Common ground on different voltage

[triise]

Hi,

Anyone who's good at electronics care to shed some light on this:
I have a quad which runs on 6S LiPos (25V), and my thermal camera on the gimbal is requiring 12V (+/- 3 V). I run the video input to the OSD (which may use 3S-6S LiPo) and power the OSD from the same battery as the FLIR (4S). The OSD outputs a the video signal to the video TX (which can take 3S-6S).
I've discovered that the Video gnd and the battery ground on the OSD is connected.

The big question is now: What happens if I power the OSD from my 6S LiPo instead of the 4S LiPo? Then the Battery ground of the 6S vil be connected to the 4S ground / FLIR ground. Is this a problem?

The the attached image for schematics.

 

[ChopperAddict]

Hi

I am not an electrician, but I do know that feeding 2 different positive voltages into a single unit via different input ports is NOT a good idea. Common ground is fine (AFAIK), so each module can ground whatever voltage it is using to the common ground, but a single module should not have 2 differing incoming voltages, unless it is documented as being able to accept such a configuration and electrically insulates each voltage form the other internally.

I hope others will join in this, and possibly confirm my thoughts/ramblings ?
Ian

 

[Cheshirecat]

That would not be a problem, in fact creating a common ground would reduce any electrical noise in the power supply.
But why not save weight by using a DC-DC converter to drop the 6S voltage to a level for the FLIR camera? one less battery to charge as well.

 

[triise]

That would not be a problem, in fact creating a common ground would reduce any electrical noise in the power supply.
But why not save weight by using a DC-DC converter to drop the 6S voltage to a level for the FLIR camera? one less battery to charge as well.

Right now I use a 1000 mah battery for the FLIR which weighs 70 grams, do you have some tips on lightweight DC-DC converters? I've looked into it and the ones I've found were actually a bit more tha 70 grams. Any tips will be very appreciated as one battery less to charge is one less thing to think about

 

[triise]

And btw, thanks for the answers. I'll be hooking up this tomorrow and give it a try!

 

[gtranquilla]

Be careful.... for a simple concept... grounding can still be very confusing.
When a common return is connected to the earth, it is a true earth grounded system.
In an aircraft there is no such thing as an earth grounded system but a common return is still necessary.
Referring to the common as a ground makes the term more ambiguous.
In the case of industrial input sensors and some output devices, the return paths often have to be isolated from each other and isolated from ground or the I/O circuits will short circuit.
You need to go inside each device schematic to see how each circuit is commoned, isolated or grounded to determine if all can be commoned together.

Hi,

Anyone who's good at electronics care to shed some light on this:
I have a quad which runs on 6S LiPos (25V), and my thermal camera on the gimbal is requiring 12V (+/- 3 V). I run the video input to the OSD (which may use 3S-6S LiPo) and power the OSD from the same battery as the FLIR (4S). The OSD outputs a the video signal to the video TX (which can take 3S-6S).
I've discovered that the Video gnd and the battery ground on the OSD is connected.

The big question is now: What happens if I power the OSD from my 6S LiPo instead of the 4S LiPo? Then the Battery ground of the 6S vil be connected to the 4S ground / FLIR ground. Is this a problem?

The the attached image for schematics.

 

[triise]

Did hook it up today (with some higher pulse) and it worked like a charm! Anyway, I did talk to the tech guys at my workplace (CIS Engineers) and they drew and discussed for some time and concluded that it shouldn't be a problem. Found a dc-dc regulator at foxtech which I have ordered and will connect, as the simplicity of one battery actually is a big plus.

Anyway, I don't say that this common ground question and my solution is valid for all situations (quote from the tech guys) but in my case it was no problem with my setup.