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I measured a few laptop AC-to-DC adapters. They have 1 MΩ between the GND pin (goes into the laptop) and the earth prong (into the wall).

Desktops are grounded directly to earth: 0 Ω.

Why this difference?

ocrdu
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    Desktop has a three-prong plug. Laptop usually two-prong and Class 2 insulation (square-inside-square marking) on the power brick. – Whit3rd Mar 07 '17 at 23:39
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    @Whit3rd Yes, often, but certainly not always. I've seen enough laptops with a power adapter with 3 prongs. My question is about the resistance between the 3rd prong (earth) and the laptop's ground. I measured 1E6 ohm. What is the purpose of that resistor? –  Mar 07 '17 at 23:44
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    Note to other readers - this seems closely related to the earlier question, where there is an on-going discussion about laptop power supplies, so reading that may give some more context: "1E6 ohm resistor inside AC/DC adapter, between GND and earth?" – SamGibson Mar 07 '17 at 23:48
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    Class 2 insulation means low-resistance ground not REQUIRED, not that it is not present. Laptops (on battery power) all are ground-not-required, so the brick is the item of interest. – Whit3rd Mar 07 '17 at 23:50
  • @Whit3rd Thanks. So, the power adapter might be double insulated and therefore a path to earth is not needed? But, in this particular power adapter, there is an 1E6 ohm path from ground to earth. What is the purpose? –  Mar 08 '17 at 00:15
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    @Marty: the 1E6 ohm path to ground (or, in US two-prong connection, a properly rated component with that resistance to neutral) is intended to provide electrostatic charge dissipation, but not AC shortcircuit fire safety. It's also seen on TV antenna inputs, which might otherwise be floating. – Whit3rd Mar 08 '17 at 01:51
  • The existence of two essentially identical questions about the same subject and posted fairly close together in time is a great hindrance to dealing with this subject well. The combined answers are over over over kill in answering the question(s) well and time and effort is being wasted by the massively overlapping duplication. 1st version and 2nd version (this one) – Russell McMahon Mar 10 '17 at 05:05

2 Answers2

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The diagram below is based on the one you provided in your duplicate of this question. Not only are duplicates not allowed they are a very bad idea as exactly what has happened here occurs - information is spread over two questions.

Your diagram was not of a power supply but of a line filter and it was left for right swapped.

I have rearranged it here and changed it to represent a power supply.
AC mains is connected at left.
DC low voltage is supplied at right.
The two series capacitors are the "Y" noise suppression capacitors required by most regulatory authorities.

IF the green dotted ground connection is connected then the centre point of the two capacitors is at ground, as is intended by the designers.

IF the green dotted ground connection is NOT connected* then the centre point of the two capacitors is at mains/2 at an impedance set by the two capacitors.

If a 1 megohm resistor is connected as shown it provides an electrostatic "bleed" from the laptop low voltage ground to either mains ground or mains/2. Either will discharge electrostatic charge.

If the 1 megohm resistor is replaced by a hard link then if the green dotted earth connection is absent* then the laptop ground will be connected via the capacitors to Vmains/2. The impedance is low enough to cause very unpleasant but technically not life threatening shocks. It is also entirely adequate as a means of semi-randomly destroying equipment connected to the laptop.

*Mains ground may not be connected because a 2 wire power cord is used, or a 2 pin plug is used, or a 2 wire mains socket is used, or because Murphy wanted to have fun. Using the 1E6 resistor rather than a hard link tends to give the best of all worlds in most cases.

AC to DC schematic diagram

Added 2022 - notes from comments:

A 0 Ohm connection )(ie Vout-ve = main ground) is used in some cases and when 2 x input Y capacitors are used from 2 x line connections to input ground AND when a 2 wire cord or otherwise unearthed supply is used you get the situation I described in my commments to this duplicate version of this question. Then Vout-ve floats at half mains with about 1 nF coupling (typically) and gives users nasty nips (AMHIK*) and may damage grounded equipment it is connected to.
(AMHIK* - Ask Me How I Know] –

An isolated secondary provides more protection generally against various issues such as Murphy destroying the primary side of the power supply mit spitzen sparken, and main voltage issues.
WITH NO connecting resistor the secondary can float at electrostatic voltages and even if the laptop is properly designed, peripherals may not be and eg mouse, general USB equipment, external monitors and more are at risk. While it would be "nice" if all such devices were 'properly designed', competent designers may go the 2nd mile and add a bleed resistor.

