won't fry anything.
However, if you somehow kick the Bitx40 into transmit mode, current could go much higher
and you may fry that protection diode.  

So a good idea to add Raj's diode.

Some have seen Nano's blown when powering the Raduino from the Bitx while plugged into a USB host.
We don't really know for sure what's on the $2 Nano clones, and we don't know exactly what fried on those Nanos.

So I'd add Raj's diode, and also shut down power to the Bitx when plugging in a USB cable to the Nano.
Not exactly sure why you would need to power down the Bitx, but then we don't know exactly what happend
in those few cases where the Nano got fried.  My guess is that Raj's diode would have prevented such trouble.

Jerry, KE7ER

On Thu, Dec 7, 2017 at 09:40 am, RCBoatGuy wrote:
When plugged into the Raduino board, the Nano is powered externally by a regulator on the Raduino board, which also has no protection diode.  So if you put 12V on the Nano Vin supply pin, you are shorting the outputs of the Nano's regulator to the Raduino's regulator, which in turn are connected via a diode to the USB supply.  The diode on the USB supply cannot provide adequate protection from the other supplies unless diodes exist on the other supplies, so when connecting the Nano via USB to a computer, the other supplies should be disconnected.

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