AC riding on DC

Discussion in 'General Electronics Chat' started by Management, Apr 14, 2010.

  1. Management

    Management Thread Starter Active Member

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    Please throw out your random thoughts on this please. Help me get my brain spinning.

    Some technologies out there use AC riding on DC for digital communications. One example is in the 4-20mA space where HART is a communications protocol that uses two frequencies for 1s and 0s coupled to the DC 4-20mA.

    There is also techs coupling high frequencies on AC power.

    If I wanted to couple high frequency content onto 100V DC what issues would I run into, granted I have the components that can do the coupling. How strong a AC would have to be and what would happen if it were even higher DC voltage.

    Thanks.
  2. rjenkins

    rjenkins AAC Fanatic!

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    There are two basic methods.

    One is a transformer with the secondary in series with a couple of small value high voltage capacitors, connected across the main supply.

    The other is a current transformer operating in reverse, so you have a turn or two or the power cable and some hundreds of turns of signal-side cable.
    With both types, the isolated (signal) side can be tuned to the carrier frequency.

    The first type (voltage feed) is better when the power side has a reasonable impedance at the carrier frequency, like with X-10 automation gear and the AC main power circuit.

    The second (current drive) works when one end of the circuit has low impedance at high frequencies, giving a return path for the current modulation.

    You can also include chokes or filters to isolate the part of the system with the carrier signal from the power source and load, so the carrier is not shorted.
  3. R!f@@

    R!f@@ AAC Fanatic!

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    U should talk about modulation than a ripple on a DCV
  4. Management

    Management Thread Starter Active Member

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    Thank you for such a detailed response.

    So which is considered safer? "Current Drive"?

    In regards to the impedance usually the lines are unterminated and run for short and long distances so the impedance can be all over the place. If the cable used for DC was always fixed and then isolated from the source then I could imagine that the impedance would also be pretty much fixed. Especially with the coupling occurring after the isolating choke/filter.

    Lets assume the carrier frequencies are in the CENELEC Band (kHz) and another method in the tens of MHz. What would the impedance of the power side be (assuming isolated) to use the first method (transform + cap. couple)?
  5. Management

    Management Thread Starter Active Member

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    Your right but I attempted to keep it simple.

    Modulation using OFDM in the above two bands. Trying to brainstorm about issues and requirements that I would run into doing this on high voltage DC lines.
  6. retched

    retched AAC Fanatic!

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    I have not looked into the technology yet, but there is an interest in IP over HV. Which is IP on high voltage AC lines.. I suppose you could also couple AC to hv DC lines to do the same, however, the deterioration of the DC signal is greater over the same distance (long distance) as AC. I dont know if you would have to sync the signals, or just seperate them regardless of AC freq. and DC freq.
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