# Laptop adapters with 2- vs. 3-prong plugs **URL:** \ **Category:** Factual Questions\ **Created:** [December 28, 2014, 2:09am UTC](https://boards.straightdope.com/t/laptop-adapters-with-2-vs-3-prong-plugs/708317 "2014-12-28T02:09:06Z")\ **Posts on this page:** 11\ **Page:** 1
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [December 28, 2014, 2:09am UTC](https://boards.straightdope.com/t/laptop-adapters-with-2-vs-3-prong-plugs/708317/1 "2014-12-28T02:09:06Z")
Why do some laptop adapters use 2-prong wall plugs, but some use 3? (In the U.S., modern 2-prong plugs are polarized so one side is guaranteed to be the neutral side. In 3-prong, a third prong is a dedicated ground.) ---
**Author:** ![johnpost](https://avatars.discourse-cdn.com/v4/letter/j/f17d59/32.png) [@johnpost](https://boards.straightdope.com/u/johnpost)\ **Post date:** [December 28, 2014, 2:16am UTC](https://boards.straightdope.com/t/laptop-adapters-with-2-vs-3-prong-plugs/708317/2 "2014-12-28T02:16:05Z")
a grounding plug (3 pin) is important if the device has a metal case. if the device has a plastic case or other construction standards then a polarized plug is OK. ---
**Author:** ![bob\_2](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bob_2/32/3341_2.png) [@bob\_2](https://boards.straightdope.com/u/bob_2)\ **Post date:** [December 28, 2014, 9:38am UTC](https://boards.straightdope.com/t/laptop-adapters-with-2-vs-3-prong-plugs/708317/3 "2014-12-28T09:38:34Z")
In the UK, all devices have three pronged plugs because of the design of our sockets (the earth pin moved a shutter that protects the other two). In fact, most, or even all, small devices only have two wires and nothing is connected to the earth pin. Earthing (grounding) is not considered necessary. ---
**Author:** ![Hari\_Seldon](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hari_seldon/32/5173_2.png) [@Hari\_Seldon](https://boards.straightdope.com/u/Hari_Seldon)\ **Post date:** [December 28, 2014, 11:16am UTC](https://boards.straightdope.com/t/laptop-adapters-with-2-vs-3-prong-plugs/708317/4 "2014-12-28T11:16:30Z")
One thing to note is that when I was in Japan I noticed that there were no three pin outlets. The place I visited, a modern building in other respects, had a “recording white board”, meaning you could make a copy of whatever was written on it. It had a three-pronged plug and they had to plug it in through a cheater. ---
**Author:** ![Mdcastle](https://avatars.discourse-cdn.com/v4/letter/m/958977/32.png) [@Mdcastle](https://boards.straightdope.com/u/Mdcastle)\ **Post date:** [December 28, 2014, 4:08pm UTC](https://boards.straightdope.com/t/laptop-adapters-with-2-vs-3-prong-plugs/708317/5 "2014-12-28T16:08:33Z")
Some laptop power supplies use the ground for EM shielding. Many modern power supplies don’t care which way you plug them in. My Sony laptop is not grounded and not polarized. ---
**Author:** ![GusNSpot](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gusnspot/32/436_2.png) [@GusNSpot](https://boards.straightdope.com/u/GusNSpot)\ **Post date:** [December 28, 2014, 6:30pm UTC](https://boards.straightdope.com/t/laptop-adapters-with-2-vs-3-prong-plugs/708317/6 "2014-12-28T18:30:31Z")
Kinda like 12V light bulbs in a car, they don’t care which-a-way the electrons are going.\* - Unless things go sideways. ---
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [December 28, 2014, 6:38pm UTC](https://boards.straightdope.com/t/laptop-adapters-with-2-vs-3-prong-plugs/708317/7 "2014-12-28T18:38:35Z")
> [@johnpost](#): > > a grounding plug (3 pin) is important if the device has a metal case. > > if the device has a plastic case or other construction standards then a polarized plug is OK. the term is “double insulated.” ---
**Author:** ![Deeg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/deeg/32/2955_2.png) [@Deeg](https://boards.straightdope.com/u/Deeg)\ **Post date:** [December 28, 2014, 10:57pm UTC](https://boards.straightdope.com/t/laptop-adapters-with-2-vs-3-prong-plugs/708317/8 "2014-12-28T22:57:03Z")
> [@johnpost](#): > > a grounding plug (3 pin) is important if the device has a metal case. > > if the device has a plastic case or other construction standards then a polarized plug is OK. Is that correct? My MBP has a long 3-prong cord but there’s a shorter attachment that uses just a two-prong, non-polorized plug. ---
