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Both mobile phones and laptop get dc voltage from ac source for charging.These chargers have transformer for isolation and rectifier + filter if i am not wrong.I guess the amount of output dc and charging current may vary.

Why do modern mobile chargers use only 2pin plug or 3pin with dummy earth pin? But Laptop chargers use 3pin plugs with earth pin?

1. Mobile charger Nokia

2. Laptop plug

Gopi
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1 Answers1

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In the EU (and probably in many/most other regions too) appliances that are double insulated, or which work from safe extra low voltages (SELV) are not required to be connected to protective earth.

In your first example, if the "wall-wart" is properly constructed, a single fault would not result in dangerous voltages being present on the low-voltage† lead from the wall-wart to the phone (and thence to you)

See also

† Typically 5V nowadays and maybe 1000 mA or so for a phone charger.

Community
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RedGrittyBrick
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  • Great. Is the dummy earth pin in class II appliances just to make sure plug fits in all possible adapters or is it a layer of insulation for protection? – Gopi Sep 19 '13 at 13:35
  • The dummy earth pin merely provides mechanical stability to the plug when plugged in. You will also find many such chargers without a dummy earth pin, just a 2-pin plug. – Anindo Ghosh Sep 19 '13 at 14:08
  • This brings up an interesting discussion on the lady in china that claims to have been electrocuted while she was charging her phone. Charger likely not built to standard if this was true. – Gustavo Litovsky Sep 19 '13 at 14:12
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    @Gustavo: She was electrocuted by a cheap non-Apple Chinese-made charger. See http://skeptics.stackexchange.com/a/16954/8755 – RedGrittyBrick Sep 19 '13 at 14:52
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    @Gopi, In UK sockets a dummy earth pin is required to open the protective shuttering over the live holes inside the sockets. In other countries the dummy earth pin may act to ensure correct orientation and hence that any switches and fuses operate on the live side and not on neutral - for safety reasons. – RedGrittyBrick Sep 19 '13 at 14:55
  • 100mA? Most phones/tablets take between 500mA to 2 Amps. Or do you mean on the ac side? – Passerby Sep 19 '13 at 15:59
  • @Passerby: [er, yeah](http://en.wikipedia.org/wiki/Brain_fart), I'll edit a more sensible value in, thanks. – RedGrittyBrick Sep 19 '13 at 16:08
  • But this answer does not explain why laptop power supplies do have earth connection. – venny Aug 25 '14 at 16:32
  • A laptop is designed to be used while attached to mains. A mobile is not supposed to be used while charging (it's in the fine print). That last point allows designers to cut corners on safety. Additionally, consumers will complain if mobile chargers came with earth pins (like they do for the nokia charger shown in picture). – Indraneel Aug 27 '19 at 10:25