Where is the Earth Pin Connected in a Laptop Power Converter?

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Discussion Overview

The discussion revolves around the connection of the Earth pin in laptop power converters, particularly focusing on the design and safety features of power supplies. Participants explore the implications of different pin configurations (2-pin vs. 3-pin) and the necessity of grounding in various contexts, including safety and electromagnetic interference. The scope includes technical explanations and conceptual clarifications related to electrical safety standards and power supply design.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants note that the Earth pin may not be connected to anything in certain power supplies, as many are double insulated.
  • Others argue that the Earth pin is a safety feature, necessary to protect against faults that could expose users to live wires.
  • A participant mentions that the Earth pin in UK plugs is required to lift safety shutters on live and neutral pins, while another clarifies that in the USA, plugs can be two-pin for non-earthed appliances.
  • Some participants discuss the construction of power supplies, indicating that a switch mode power supply may have a metal case for electromagnetic shielding, which could necessitate grounding.
  • There is a suggestion that the negative lead of the DC output may also be earthed, but the laptop could function without this connection.
  • One participant raises the point that confusion may arise from the presence of plastic versus metal cases in power converters.
  • Another participant describes the presence of capacitors in the power supply that connect to ground for filtering purposes.

Areas of Agreement / Disagreement

Participants express differing views on the necessity and function of the Earth pin in laptop power converters. There is no consensus on whether the Earth pin is essential for all configurations, and the discussion remains unresolved regarding the specifics of grounding practices in different regions.

Contextual Notes

Some participants mention the potential for confusion regarding the materials used in power supply casings (plastic vs. metal) and the implications for grounding. There are also references to safety standards that vary by region, which may affect the design of power plugs and connectors.

likephysics
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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?
 
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It may not be connected to anything. Most power supples are double insulated, in any case and often have a plastic third pin. (UK)
 
Th plastic Earth pin is there because it is needed to lift the safety shutters on the live+neutral pins on a UK socket.
 
Natch.
I meant that the third pin is plastic rather than brass.
 
I was just explaining why they bother fitting a 3rd pin if it's plastic - other than BS1363 requiring it
 
The 3rd pin is not plastic. It is metal, just like the two other pins. The laptop was bought in the USA.
 
likephysics said:
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?

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.
 
likephysics said:
The 3rd pin is not plastic. It is metal, just like the two other pins. The laptop was bought in the USA.
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 doesn't 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 .
 
vk6kro said:
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.

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.
 
  • #10
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.
 
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