Differing voltages with the same ground?

Question

I have a battery powered radio controlled aircraft. The aircraft runs at 16v.

I want to add a video transmitter that is built for 12v. I plan to use this regulator to supply the 12v.

+16v-------->|  D24V5F12  |+12v---------> Video
 GND---------|  V. Reg.   | GND---------- Trans

The 16v side also drives a camera and telemetry equipment built for 12-24v. That subsystem outputs a video signal. I have 2 questions..

I have this notion that subsystems running at different voltages must be isolated form one another, say, by a transformer. But I think the voltage regulator gets wired with a common ground, so it seems I am incorrect. Can subsystems running at differing voltages safely share a common ground?

Am I safe simply connecting the video out from a 16v subsystem to the video in of a transmitter running at 12v? The final wiring would be:

+16v-------->|  D24V5F12  |+12v--------->Video
 GND-------------------------------------Trans
Vsig------------------------------------>Vsig

Answer 1

Isolation is unlikely to be required and several systems sharing the same 0V is very common and I would say typical. Problems can occur though and usually the problems are related to what is called "ground bounce". This might be a problem in a lightweight aircraft because you will want to keep weight down as much as possible. This might mean that you are intentionally keeping the weight down by NOT using thick cables for 0V wiring and NOT using star-point wiring for 0V. Star point wiring takes each sub-systems 0V directly back to the battery/generator to avoid currents in some sub systems causing ground-bounce on other sub systems.

So you ought to do tests to ensure you are OK but, my gut tells me that you should be able to make it work OK. The video signal max voltage will need to be checked to ensure it doesn't exceed 12 volts of course and this is easily fixed if it is.

Another problem is the regulator which I'm sure you are aware of - it's drop-out voltage isn't that great despite what their marketing boys say and on lean and mean wiring without decent star points this could cause a problem as possibly would the 36 volt max input to the regulator.

Answer 2

A voltage is a "difference between two points". One of those points can be called "Ground" even if it's in a flying plane. Like the chassis of a car is hooked to the (usually) negative terminal of the battery: the ground connection is common to ALL electronic circuits.

Your concern about voltage levels stems from older 5V logic signals, going into 3.3V circuits. Some Arduinos aren't 5V tolerant, and the Beaglebone Black is 3.3V logic only.
In your case, you will have a 12V-powered video transmitter, getting video from a 16V-powered camera. The video signal is unlikely to reach 16V, but if you're really worried you have 2 options:

1. put some kind of attenuator between the camera and the transmitter, or
2. power the camera from 12V, too.