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Combining DC and AC signals to one

Eavesdropper
What are some strategies in combining two low voltage DC & AC signals on a single transmission line? IE: 5VDC @ 20A + a small AC signal 0.5VAC 
 
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    Powering devices over the signal cable is a technique that is often used.
    For instance to power low noise converters in satellite dishes.
    However I would not recommend using 5 Volt @ 20 A over a  signal cable.
    20 A is too much for most cables.
    A better way is to apply a higher voltage, like 50 Volt, on the signal cable and place a DC-DC converter close to the load to make a stable  5 Volt.
    To separate the DC and the AC-signal an inductor should be used in the DC path and a capacitor in the AC-path at both ends.
     
    To power devices over twisted pairs, you can use the phantom or the superphantom circuit.
    This way more wires are used in parallel for the DC-current.
    See schematic.
     
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  • 0 in reply to jvdberg@ieee.org

    JV,

     

    Thank you for the reply and answer. It is very clever. What would you suggest if the DC signal was at 5V@3A?

     

    E

  • 0 in reply to Eavesdropper

    That depends on the length of the cable and the size of the wire.

    In a 5V circuit you cannot afford much voltage drop. For a very short cable this could be done.

    The inductors to separate the AC and DC signal must also have a low resistance to minimize the voltage drop.

    At a higher voltage the current will be lower and you can afford a higher voltage drop.

    At 50V input voltage you could have a voltage drop of 5V over the cable and the inductors and you can still make a stable 5V from the remeining 45V.

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  • 0 in reply to Eavesdropper

    That depends on the length of the cable and the size of the wire.

    In a 5V circuit you cannot afford much voltage drop. For a very short cable this could be done.

    The inductors to separate the AC and DC signal must also have a low resistance to minimize the voltage drop.

    At a higher voltage the current will be lower and you can afford a higher voltage drop.

    At 50V input voltage you could have a voltage drop of 5V over the cable and the inductors and you can still make a stable 5V from the remeining 45V.

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