**Author:** ![tellyworth](https://avatars.discourse-cdn.com/v4/letter/t/977dab/32.png) [@tellyworth](https://boards.straightdope.com/u/tellyworth)\ **Post date:** [December 29, 2014, 2:19am UTC](https://boards.straightdope.com/t/laptop-adapters-with-2-vs-3-prong-plugs/708317/9 "2014-12-29T02:19:16Z")
> [@Deeg](#): > > Is that correct? My MBP has a long 3-prong cord but there’s a shorter attachment that uses just a two-prong, non-polorized plug. The MBP’s power supply is double-insulated. The third prong is optional. It will ground the PS if you use the long cable, but that’s not strictly necessary, just an extra precaution I guess. ---
**Author:** ![Isilder](https://avatars.discourse-cdn.com/v4/letter/i/cc9497/32.png) [@Isilder](https://boards.straightdope.com/u/Isilder)\ **Post date:** [December 29, 2014, 11:25am UTC](https://boards.straightdope.com/t/laptop-adapters-with-2-vs-3-prong-plugs/708317/10 "2014-12-29T11:25:21Z")
> [@CookingWithGas](#): > > Why do some laptop adapters use 2-prong wall plugs, but some use 3? > > (In the U.S., modern 2-prong plugs are polarized so one side is guaranteed to be the neutral side. In 3-prong, a third prong is a dedicated ground.) The devices make little use of the fact that the neutral is fixed to being the one pin… You are unwise to touch “neutral” conductors ,but for old devices like lights, they do allocate neutral to the most dangerous conductor, if the design has one that is more dangerously exposed than the other, for safety reasons. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [December 31, 2014, 6:20pm UTC](https://boards.straightdope.com/t/laptop-adapters-with-2-vs-3-prong-plugs/708317/11 "2014-12-31T18:20:37Z")
Just saw this thread and decided to google schematics of power adapters for laptop/notebook computers. The vast majority of schematics show a two-wire (non-grounded) input and a two-wire output. The output voltage is isolated from the input. (Well, not 100.000% isolated. But you know what I mean.) After much digging I finally found a couple schematics that showed a three-wire (grounded) input. In [this](http://www.datasheetdir.com/65-Notebook-Adapter+Application-Notes) schematic, the common (negative/return) on the DC output is hardwired to the ground wire on the input. On page 11 in [this](http://www.st.com/web/en/resource/technical/document/application_note/DM00044787.pdf) pdf, the ground wire on the input is _only_ used to ground the two common-mode EMI filter caps (a.k.a. Y-caps) on the input. I then looked around our lab and found a couple power adapters for HP/Compaq laptops w/ grounded inputs. On each, the resistance between the ground prong on the input and the common on the output was 1.0 MΩ. This is too low to be leakage resistance, thus I would assume the design incorporates a physical 1.0 MΩ resistor to bridge across the isolation. So if your laptop power supply has a grounded input, then one (or possibly more) of the following is likely: 1. _It is strictly there for input common mode noise protection_. Though if everything else is isolated, I’m not sure why they would care about this. 2. _The common on the output is hard-wired to earth ground_. I suppose this could be beneficial if the laptop has a metal case. But it may also be a liability, in that it grounds the user, and the thus the user could receive a shock if another part of his/her body comes in contact with a live voltage from somewhere else. Or perhaps it’s for older supplies w/ RS-232, which (I believe) need to be ground referenced. Not sure. I’m also thinking that this may be more common for “general purpose” power supplies, wherein the power supply manufacturer doesn’t know what it will be plugged into, and grounds the common on the output “just to be safe.” 3. _A high-value (e.g. 1 MΩ) resistor is connected between the ground prong on the input and the common on the output._ This could make sense for a couple reasons: a) If the power supply doesn’t have “good” isolation, and if the laptop has some exposed metal, it can keep the output from “floating up” to a high voltage and causing a tingling sensation. b) It _might_ help bleed of static charge that builds up on the laptop and/or user. > [@Mdcastle](#): > > Some laptop power supplies use the ground for EM shielding. Despite the claim by some EEs, connecting a shield to earth ground does not improve EMI performance.