Why does a laptop power supply have a three-prong grounded power cord? (Dell XPS 13 2-in-1)

by philg

This is kind of an embarrassing question considering that I studied electrical engineering, but under the theory that “there are no stupid questions, only stupid people” here goes: Why does my laptop’s USB-C power supply have a three-prong grounded power cord? Why wouldn’t a two-prong cord work just fine?

[Separately, this Dell XPS 13 2-in-1 has now racked up about 10 hours of Dell tech support time (previous post). My advice: make sure if you get one you have a phone with an extended battery life (during the latest phone call it was 38 minutes of hold time and transferring from one agent to another before a tech support person finally began work). One problem that seems to be beyond Dell’s capability to solve is that the machine cannot stay connected to a Bluetooth mouse (seems like a software problem because the severity varies). It looks slick, but it cost $2,400 with tax and has never worked properly. Who has experience with returning products to Dell that they can’t fix? (I waited for two hours on various kinds of hold with the returns department, but they eventually hung up on me. When I called back they started with an automated attendant asking basic questions and for the order ID, kept me in queue for about 20 minutes, and then dumped me back at the first menu without ever connecting me to a human. It seems like it might be a brilliant scheme to make sure that nothing ever comes back, but on the other hand how many repeat customers can they get?]

14 thoughts on “Why does a laptop power supply have a three-prong grounded power cord? (Dell XPS 13 2-in-1)”

  1. Possibly to ground the RF shield to keep all the electrically noisy pulse width modulation inside?

  2. Probably to placate those who think that 3-prong plugs are safer than 2-prong plugs when used with plastic enclosures. (Which they aren’t.)

    It’s possible it could be better shielding, but I’m not convinced of that.

  3. I am pretty sure it is a UL requirement to ground the transformer shield for safety reasons. So the setup is two connections to the transformer terminals and a ground to the case shield. So three prongs.

    I suggested and now I repeat the suggestion you buy a HP Spectre 2 in 1 laptop. It is just a better machine. I have two of the HP Spectre machines and they are great. I have the older models with the three regular USB ports. Dell as a company is falling apart. I am not sure why their products have gotten so bad but they are just junk now.

  5. I forgot to mention we have had issues with the internal bluetooth software inside one of our HP machines. That connection drops out or cannot be started sometimes. It is related to the sequence of starting things. My wife uses bluetooth to drive her Bose Mini II for listening to music. She has to start the Bose first and then start Itunes and so forth. I don’t remember the exact sequence. But only one of my HP machines has internal bluetooth and the other does not so I only see this issue on my wife’s machine. I think the bluetooth machine has slightly different hardware and that means it uses different software drivers. She uses a plug in dongle to drive her bluetooth mouse.

    That machine also has issues with the standard RCA jack we use for our headphones for music. That RCA plug only works correctly if it is plugged in BEFORE the machine is powered on from a cold start. Then the headphones work OK and speakers are off. If the headphones are unplugged after the computer is on they will not work correctly nor do the speakers turn off. So it sorta works.

    Good Luck.

  7. Bill: How can Bluetooth on laptops be this bad? It works perfectly on phones, right? You start your car, the phone connects to the car, you call your mom. You can repeat that process 100 times without any issues. If it works on a $100 Android phone why doesn’t it work on a $2,400 Dell laptop?

  8. defaults write com.apple.BluetoothAudioAgent “Apple Bitpool Min (editable)” -int 80
    sudo killall bluetoothaudiod

    I had to do that in order to get my $2k+ Macbook to play audio over bluetooth without skipping.

    I thought the $20 bluetooth audio stereo amp I got was the problem, but it was in fact the operating system on my laptop.

  9. Here’s one reason to have a power adapter with a three-prong cord:

    http://apple.stackexchange.com/questions/32417/how-can-i-avoid-my-macbook-pro-giving-me-minor-shocks

    This sort of effect has also been known to fry other grounded equipment the moment you plug it into the laptop — notably audio equipment, since the common 3.5mm jacks make all sorts of strange connections when you’re plugging them in; they don’t connect ground to ground first, which is best. (RCA jacks also make the signal connection before they make the ground connection.) Basically, with a 2-prong jack your laptop is floating (ungrounded), except that there is some capacitance to one or both of the 2 prongs, and through that capacitance small shocks can be delivered: not enough to be dangerous, but enough to be annoying or to fry sensitive input transistors on connected equipment. Sometimes they put in the capacitance intentionally, to meet EMI goals; sometimes it’s just transformer winding-to-winding capacitance.

  11. @Colin
    You will note the cable in question ends in an ungrounded IEC C7 connector.

    2Phil
    I suspect your BT problems are a legacy of Microsoft’s belated and hands-off integration of Bluetooth in Windows, just as TCP/IP wasn’t fully integrated until Windows 95. Contrast with Mac OS X, iOS and Android that always had it tightly coupled in the OS, with the flip side being only a small number of BT chipsets were supported. This kind of architectural compromise is very hard to reverse and that’s why BT is so flakey in Windows.

  12. Phil,

    I think Demetri is probably right, in order to pass radio emissions certification (FCC) they probably had to ground the rf shield over the power supply. Why does Apple not have a grounded plug? Apple generally uses switching regulators which are less noisy but cost more. Apple probably designed the usb-c power supply themselves while Dell probably had an ODM design and build the regulator. The ODM needs to get the thing to pass in order to sell it to Dell and get paid, hence they want to keep costs as low as possible and yet make sure they will pass. “Xingfu” brand regulators from Ebay usbwere not tested properly.

  13. A few years ago someone took apart a bunch of phone and tablet chargers and posted the results on a web page – there was considerable variation in quality from high quality chargers from Apple down to $1 models from China on ebay. Some emitted considerable electrical noise, others had deficient isolation between the high and low voltage sections. I imagine that laptop chargers also display this kind of variation.

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