FPV-Japan.com

If you haven’t yet read up on ground loops or multiple return current paths please do so.
Also please note that there is nothing wrong with the design of current sensors, OSD/UAV modules or indeed ESC with onboard BEC (even though the idea of a BEC on an ESC is poorly though out). ESC with on board BEC where never designed with FPV in mind, or rather, as engineers we would certainly would like to think so!
The problem we will cover here in detail is a simple issue and one that should be obvious and definitely avoidable.

Lets take a look at the ESC/BEC ground loop or more accurately “multiple high current return path” issue.
Fixes to follow shortly.

If you’re wondering just how much power can pass through such a ground loop then several factors must be understood. First the thickness and in turn, resistance of the main power wires and then the ESC servo wire thickness. Then the overall length in relation to each other.
A typical system peaking at 30A will push roughly 27Amps through the main ESC to current sensor route while 3Amps “escapes” down the ESC servo wire, through your RX ground plane, through your OSD ground plane and then back up the current sensor black wire where it meets thin PCB track to final GND junction.

Ouch.

The easiest way to avoid this risk is to use a dedicated BEC for system power.
But care is required even with BEC and where to tap into the main power system.

– more details to follow –
topics to cover include – how to safely use an ESC/BEC for dedicated subsystem power
why we prefer linear power supplies for sensitive and important low current equipment
long cable runs and supply voltage losses at servos