I am in the middle of making my own ROV with six DC motors as the thrusters.
The main controller inside the ROV is a microcontroller that is connected to Raspberry Pi that can send and receive control command over TCP. Power and control from-and-to the surface is established using tether cable which consists of power cable and Cat-5 cable for ethernet connection. The power transmitted is 36V with maximum current rating of 20A. Inside the ROV, the transmission voltage is converted to 12V for motor usage and 5V for other electronics (microcontrollers, Raspberry Pi, other sensors).
The problem is every time I start the motor, Raspberry Pi seems to be browning-out (flickering power indicator) and then turned off. I have tried using separate source (e.g. battery) for the 5V devices and it works seamlessly.
Do you guys have any solution to this? I have thought about using optocoupler, but then again the power came from the same source (tether cable) so I thought it will result in the same way. The best that I can think of right now is to find isolated DC-DC converter for the Raspberry Pi.
Edit: I made my own driver, which consists of a single NPN transistor (TIP142) acting as a switch. The motor will draw around 7A To convert 36V to 12V, I use this http://www.ebay.com/itm/DC72V-36V-48V-60V-to-12V-240W-16-20A-converter-Electric-Storage-Battery-car-/221381653107?pt=LH_DefaultDomain_0&hash=item338b5ff273. As for the 12V to 5V converter, I 'm using Turnigy UBEC 5A