Electronic – Is the laptop’s power adapter output negative terminal grounded

adapter

Today I find my lenovo laptop power adapter's output negative terminal is not grounded (the input ground terminal is not connected with output negative terminal, measured with DMM). But another old one is just opposite, the output grounded. Why? There is any standard for power adapter? If the laptop is not grounded, is it safe?

Best Answer

My experience is that the power supplies for the older laptops in which the backlight was CCFL were usually grounded on the DC side as well. I suspect it was because of the high-voltage inverter needed for the CCFL (approx. 600V DC output), but I'm not entirely certain that was the determining factor. There's a concrete example of an older HP I have that does have minus grounded; details on that in this superuser post.

The newer laptops with LED backlight tend not to have the minus pin grounded. There can be static buildup and leakage currents on the non-grounded laptops [the latter particularly on metal cases] which can be irritating to some people. See this discussion on MacBook and this video of an actual measurement; he only measured AC voltage from case to ground as 78V AC with no grounding, which is probably enough to feel a slight tingle on a leakage current. A better test would have been to measure the leakage current as well... but I guess you need to try harder finding EE pros among Mac fans.

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