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This adapter is clearly not capable of delivering a null, a power and a ground to the laptop because the connector only has two surfaces. The adapter itself is plastic. So why is it grounded? (Note that this is not specific to this particular brand or model, I see many such adapters.)

I was thinking that maybe on DC side the adapter the ground and the negative is connected but isn't dangerous to connect power to the ground? At another point, say the bathroom you touch a pipe which is connected to the same ground and poof, shock.

Dell adapter

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    \$\begingroup\$ The connector actually has 3 surfaces. The outside of the barrel, the inside of the barrel, and the pin. \$\endgroup\$ – mfarver Aug 6 '13 at 2:01
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    \$\begingroup\$ cos = cosine of the connector? What? \$\endgroup\$ – JYelton Aug 6 '13 at 3:16
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    \$\begingroup\$ fixed. 'cos == because. \$\endgroup\$ – chx Aug 6 '13 at 7:03
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    \$\begingroup\$ @mfarver that renders the question mostly moot. I didn't realize the inside and the outside is not the same metal barrel but are insulated from each other. \$\endgroup\$ – chx Mar 26 '16 at 18:33
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In the U.S., we refer to the three pins on the AC plug as Hot, Neutral, and Ground. Sometimes the Ground is more accurately called a "Safety Ground".

On the DC Plug going into the laptop, the ground conductor is usually (but not always) connected to the Safety Ground. Doing this makes it easier to pass regulatory testing approval (EMC, ESD, Etc.). It also reduces leakage current through the "isolation" barrier inside the power supply. Some people complain that when laptops are placed on their bare laps, they get a tingling sensation where the screws of the laptop touch their skin. Having those screws connected to the safety ground of the AC plug mostly gets rid of that issue.

Also, even if the safety ground is not connected to the ground conductor of the DC jack, it could be providing some additional safety benefits inside of the power supply itself.

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  • \$\begingroup\$ Hot, Neutral, Ground, OK, learning :) I am confused. Power only exists between two conductors. So you get the AC power between Hot and Neutral, have an AC-DC converter and now you have power between Hot and Ground? Is Ground and Neutral connected in the adapter...? If yes see the concern on shocks. If not, well, I sucked in physics in high school :) \$\endgroup\$ – chx Aug 6 '13 at 0:24
  • \$\begingroup\$ @chx Normally neutral and safety ground are at the same voltage (zero volts). They are connected together "somewhere in the building" (I don't know exactly where), but not in the device. The device is powered from the hot and neutral, and almost zero current is on the safety ground wire. Only if there is a fault, or damage can there be current on the safety ground. \$\endgroup\$ – user3624 Aug 6 '13 at 1:58
  • \$\begingroup\$ I have experienced this with ruggedised laptops like the toughbook - they have metal cases, unlike most, and at one stage our employer changed out thousands of 2-pin laptop supplies for 3-pin ones as all the engineers were measuring ~60v float above ground on the laptop case (about the level you start to get the tingle). \$\endgroup\$ – John U Aug 6 '13 at 7:42
  • \$\begingroup\$ @chx: For what it's worth, outside the USA some English speaking countries refer to "live", "neutral" and "earth". \$\endgroup\$ – RedGrittyBrick Aug 6 '13 at 8:26
  • \$\begingroup\$ @user3624: For the record, the neutral and the ground are connected at the service panel. Usually it's a big metal box in the basement with all the breakers in it. \$\endgroup\$ – BlueRaja - Danny Pflughoeft Sep 21 '15 at 1:28
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The third pin on the DC power connector is to allow the power supply to communicate its specifications to the computer so the computer knows how much power it can draw. Some use a resistor to indicate the current draw and others send a digital signal over the extra pin. The extra pin can cause issues with third party/generic power supplies.

The negative on the connector may be connected to AC ground. This grounds the laptop, which can be a good thing or a bad thing. If the laptop wasn't grounded, the decoupling capacitors (to clean up radio frequency noise) between hot and ground and neutral and ground on a 3 prong AC plug based laptop supply would form a capacitive voltage divider which would cause the laptop to float at around 60V (120V divided by 2) with weak current - this will give you a little buzz when you touch it. Grounding the laptop, however, can cause ground loops when connected to other devices that are also grounded. When high current devices such as motor drivers and two way radios are connected to computers, the voltage drop between the external device and its own grounded power supply can easily result in a 1V at potentially high current being injected into the USB or other ports on the computer through the ground and/or as common mode on the data lines. Connected to audio equipment or test equipment, you may get hum. Floating the ground on the laptop allows you you to ground the laptop externally in accordance with your grounding needs for the overall system.

If the laptop power supply has a 3 prong plug, there is a good chance it will ground the laptop while a 2 prong plug should not.

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That looks like a Dell power supply in that case the center pin of the connector goes to a memory device within the power supply (a Dallas Semiconductor One-Wire device) that is interrogated by the computer so it can determine what power supply is connected.

The computer then knows how much power it can consume - it also can refuse to charge or only run at low speed if it determines it is not a certified power supply.

Be careful about probing the center pin with a voltmeter - if you short the center pin to the inside of the barrel it will destroy the ID memory device and cause the computer to think that the power supply is not certified (been there done that :-().

You can read more about how it works Inside the Dell Power Supply

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The ground in most electronics is primarily used as an RF shield around the device. This keeps noise out, and emissions in. In the case of a DC powered, low voltage device the ground is usually tied to the DC negative input.

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