Homebuilt ROVs Forum

I am reading a lot of posts where people are talking/asking about voltage drop, or should I power on the ROV or at the surface, etc.

Now I am new here, and certainly new to any type of ROV building. But I am not new to engineering or electronics. So let me toss this information out here and respectively ask for comment from my peers who have direct experience.

Lets talk Cat5 cable and power basics and try to keep it simple.

In regards to voltage drop over long distance with any wire you want the voltage at the end to endure less that a 3% drop to be optimal. So, 13.8v on the wire at the surface would be optimal if it was no less than about 11v at the ROV. Most 12v devices will so just fine with voltages of 10.8v to 13.2v.

Cat5 is about 24 AWG, 2.5 ohms cable resistance, and power dissipation of 0.01 watt per foot. On 200 feet of cable if we supplied 14.5 volts to the end we would have about 12v at the ROV. Sounds good. But, I see a problem here. We have to regulate voltage based on cable length and calculate what the optimal input voltage is every time you alter the cable run.

So, to correct this why not take a page from a lot of other electronic vendors. When long runs are needed on Cat5, for example. It would be real easy to have a 24vAC (voltage of choice for may cameras, alarms, etc, that run power long distances over Cat5)input feed at the topside. Run the 24vAC down the cable and on the ROV and convert that to 12vDC at the ROV. This solves the problem of variable voltage and a converter will typically accept between 18vAC and 28vAC, allowing for much greater voltage drops and the ability to maintain 12vDC power.

All these converters are very cheap, less than $15 dollars.

Has anyone tried this? Is my reasoning flawed? Or am I way off base?

I'm with you on this one Joey.
(also an electronics eng) and thats the way the big boys do it. We used to run 2kv ac down 1000m of umbilical and convert down to 15v dc at the bottom for the electronics.
Sadly i havn't got room for the extra transformers in my build but for bigger ROVs, thats the way i'd go.

The one thing that needs to be brought up is current. Actually if you put in 12vdc, you will get 12vdc at the output, with no current flowing. Once you put a load on, things will start to go bad. It seems everyone was talking of putting in 12vdc, but when they started running motors,etc, it would drop to 5vdc or so. I was trying to bring up in my post, that surface power was fine as long as you had the gauge wire to support the current you required. For me I thought it was a lot of trouble to have bigger gauge wire or converters,etc., when I could just put the batteries on board. When all motors and other electronics are running we are talking amps and however you do it you need to have the wire to support it.

Chuck

On board batteries is IMHO the best for ROV's. Control on the surface via some form of communications link. RS232 RS485, PWM, PCM PPM. No line loss to calcultae no stepup TX's required on board. I just makes good sense. And using a $10 OSD you can monitor your batteries at the surface on your video display.