On-Board Battery

[AHarris]

If on-board batteries reduce the voltage drop when in association with relays.... how does it work?

For the switch to work, electricity needs to flow through the switch, but if that is running through Cat5 cables, what makes this different to voltage drop from sending it through Cat5 cable at the start?
If that makes sense...

Basicly why does having a relay reduce Voltage Drop and allow for a thin tether when the electricity still needs to run accross it?

[SoakedinVancouver]

Ohm's law is the problem, and the path to the solution.

E (voltage) is equal to I (current) times R (resistance)

Let's say you supply your thruster from topside:

The voltage dropped (creating heat) by your umbilical is caused by the resistance of the umbilical multiplied by the current required (demanded!) by the thruster. That is a high current, therefore the voltage drop (E) is high.

Now, let's say you are controlling relays from the top side, while the batteries for the thruster are but inches away from it... You can see where this is going, eh? The relays are demanding very little current (I) to activate and even if your umbilical is now built with smaller wires (higher resistance, R ), the much smaller current (I) is what makes the difference, you have less voltage (E) dropped along your umbilical while at the same time the very short length of wire from your batteries to your relays plus the other short length of wire from your relay to your thruster is not resistive enough to drop anything like the umbilical used to, PROVIDING you do use "heavy" wires from the batteries and to the thruster.

Relay current demand is more likely in the 0.05 amps range, versus about 2.00 amps for a 500GPH bilge pump thruster. If you look up wire gage resistance tables, you will see that the R ratio is not as heavy on the equation's results (smaller factor).

Control: small current, small wires;

Controlled ("load"): large current, large wires.

All this to waste less electrons to heat the lake. And have a longer runtime. It's worth it.

[AHarris]

Thanks for the help, I think I understand it now.
I however am having trouble getting relays and I wondered if there was anyway around it?
Also, I am using a motor which draws a current that is only 500Ma... does this mean I shall have minimum voltage drop?
I originally thought of a parallel circuit but that would work because you can't send it both ways around a circuit at the same time (for forward and reverse at the same time).

[Rover]

Try Radio Shack or All Electronics (http://www.allelectronics.com/make-a-st ... ays/1.html)

Is the 500ma load with the motor under load, moving water, or running in air?

[AHarris]

It is in air, but in water it shouldn't increase by too much?
I also can't get the relays online due to where I live and that the majority of places don't deliver. If they do then I end up paying about £20.00/$40.00 for shipping which is a stupid price considering the relays shall cost about £10.00/$20.00.
I am however going to go into a local model shop and hope that they have some... otherwise I guess I shall have to buy one online.

[Rover]

The current draw with the motor under load could go up by 4 to 5 times the current draw when running the motor in air, due to the restiance of the water. Hobby shop may not carry relays, look on google or the phone book for electrical stores, or surplus dealers.

Good luck.

Jim

[AHarris]

Thanks for all the help guys,
The model shop unfortunately didn't sell any and considering the island I live on is small enough to walk around in a day... we don't have any massive DIY stores or anything like that so everything has to be shipped in.
Due to this we unfortunately have to pay a lot more in shipping costs, I have however ordered some and they should hopefully be here in the next couple of days.
I just hope they are the right ones...

[derelicte]

In my experience, the difference in current draw between operating in the air and underwater was about 10x.

With the rov out of the water and all three motors running full speed, the current draw was 1.2A. In the water, all three motors drew 12A. The start up current was closer to 15A.