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Author Topic: common negative?  (Read 1359 times)

tobyker

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common negative?
« on: October 30, 2007, 09:03:01 PM »

In the current project I will need both a 12v and 6v supply for various things. Q1 - To simplify the wiring, can I have a common negative wire running the length of the boat, and separate positive wires? I'm thinking of busbars running the length of the boat so I can tap into them where I want to. I'm sure I've seen circuit diagrams with one track marked 0v and others marked 5v and 9v. I would not be connecting the radio to these supplies.  
Q2 - In such a situation, which has less resistance, multistrand wire or solid copper as found in modern house wiring cable? Obviously I'd use multistrand where there was any likliehood ov vibration/work hardening etc.
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Telstar

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Re: common negative?
« Reply #1 on: October 30, 2007, 10:26:30 PM »

Hi Tobyker
Basically the answer to Q1 is yes, but you must remember the common return (negative) wire will be carrying more current than the other(positive) wires. For example if the 12v supply is providing 5A  and the 6v supply is providing 5A  then the negative wire will carry 10A,(the sum of the supply current) So the common return must be capable of handling higher current.
Q2 providing that the wire is the same cross sectional area (1mm or 2.5mm etc) there is virtually no difference in the resistance per unit length. Stranded wire is normally more expensive, so solid is used where movement or vibration is not a problem
cheers telstar
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wombat

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Re: common negative?
« Reply #2 on: October 30, 2007, 10:32:29 PM »

HI Tobyker,

It is OK to use a single ground and separate positives. However as Telstar says, make sure you rate the ground line for the combined current of the two positives. I would suggest that if you are going to use bus-bars which you tap off along the length, then use bar with a cross sectional area as large as you can manage, otherwise you could see problems bacause of resistance in the line. I much prefer a "star" arrangement where each load brings positive and negative to a central distribution point - it means more wires but limits the interaction between them.

Wom
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malcolmfrary

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Re: common negative?
« Reply #3 on: October 31, 2007, 02:43:24 PM »

The common return should be as knobbly as possible.  To get the required current rating, add exerything else together, multiply bt two, and go for the next size above.  The voltage dropped along the length of anything less will be miniscule and probably impossible to see on a meter, but could insert some interesting glitches into the radio setup.
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tobyker

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Re: common negative?
« Reply #4 on: October 31, 2007, 10:05:25 PM »

Thank you all very much. Excellent forum, helpful members, high tolerance of ignorant people like me.
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