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Laptop SMPS grounding

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hithesh123

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Why do some laptop power supplies have a 3 pin AC connector and some have 2 pin (IBM T42).
The 2 pin has no neutral line connection. Does this mean, it is more prone to shock than the 3 pin?
 

3 are for Line, neutral and earth. I guess two pins are line and neutral. With another words there is no earth. I dont know, but such connection is not allowed in certain countries (e.g EU).
 

There are conditions in EU and other countries where an Earth wire is not essential. Generally, it is omitted when there are two layers of insulation between the incoming AC side of the power supply and the laptop connections. This is called "double isolated" and is marked on the PSU with a symbol of two concentric squares. A high voltage test, typically 1.5KV for 30 seconds is carried out between the AC and DC sides to check the insulation is up to standard. Sometimes the Earth is still needed when double isolated if local safety regulations demand it.

Brian.
 
Double isolation! I thought only medial devices required double isolation. How do they double isolate it?
 

It's very simple, the primary and secondary windings on the transformer are on their own plastic formers or at least have a dividing wall between them on the same former. This is in contrast to the conventional transformer design where one winding is on top of the other and therefore only isolated by the insulation on the winding wires. Keeping the windings apart not only greatly decreases the chances of leakage between windings, it also minimizes the capacitive coupling between them.

Brian.
 
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