How can a laptop charger be dependent on an earth connection to function?

5

I recently bought a new laptop, and it came with an AC/DC adapter with a 3-prong mains plug. It works fine when connected to a 3-slot outlet (i.e. with earth connection).

Last month, while I was travelling, I tried to use my laptop charger on a 2-slot outlet without earth connection (via a travel adapter where the earth is left floating), and the laptop failed to charge. I was puzzled, since I do not understand how the charger, which is supposed to act as an isolating transformer, can depend on the earth connection to function. Also, I never observed this behavior with my previous laptop.

Back home, I did some measurements with a multimeter. Using the ohm-meter, I did not detect a direct connection between the earth pin of the mains plug and any of the surfaces of the laptop plug. When plugged in with a "proper" 3-prong connection, the inner surface of the laptop plug is at 20.6V with respect to the outer surface. The inner surface is at 10.3V w.r.t. the earth pin of the outlet, and the outer surface is at 0V w.r.t. earth. When plugged in without the earth connection, the charger does not produce a voltage drop between the outer surface and the inner surface.

What explains that the charger doesn't work without the earth connection? How come that this charger behaves differently from the charger of my previous laptop in this respect? Is it possible to fix this without buying a new adapter?

derren

Posted 2015-11-22T12:08:14.107

Reputation:

Did any other device work on the 2-slot outlet ? – None – 2015-11-22T13:26:03.050

Yes, the laptop charger from my older laptop (Sony) worked fine. All other devices (phone chargers, hairdryer, etc.) also worked fine. – None – 2015-11-22T13:46:01.787

Does the charger work when connected to a grounded outlet but with live and neutral swapped? – plugwash – 2015-11-22T14:13:56.163

Answers

1

Where did you travel to? I don't think your laptop not charging had anything to do with the earth pin. My guess is the voltage or current you where getting through that outlet was below the specs of the charger and that's why it didn't charge.

Another hipothesis is the two connected pins failed to properly come in contact with the outlet, or the adapter was broken. Make a test with the adapter to check the latter one.

Other idea: maybe you accidentally reverse polarity by connecting the positive and negative the other way around, when using the adapter?

jotadepicas

Posted 2015-11-22T12:08:14.107

Reputation: 126

1I re-tested the adapter separately, and indeed it is broken. It has intermittent problems with the contacts, causing it to work with the plugs of some of my devices but not with others. Thank you for your helpful suggestion. I realize this is a silly end to this question, so sorry for taking your time. I will remove the references to the brand of my computer above, because this is not relevant anymore. – None – 2015-11-22T16:11:06.493

For science, I also re-tested the laptop charger with a homemade plug where I left the ground disconnected, and it now works. – None – 2015-11-22T16:19:43.967

@derren glad I could help! troubleshooting tends to end like this sometimes :P – jotadepicas – 2015-11-22T17:50:16.070

Asked: 2015-11-22T12:08:14.107

Viewed: 528 times

Active: 2015-11-23T04:22:06.597