Hi dear FPV users!

I had no luck recently and I have now 2 new AIO flight controllers that are broken even before the maiden flight! And with different electronics.

I go baby steps, testing resistance and electronic after each solder. The one I’m trying to fix is a GePRC F411 35A AIO.

The 5v bec died after adding the buzzer and there was no short! It worked for couple of seconds and the FC started to reboot up to a point where it failed. I measured just over 1v at 5v pad. Since it is working from USB port, I added an external micro 5v bec and it’s now working (no buzzer.. I’m afraid to break it again).
The external bec is connected to the BAT pad and the output to a 5v pad. When the output is connected to the 5v pad, the USB port will not work and the bec gets warm!

Am I doing something wrong or do I have to disconnect external bec each time I connect through USB? Is it ok to connect the bec input to a BAT pad or will it take too much amp?

Thanks

There is usually a diode that prevent a back flow of current. I wonder if you are shorting out the USB when you connect your BEC to 5V. The 5V is meant to be an output not input.

(25-Jul-2023, 05:36 AM)voodoo614 Wrote: There is usually a diode that prevent a back flow of current. I wonder if you are shorting out the USB when you connect your BEC to 5V. The 5V is meant to be an output not input.

You mean a diode on the external BEC or on the FC?  The USB will always output 5v to the 5v PADs, so the BEC will get 5v from its output lead, so I'm wondering how it could work without a diode on the BEC.

I also noticed that this board is outputting voltage to the batt pad with 4v when USB is connected. I could see that my Vista is getting voltage.

Speaking generic electronics here, not something specific on this particular problem.

When a diode fails generally it will be a short. Hence no longer blocking voltage from the wrong direction.

Also when you see a 5v supply drop to 1v, you're device is pulling too much current or there is a short or near short going on somewhere.

(25-Jul-2023, 04:04 PM)MajorFPV Wrote: You mean a diode on the external BEC or on the FC?  The USB will always output 5v to the 5v PADs, so the BEC will get 5v from its output lead, so I'm wondering how it could work without a diode on the BEC.

I also noticed that this board is outputting voltage to the batt pad with 4v when USB is connected. I could see that my Vista is getting voltage.

Generally, there are two diodes to prevent back flow. One to the USB port and one the on-board 5V regulator. But since you tapping into the 5v rail via the 5V output pad without a diode. 5v from the USB is back flowing into your BEC. This is probably the reason the BEC gets hot. As for why the USB does not work, the wiring might be causing some kind of short circuit and preventing the USB from working.

If you put a schottky between the BEC and the 5V pad, there is a chance you might solve the problem. But without understanding how your FC shorted in the first place, it is a risk you take.

For visual context, below is a basic diagram showing how a Schottky diode is usually designed on an FC circuit to prevent voltage from the 5V regulated LiPo feed being sent back down the wire to the USB Port. It can only travel forwards through the output and not back out through either of the inputs, unless of course the Schottky diode has an internal short. Either a Schottky diode IC package can be used, or two individual Schottky diodes with their output legs connected together to create the same relative circuit as a single IC package.

Thanks for your input  

From your diagram, I think the diode between 5v bec and USB port is either defective or missing by design.  This is a popular board since it's in the Smart35 but I have it for a custom build.

So I will either add a diode between bec and 5v pad or use a micro connector.  Everything works fine if USB is not connected and all components are cold so I'm not too concerned by a short.  Also if I disconnect the bec 5v lead and connect USB and lipo I am able to configure all Motors / ESC.

In the picture we see the 2 input leads from batt/ground pads and the pad where I connect the 5v output lead (not connected in this picture)