# long range dc transmission

Discussion in 'The Projects Forum' started by thecrazzyman, Oct 27, 2009.

1. ### thecrazzyman Thread Starter New Member

Oct 27, 2009
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Hi im working on a project and i need to get 12v dc down 1000ft of cat 5 cable on only 2 pairs.
I am currently testing using a 49vdc psu to push the voltage down the total 1000ft. Then use some sort of regulator to keep a clean 12v signal.
So i am trying to use a lm340t12 but am having some issues.
I need to try and simulate a 12v 500 ma load beacause i dont want to hook up the device and have wrong voltages destroy it.
The device runs off 12vdc and consumes 400ma or so.

Any suggestions for a dummy load to simulate the device?
And any other input on this situation would be greatly appreciated.

Thank You
alex snell

2. ### russ_hensel Distinguished Member

Jan 11, 2009
820
47
Why 49 volts? To high for many regulators. What is the resistance of the wire and the maximum voltage drop. This plus 12 volts plus the drop of the regulator plus a few extra volts is what you need as an input. You do not need to simulate to avoid the wrong hook up ( i hope ).

3. ### thecrazzyman Thread Starter New Member

Oct 27, 2009
4
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i was testing with 3 diffrent voltages (12,24,49) and looking at the drop off. i figured if i used 49volts to counter any voltage dropoff caused by the 2pair 1000ft run of 24awg cable. i think the voltage i should go for is around 36 volts but havent found a power supply at that voltage
12.3 ohms is the resistance of each power lead.

Jul 7, 2009
1,585
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Here's a result of typing your numbers into a spreadsheet I used to calculate wire resistance. I adjusted the diameter to get the same cross sectional area as two 24 gauge wires have.

If you use this to power a load, you can see you'll have to provide 12.8 volts more than the load requires to counteract the effect of the wire's resistance. The power loss per unit length is small, so you don't have to worry about self-heating effects.

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5. ### CDRIVE Senior Member

Jul 1, 2008
2,223
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Why in the world are you using CAT5 for this operation? It was never intended as a power bus.

6. ### thecrazzyman Thread Starter New Member

Oct 27, 2009
4
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first cat 5 is cheap and im broke
and thanks for the info someonesdad could you do that again but at 0c? this is for a underwater camera project im doing a a nill budget.

edit
also i wanted to make a dummy load because when i apply what ever voltage 12,24,49 when i put my multimeter on the other end it only shows 1 or 2 v drop but when i attached a .8a load to the far end of the cable voltage goes way down.

Last edited: Oct 27, 2009
7. ### mik3 Senior Member

Feb 4, 2008
4,846
63
This because when there is no load the current drawn by the regulator and thus the current through the wire is low. Therefore, the voltage drop across the wire resistance is small. As the current drawn by the load increases the voltage across the cable will increase too.

It is just Ohm's law.

V=I*R

8. ### CDRIVE Senior Member

Jul 1, 2008
2,223
99
You should be considering something like this but still not with CAT5.
Note the third (Sense) conductor.

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9. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
Our OP would need a +VS and -VS to determine the voltage at the load. However, there is so much wire between the supply, the load, and the sense return lines that attempts at regulation that way would be futile. There's simply far too much inductance involved; you'd wind up with a low frequency oscillator.

10. ### CDRIVE Senior Member

Jul 1, 2008
2,223
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Yes, I also should have included (-) Sense. As for the possibility of this long line causing LF osc, .... perhaps. So what's the solution? How about putting the Vreg at the end of the line?

Jul 7, 2009
1,585
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You can get the spreadsheet here. Or just look up the temperature coefficient of resistance of copper and make the small correction.

12. ### russ_hensel Distinguished Member

Jan 11, 2009
820
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Why can't you use all 4 pairs, this would cut the resistance to 1/4 of what you have now.

13. ### CDRIVE Senior Member

Jul 1, 2008
2,223
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Yes it would a great improvement but it's still a poor choice. PA cable is definitely not as cheap as CAT5 but it would be far more efficient. You can get it in 18 to 14AWG and maybe heavier.

14. ### thecrazzyman Thread Starter New Member

Oct 27, 2009
4
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im working on getting other cable but for this test i have a full box of cat 5 cable to laying around. there are 2 cameras and a light. the cameras need 12v i already burnt out my third camera for other uses with a faulty 12v psu and the light can take 8-30vdc and uses 205ma at 12v. if i had money i would get one of thiese. http://www.foresight-cctv.com/VDS2500.htm