There is no certainty that worst case electrostatic charge will not exceed standards models. Laptops may have substantial common grounded interconnecting metalwork and you can usually expect continuity between both the expressly grounded outers of some connectors and the metal mountings or most other connectors.

I just took the nearest laptop [HP 2540P Elitebook] and tested continuity of metal parts on external ports & parts. "Grounded" continuity exists between metal parts on 11 of the 13 connectors (2 have no accessible metal), the 12 or so base screws plus the DVD drive large metal plate plus the DVD metal slide rails. Sitting this laptop on ones lap with a shunt linked supply and no ground on the power lead would lead to very annoying shocks (AMHIK) and potential (pun noticed) damage to or destruction of peripherals (AMHIK). –

Russell McMahon
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    Using the 1E6 resistor rather than a hard link tends to give the best of all worlds in most cases. I tend to agree with you on that after reading your comments on my answer. I expected the Dell adapter (hard link) to have the more sensible solution compared to the Acer (1 M resistor) yet the opposite is true. Fortunately I have yet to experience grounding troubles similar to yours even though I mix and match all kind of equipment. – Bimpelrekkie Mar 09 '17 at 13:15
  • IF the green dotted ground connection is NOT connected then the centre point of the two capacitors is at mains/2 at an impedance set by the two capacitors. can you explain this? If the dotted green connection is not present shouldn't the center node float to any potential? – vrleboss Apr 09 '17 at 17:26
  • @vrleboss If the two capacitors were instead eg 10k resistors then they would form a voltage divider so their centre point was at Vmains/2. Yes? | Wityh AC voltages the capacitors have an AC resistance = impedance equal to about Z = 1/(2 x Pi x Freq x C) or ABOUT (30,000 Ohms)/C for C in uF ie about 3k for 1 uF, 300k for 0.01 uF and 3 Megohm for 0.001 uF. The two capacitors divide the mains voltage by 2 for equal caps, and impedance is approx Zcap/2. – Russell McMahon Apr 10 '17 at 09:24
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I have here a Dell laptop power adapter where I measure 0 ohms between the mains ground and the ground of the low voltage output.

And I also have an Acer adapter where there is 1 Mohm like you measure.

So it is not a "universal truth" that all laptop adapters are grounded via 1 Mohm.

Indeed all desktops are grounded via 0 ohms.

For desktops the 0 ohm makes sense, if for whatever reason the metal case becomes live a 1 Mohm series resistor will not make enough current flow to ground in order to protect you, the user, when you touch the case or anything connected to it. Nor will it trip the RCD or earth leakage circuit breaker.

Remeber: metal case so 0 ohm grounding is required !

Laptops are a different class of devices as there is no mains voltage present in the laptop itself. The power adapter has a plastic housing so it cannot become live so 0 ohms grounding is not needed for safety.

This leaves it up to the manufacturer to make the choice of not grounding the low voltage side (mains side uses a figure-8 type power connector) or ground via 0 ohms like in a desktop or ground via 1 Mohm.

My guess for the reason to ground via 1 Mohm instead of 0 ohm might have to do with ESD discharging. When you are electrically charged and touch a metal part of the laptop, the discharge would flow through the laptop's ground. When this ground is 0 ohms you might feel a "tingle". With the 1 Mohms the current will be limited so the discharge will still take place (and take slightly longer but still only a fraction of a second) but you would not feel it.

