3 pin vs 2 pin power plug

  1. I was looking at my laptop power plug - one has a 3 pin and the other is 2 pin.
    The power coming into the laptop has only 2 connections - GND and +DC
    If I remember rectifiers, we only used the AC mains to convert AC to DC, not the earth.
    I am wondering, where is the the earth pin connected in the laptop power converter? To the power converter chassis?
     
  2. jcsd
  3. sophiecentaur

    sophiecentaur 13,700
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    Gold Member

    It may not be connected to anything. Most power supples are double insulated, in any case and often have a plastic third pin. (UK)
     
  4. mgb_phys

    mgb_phys 8,952
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    Homework Helper

    Th plastic earth pin is there because it is needed to lift the safety shutters on the live+neutral pins on a UK socket.
     
  5. sophiecentaur

    sophiecentaur 13,700
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    Gold Member

    Natch.
    I meant that the third pin is plastic rather than brass.
     
  6. mgb_phys

    mgb_phys 8,952
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    Homework Helper

    I was just explaining why they bother fitting a 3rd pin if it's plastic - other than BS1363 requiring it
     
  7. The 3rd pin is not plastic. It is metal, just like the two other pins. The laptop was bought in the USA.
     
  8. vk6kro

    vk6kro 4,059
    Science Advisor

    Are you referring to the external power supply that converts your mains AC to about 17 volts DC for the laptop?

    Mine is a switch mode power supply and it is in an earthed metal case, possibly to reduce radiated interference. The negative lead of the 17 V DC is also earthed, but the laptop would still function if it wasn't earthed.

    These power supplies contain high frequency oscillators that generate a lot of harmonics and these need to be filtered and shielded to avoid interference in nearby equipment.
     
  9. mgb_phys

    mgb_phys 8,952
    Science Advisor
    Homework Helper

    Sorry got a bit off topic.

    The earth pin is a saefty feature, any exposed metal parts of a product must be connected to earth so that if a fault allowed the live wire to touch the metal your would be protected. Since the laptop isn't made of metal - and doesn't have any high voltages inside it there is no need of an earth.

    The power brick probably has a 2 pin connection (a figure of 8 DIN socket) because it is also plastic and doesn't need an Earth.
    UK plugs always have an earth because the third pin is needed to open the safety shutters on the live/neutral holes, if the equipement doesnt need the earth this pin is sometimes plastic to save money - but an unused metal earth pin isn't a problem.

    US plugs that connect to non-earthed applicances can be just two pins.

    If the laptop power brick has a 3pin socket then its a bit naughty to supply a 2pin plug .
     
  10. That's what I thought. But I don't think there is a metal case, only plastic outer case. which is why the confusion started. Maybe some of the power converters have them.
     
  11. vk6kro

    vk6kro 4,059
    Science Advisor

    Mine has a plastic case, but it seems to have a metal inner casing which might be for EM shielding. I can attract it weakly with a magnet, but I haven't taken it to bits.

    If so, it would need to be earthed.

    On the outer plastic there is a caution: "connect only to a grounded outlet". Since there is no exposed metal, this would be for the shielding to be effective.

    Also, there would be filtering on the input and this would have capacitors bypassing the input lines to ground. So, the ground would have to be brought into the box.
    Mine has 8nF from both active and neutral input leads to the ground lead and this is presumably from filtering at the input of the power supply.
     
    Last edited: Dec 11, 2009
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