How can I detect which DC power source to send to an underwater camera housing?

Question

I have a small problem and hoping someone can help:

I am trying to design and build a reasonably universal underwater housing for some different cameras. there is a breakout box on the surface and the housing is cabled to this box for things like a video feed, ethernet control of the camera etc.

My current problem is power. As well as onboard batteries, I also want to feed power to the camera from larger DC block batteries located on the surface. There are two inputs on the breakout box, one accepts 12 V DC power + and - from a block battery and one accepts 24 V DC power + and - from different type of block battery. Inside the camera housing underwater, there are two power connectors (one 12 V with + and - terminals and one 24 V with + and - terminals) as I would like to support the use of both 12 V and a 24 V camera. These power connectors are different styles to avoid connecting a 12 V camera to a 24 V supply!

I only have three wires left on my underwater cable to carry power so can't just hard wire the 12 V input to the 12 V output and the same with the 24 V input/outputs as this would require four cables (this is an assumption on my part: You can't have 24 V positive going down cable 1 with 12 V positive down cable 2 and have them share the same negative cable 3. Please correct me if I'm wrong!).

I will only need one of these sources depending on whether a 12 V or 24 V camera is being used.

To solve this problem and make it the least amount of work for the user possible, I have the following solution:

inside the surface breakout box have a circuit that detects which power input has a battery plugged into it. If it's the 12 V battery, use cable 1 to the housing as positive and cable 3 as negative. If it's the 24 V volt battery, use cable 2 as positive and cable 3 as negative. If the user has plugged both a 24 V and 12 V battery, it should favour the 24 V and use the logic above but isolate the 12 V battery to prevent it overloading.

Down at the housing end, another circuit can detect whether positive current is coming down cable 1 (this would be the 12 V battery according to the logic above) or cable 2 (the 24 V). Based on this detection, it would the connect the power to either the 24 V or the 12 V outputs to power the camera.

I don't want to get into voltage measurement as 24 V and 12 V are expressed as simplifications. Actual voltages could vary (but would remain within camera tolerances). Also I wouldn't want to smooth voltage to a consistent level as the video feed from the camera includes an overlay that expresses voltage of the input power source that people use as a gauge of when to change the battery.

My questions are:

a) is this possible?

b) is there a better solution to my problem?

c) is it possible to just have voltage convertors at the camera housing end? (e.g. one that would take 10-30 V and convert to 12 V and one that would take 10-30 V and convert to 24 V). Overheating is a consideration for a camera in an enclosed environment but if this were a potential solution, I would accept voltage smoothing, contrary to what I mentioned above.

Answer 1

You can absolutely share the same ground for as many voltages as you like.

Current flows to whatever loop is closed, so as long as you don't connect both positive and negative together the batteries won't even know about each other. That would be the easiest solution:

^{simulate this circuit – Schematic created using CircuitLab}

R1 sees only 12V, R2 sees only 24V and neither battery can ever see the other.

And you can also just connect them like this, same idea:

^{simulate this circuit}

Just make sure the load doesn't connect between the 12V and 24V, you'll get 12V, but it will run through both batteries and cause trouble when one is empty before the other.

Answer 2

If I understand this question correctly, the simplest solution would be to combine the 12/24 at the surface with two diodes and then have two DC-DC converters at the camera end. For example, these with 9-30V input and 12 or 24V output at 10W. That may not be enough for you, there are plenty of other options, just search a distributor such as Digikey for DC-DC converters.

That way you have only a single cable, the higher input voltage dominates automagically, and there is no danger of damage, in fact you wouldn't even need different connectors topside.

That give you both 12 and 24V regulated inside the camera housing - what you do with those is up to you.

If you wanted redundancy you could stay with two cables, put 10-30 on either or both (they would be the same as each other) and combine the voltages with two Schottky diodes inside the camera housing, then feed that to the two DC-DC converters to get 12 and 24V simultaneously.