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Remote TV Camera Power

A while back I received an email from a company with a problem. They needed a security camera installed at some remote location.  The camera was a high quality color camera sending out standard NTSC video through a long 500 foot 50 ohm coax cable.  With some difficulty they got the coax strung.  They were told that there would be AC power near the camera location to power that device.  But, they then learned that in fact, there was no power available.  They thought about using a solar panel and a battery but they decided to call me for help instead.  Was there some way to power the camera without stringing another cable?

The circuit below is what I suggested they use.  It uses an old trick of mixing DC to run the camera with the video signal.  Two coils are used to isolate the two signals.  One coil allows 15v DC to be injected into the 500 foot long cable and a second coil pulls out the voltage, which is fed to a low voltage drop 12v regulator.  With a typical 6MHz bandwidth, the impedance of the coil would be high to the video signal but would have a low DC value to the power supply. The camera requires a clean 12v and pulls about 300ma of current.  I suggested at 15v power adapter with at least a 400ma current rating.

Click on Drawing Below to view PDF version of Schematic

Remoter TV Camera Power circuit designed by Dave Johnson

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