Bimpelrekkie
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  • Thanks! So, you think it's to protect the user? I was also thinking about ESD, but also for a slightly different reason: to protect the laptop. When the earth wire from the wall would be negative charged for whatever reason, then the resistor can slow down incoming electrons. Or the other way around, when the earth wire is more positive than the laptop's ground. Am i thinking in the right direction? –  Mar 07 '17 at 20:29
  • I think you might be onto something, but "by definition" earth isn't charged. What can certainly happen is a user with rubber shoes on a polyester carpet carries the laptop around, building up a charge (positive or negative, doesn't matter) on the laptop, and that would spark to ground when the laptop is plugged in. – pjc50 Mar 07 '17 at 20:37
  • Actually the MegOhm protecting from ESD is a fallacy. If it were 0Ohm it would protect against ESD discharge, as all PC connectors are prescribed to be shelled to system ground (which may also be the system's 0V). 1Mohm will present a much higher resistance than just about any other connection, increasing the statistical risk of the spark actually traversing electronics through something else connected. Though the good news is that well designed laptops would stick to the spec of having ESD protection on all external connections. Well designed ones. – Asmyldof Mar 07 '17 at 21:27
  • In my case the power adapter (with 1E6 ohm) belongs to a pretty decent laptop. So, i think the resistor is meant to be there. But what is the purpose? –  Mar 07 '17 at 23:35
  • @Marty A stupid myth about ground loops being bad, regardless the situation. – Asmyldof Mar 08 '17 at 21:42
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    @Asmyldof No. A 0 Ohm connection )(ie Vout-ve = main ground) is used in some cases and when 2 x input Y capacitors are used from 2 x line connections to input ground AND when a 2 wire cord or otherwise unearthed supply is used you get the situation I described in my commments to this duplicate version of this question. Then Vout-ve floats at half mains with about 1 nF coupling (typically) and gives users nasty nips (AMHIK) and may damage grounded equipment it is connected to (AMHIK) [*AMHIL - Ask Me How I Know] – Russell McMahon Mar 09 '17 at 08:20
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    @Asmyldof And :-) an isolated secondary provides more protection generally against various issues such as Murphy destroying the primary side of the power supply mit spitzen sparken, and main voltage issues. | WITH NO connecting resistor the secondary can float at electrostatic voltages and even if the laptop is properly designed, peripherals may not be and eg mouse, general USB equipment, external monitors and more are at risk. While it would be "nice" if all such devices were 'properly designed', competent designers may go the 2nd mile and add a bleed resistor. ... – Russell McMahon Mar 09 '17 at 08:25
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    @Asmyldof ... Also - there is no certainty that worst case electrostatic charge will not exceed standards models. || Laptops may have substantial common grounded interconnecting metalwork and you can usually expect continuity between both the expressly grounded outers of some connectors and the metal mountings or most other connectors. || ... CHECKS ... – Russell McMahon Mar 09 '17 at 10:00
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    @Asmyldof ... I just took the nearest laptop [HP 2540P Elitebook] and tested continuity of metal parts on external ports & parts. "Grounded" continuity exists between metal parts on 11 of the 13 connectors (2 have no accessible metal), the 12 or so base screws plus the DVD drive large metal plate plus the DVD metal slide rails. Sitting this laptop on ones lap with a shunt linked supply and no ground on the power lead would lead to very annoying shocks (AMHIK) and potential (pun noticed) damage to or destruction of peripherals (AMHIK). – Russell McMahon Mar 09 '17 at 10:01
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    @RussellMcMahon All fair points. Though somewhat drawn-out for a comment section, I'll happily take it on the chin. As a person with only grounded outlets in and around computer capable areas I forgot to consider normal (read: clearly insinsible) people who do not. – Asmyldof Mar 09 '17 at 10:41
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    @Asmyldof :-). Thanks - after 5 decades of 'playing' I've been bitten by most of the things that go wrong and not a few that shouldn't. An annoying weak link in the above situation is cords with either no ground pin or no ground lead. Then if the psu is correctly wired with y caps and a hard prim-sec link the powered device is very unpleasant to work with in a 230 VAC system. Laptop on lap with shorts bites bare legs and fingers when brushed against "earthed" metal if even high R user earth part is present. I had a new printer destroyed because it had a bad ground and the PC was grounded. – Russell McMahon Mar 09 '17 at 10:58