DC power over coax cables?

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DC power over coax cables?

Postby Fry-kun » October 5th, 2015, 3:00 pm

I have a house with built-in coax cabling to each room (2 cables per outlet, all coming to a single junction box on the other end). I don't use them for anything, so I had a [potentially] stupid idea -- would it be reasonable to use those cables for some DC power? Perhaps I could install a USB outlet everywhere there's a coax plug right now (to charge various devices).
How much power could I transfer over those cables? What other considerations are there?
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Re: DC power over coax cables?

Postby st2000 » October 5th, 2015, 8:51 pm

Have no idea. But USB devices are not suppose to pull more than 500ma w/o negotiating for more. So it might work. Let's see. If 100 feet of coax has a resistance of (I am guessing) 1 ohm, then you should expect to get a half volt drop for every 100 feet (v=ir, i is 0.5a and r is (again guessing) 1 ohm that make v 0.5volts). That is, if my guess is any good, you should see about 4.5 volts at the other end of 100 feet of cable if your device pulls 500ma. If your usb device draws less current, the voltage will be closer to what you pump in at the far end. Supposedly 5 volts. So it may work with some devices. But you may have problems with others. Especially devices that draw a lot of current.

There are cable resistance calculator all over the web. Basically, the thicker the wire the less the resistance.
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Re: DC power over coax cables?

Postby asheets » October 6th, 2015, 10:47 am

Don't know about the particulars involving Ohm's Law, but I've seen multiple instances where DC power is fed over coax. In particular, my LNA/Downconverter and my TV antenna amplifier are fed 12VDC via coax.
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Re: DC power over coax cables?

Postby AnalogAnomaly » October 27th, 2015, 12:29 am

coax has either 50 or 75ohm resistance depending on the cable.
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Re: DC power over coax cables?

Postby k-ww » October 27th, 2015, 3:21 am

Sorry, the 75 or 50 ohms is the 'AC' impedance of the coax, not the DC resistance, which varies according to the length of the coax cable & it's wire gauge.
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Re: DC power over coax cables?

Postby SQTB » January 21st, 2016, 8:39 pm

Fry-kun wrote:I have a house with built-in coax cabling to each room (2 cables per outlet, all coming to a single junction box on the other end). I don't use them for anything, so I had a [potentially] stupid idea -- would it be reasonable to use those cables for some DC power? Perhaps I could install a USB outlet everywhere there's a coax plug right now (to charge various devices).
How much power could I transfer over those cables? What other considerations are there?


You could but you can't use any house current voltages like 110 V in USA or 220 in other countries. I would keep it at a low 12 volts. But of course you realize that there is an intrinsic resistance in 75 Ω cable TV coax. Of course that is impedance for AC voltages. But there is a much smaller DC resistance present too. Couple that with the intrinsic resistance for long runs you can expect your resultant output voltages to be much smaller than 12 volts. So you will need a variable power supply that can compensate for the resistance.

What I would do with it would be to use it for a CCTV system to watch for burglars or check on the kids in the play room. You could use it for a closed circuit room-to-room simplex video intercom. You could use full-duplex if you use surplus Ch 3 and Ch 4 RF modulators. You can't run several CCTV cameras without a video sequencer or variable RF channels.

You could also use it for a extension for your POTS telephone - to a hard to wire room that already has coax but no telephone wiring. They use about 5-9 volts DC for speech and dialing. About 48 volts AC for ringing.

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