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The cord connecting my laptop to the charger is a coaxial cord. It seems to me that other laptops also have the same type of cord.

I get that coaxial cords are good for connecting my TV to an antenna, or to transmit other signals, but do we need coaxial cords to transmit DC power?

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  • \$\begingroup\$ I had an old Dell laptop with a non-coaxial cord, and it was annoying to plug in because you had to get the orientation right. \$\endgroup\$ – markrages Mar 25 '14 at 18:53
  • \$\begingroup\$ Well, that does seem to be a problem of the plug, not of the cord. The plug should only allow to be plugged in the right way. \$\endgroup\$ – Quora Feans Mar 25 '14 at 18:56
  • \$\begingroup\$ it would only plug in one way, but the coaxial plug plugs in any which way. \$\endgroup\$ – markrages Mar 25 '14 at 18:58
  • \$\begingroup\$ Could it be that a coaxial cable radiates less EMI and helps to pass the tests? Another possibility, laptops have coaxial DC cord for ergonomics reasons, because coax is equally flexible in every direction. 10 years ago, I had a laptop with a side-by-side cord for DC. \$\endgroup\$ – Nick Alexeev Mar 25 '14 at 19:03
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    \$\begingroup\$ The cord isn't coaxial, just the plug at the end. The coaxial shape is the most idiot-proof; it only goes in one way. \$\endgroup\$ – Dave Tweed Mar 25 '14 at 19:17
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The plug is coaxial because it can't be easily shorted when pressed in any orientation against a metal surface, and there's no orientation issue when inserting it into the laptop.

The cables are rarely coaxial, but when they are it's often more for the aesthetics of a very thin round cable than it is for anything electrical. You can make non-coaxial cables that have round outer profiles, but they are thicker, as the cable has to have a diameter at least twice the diameter of the two wires inside the cable.

If you're carrying three wires, as with grounded outlets, the diameter of the cable isn't going to be much less if it's coaxial, so there's little benefit to making it coaxial.

However, if you have only two wires to carry of a specified gauge, then you can have a thinner, round cable profile if it's coaxial.

It does increase safety, slightly. If the grounded conductor is the outer conductor, then any cutting or piercing of the cable will result in a ground contact first, before hitting the (typically) low voltage core wire. These supplies are isolated anyway, so it typically won't matter, but it does mean that if your cable insulation is damaged and that comes into contact with a metal part of the laptop, you won't see sparks, kill your laptop, or the power supply.

EMI concerns should be negligible on this cable, but it may help with them in some small degree. Typically you put filtering on the laptop and the power supply so nothing but the intended signal goes out on the wire, and nothing received on the wire goes into either device.

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IEC61000-4-6 is the EMC spec for conducted RF testing (immunity testing via injection) and it clearly states that all "ports" should be tested. "All ports" I take to mean the following: -

  1. Power AC input
  2. DC output to "whatever"
  3. Any other port such as USB, audio, external control, really anything

No distinction is made between screened and unscreened cable. Life is hard for the unscreened! For a laptop charger, I don't see any way of not testing the DC power lead because it is sold as a separate product in its own right. An outer earth screen on the cable will make it a lot more resilient to the transients they can inject in the testing.

enter image description here

Shown above are typical examples of how coupling is made to cables - the cable is placed in the box and the lid closed. Inside the box (voltage injection), and surrounding the cable is a conducting hinged plate that touches the outside of cable thus capacitively coupling itself to the cores of the cable. Other types use current injection and therefore have split ferrites to make surrounding the cable relatively easy - this is called current injection.

What do they inject? This is a good document to explain what is done but in short: -

Continuous conducted RF immunity testing involves injecting RF voltages or currents into each of the cables associated with the equipment under test (EUT). The purpose of the test is to simulate the proximity of the EUT and its connected cables to radio transmitters and RF manufacturing equipment operating at low frequencies. These frequencies are not easy to test using the radiated RF immunity techniques described in 2. It is hard to generate uniform fields in typical test facilities at frequencies much below 80MHz, but for typical sizes of apparatus the immunity problems at frequencies below 80MHz are normally associated with cable coupling, so conducted testing of the cables is seen as a reasonable alternative to radiated methods at such frequencies.

A good document that has helped me is this from Laplace Instruments.

So, do you use a screened cable whose outer screen is earthed or do you trust to probably more expensive solutions at either end of the cable?

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