Common ground on different voltage
 

Hi all!

Got an electrifying question for the experts on this subject. I'm after detailed theory learning for future setups so I can take ground issues into consideration. I got the need to know how my things work, not just that they do... It's a curse.

Example
I power the RX from an ESC internal bec, since it can only deal with 6V.
I power the steering servo from an external CC BEC 10A at 7.4V since I want to use its full potential.

What I'll do
My first thought was to just take the signal wire from the servo to the RX. That's it.

But wait!
Then I learned about ground loops. If there is no ground back to the RX for the signal voltage, it will take the path through the BEC, ESC, until it gets back to the RX. Which I guess would work, but be a very noisy path since the ESC throttle signal is in the same path. After a lot of googling I have found that people solve this issue by taking ground from servo back to BOTH BEC AND RX with an extra wire, split or whatever. Some receivers don't need this, others do. Either way it should lessen noise in the system.


I need to understand this:
What I'm having a difficult time wrapping my head around is how the electric traffic is handled. 7.4V comes into the servo from BEC, so I assume the ground carries a similar negative charge back out through ground from the servo - and that the 6V signal from RX travels in the same stream. How does this effect eachother? In details please!

This leads me to wonder if I split the ground from servo - won't the RX be hit with 7.4V negative charge as well as the signal? Will the RX be required to handle 7.4V current, or does it not work that way? I'm a bit unclear on how negative charge acts opposed to positive... And honestly feel kinda dumb about it. So any clarification is greatly appreciated.

A ground does not "carry" the voltage. In a sense, it's what creates the resistance to operate the electronics. A common ground from the same power source can operate at different voltages without harming them. I'm no expert by any stretch of the imagination but have work on equipment with voltages ranging from 1.5 to 24 volts, all coming from the same power source. I do know that a well established ground is important to prevent voltage feed back. Hope this helps a bit....

In my converted computer power supply for my charger there is a 5v circuit and a 12v circuit both sharing the same ground block and it works like a charm

The ground doesn’t carry the different voltages, but it does carry the return currents. This can cause slight voltage differences in ground at various points, but is generally not a problem for well designed components.

The RC servo signal is a fairly high voltage pulse where the duration/width is important and is typically reasonably immune to slight ground voltage variation/noise.

It’s considered poor practice to have any ground loops, they can cause issues. The grounds should either be connected daisy chain style, the normal RC way, battery to esc to receiver to servo, but not servo back to battery which creates a ground loop. Or the star method, from battery to each component without any interconnecting grounds.

If the typical daisy chain style doesn’t provide the servo enough power, perhaps try running a ground to it directly from the BEC, but remove the receiver to servo ground.

Thanks for inputs.

In my example the BEC -/+ goes directly to the servo (star method), as it powers it, sorry if it was unclear. With only the signal wire going to the receiver. But this means the return current for the signal has to travel a long and noisy path to get back to the receiver. My research indicates some receivers, especially Spektrum based ones, can't handle this. I believe it also disturbs the ESC by creating a potential difference on its way.

If I choose to Y-split the Gnd from servo to both BEC and receiver, I'll make a ground loop if I'm not mistaken, not favorable as noise will occur around it and voltage references would be all over the place if I'm right.


Am I wrong in concluding that running a bec directly to servo outside receiver in a star style will be inferior either way, opposed to running bec directly to a receiver that can handle the same voltage? I believe you called it daisy style :) At least no return currents will end up having to go through the noisy ESC mains that supplies the motor that way.