1
\$\begingroup\$

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 12v DC power + and - from a block battery and one accepts 24v DC power + and - from different type of block battery. Inside the camera housing underwater, there are two power connectors (one 12v with + and - terminals and one 24v with + and - terminals) as I would like to support the use of both 12v and a 24v camera. These power connectors are different styles to avoid connecting a 12v camera to a 24v supply!

I only have three wires left on my underwater cable to carry power so can't just hard wire the 12v input to the 12v output and the same with the 24v input/outputs as this would require four cables (this is an assumption on my part: You can't have 24v positive going down cable 1 with 12v 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 12v or 24v 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 12v battery, use cable 1 to the housing as positive and cable 3 as negative. If it's the 24v volt battery, use cable 2 as positive and cable 3 as negative. If the user has plugged both a 24v and 12v battery, it should favour the 24v and use the logic above but isolate the 12v 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 12v battery according to the logic above) or cable 2 (the 24v). Based on this detection, it would the connect the power to either the 24v or the 12v outputs to power the camera.

I don't want to get into voltage measurement as 24v and 12v 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? (eg one that would take 10-30v and convert to 12v and one that would take 10-30v and convert to 24v). 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.

Thanks!

Sam

\$\endgroup\$
  • 1
    \$\begingroup\$ You might consider simplifying this question to attract more answers. \$\endgroup\$ – Andy aka Aug 1 '15 at 9:54
2
\$\begingroup\$

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:

schematic

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:

schematic

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.

\$\endgroup\$
  • \$\begingroup\$ Nice one,can he also use switch just above R1 and and R2 to just shut-off the camera's?what impact of +12v will have though no current flows? \$\endgroup\$ – MaMba Aug 1 '15 at 13:19
  • \$\begingroup\$ @MaMba I'm not sure if I read your question right, but of course you can add a switch, as long as the wire that is unique to a voltage gets interrupted. If you break the common with both voltages present both load together will see a 12V differential. \$\endgroup\$ – Asmyldof Aug 1 '15 at 14:09
  • \$\begingroup\$ Thanks to Asmyldof and Spehro Pefhany for your responses, I will test with Asmyldof's solution first (and confirm results here) as it's the simplest! Your help is much appreciated. \$\endgroup\$ – Sam Spurgeon Aug 2 '15 at 11:03
1
\$\begingroup\$

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.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.