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An idea I have for better magnetic shielding...

  1. Dec 4, 2016 #1
    To try to contain a magnetic field in a pair of wires carrying a 6 volt at 2 amp current I twist the wires together. Could a braided shield surrounding those two twisted wires connected to ground help in that process?

    Using a braided cable such as RG 174 as a audio signal wire with the braid connected to ground on one end I assume captures the EMI and sends it to ground. Can that current be dissipated as heat by placing a resistor on the grounded end?

    Just thinking out loud...

  2. jcsd
  3. Dec 5, 2016 #2


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    • Twisting the wires helps reducing the electromagnetic field
    • Shielding the wires helps reducing the electrostatic field
  4. Dec 5, 2016 #3


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    Agreed. @Planobilly -- why do you want to shield that DC magnetic field?
  5. Dec 5, 2016 #4


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    The important thing about a return circuit is to prevent radiation by keeping the area of the circuit loop small. By twisting the wires, each half twist reverses and so cancels the area of adjacent half twists.

    At low frequencies, twisting the two wires of a return circuit will cancel the radiated magnetic fields, but any imbalance in the currents will radiate. Symmetrical voltage fields will also cancel. Feeding a twisted pair through a ferrite toroid will block unbalanced RF currents, but not affect the balanced currents.

    A braided screen will stabilise the transmission line impedance of a twisted pair, but impedance is not important for such short wires at low frequencies. A braided screen will not significantly reduce low frequency radiation from a twisted pair.
  6. Dec 5, 2016 #5
    I one case I have 6.3 volt AC wires going to the heaters of the tubes. We deal with that by twisting the heater wires to reduce the the hum caused by those wires. I am looking for more effective ways to deal with the issue.
    The second case is a audio signal wire going from the board to a pot for example. I use RG174 braided cable with one end of the braid connected to ground. Again looking for more effective ways to deal with the issue.
    In the first case I am looking to keep everything in the wire and the second case prevent things from entering the wire.

  7. Dec 5, 2016 #6


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    Can the wire runs . Earthed ferrous metal screens , troughs , boxes or conduits .
    Last edited: Dec 5, 2016
  8. Dec 5, 2016 #7


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    There is really no problem as there will be very little radiation from the twisted pair.
    But if you have high impedance cathode circuits, it may be important to balance the filament wires about ground so as to prevent differential capacitive coupling into the cathode. If you use two fixed resistors to ground you will be unable to adjust the fine balance. The original hum cancelling controls were adjustable, some also had a quadrature phase correction.
  9. Dec 12, 2016 #8
    Can you convert the 6.3 V AC power to the heaters to DC?
  10. Dec 12, 2016 #9
    Sorry guys, I have been away and super busy.

    I have some ideas I want to draw up soon and will post them.

    skeptic2...yes DC is a possibility but there are other issues involved with DC.

    Anyway, thanks for the feedback and I will be back in a few days.


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