I measured a few laptop AC-to-DC adapters.

1E6 ohm between GND pin (goes into the laptop) and earth prong (into the wall).

Desktops are grounded directly to earth: 0 ohm.

Why this difference?

  • \$\begingroup\$ Desktop has a three-prong plug. Laptop usually two-prong and Class 2 insulation (square-inside-square marking) on the power brick. \$\endgroup\$
    – Whit3rd
    Mar 7, 2017 at 23:39
  • \$\begingroup\$ @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? \$\endgroup\$
    – user127725
    Mar 7, 2017 at 23:44
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    \$\begingroup\$ 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?" \$\endgroup\$
    – SamGibson
    Mar 7, 2017 at 23:48
  • \$\begingroup\$ 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. \$\endgroup\$
    – Whit3rd
    Mar 7, 2017 at 23:50
  • \$\begingroup\$ @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? \$\endgroup\$
    – user127725
    Mar 8, 2017 at 0:15

2 Answers 2


The diagram below is based on the one you provided in your duplicate of this question. Not only are duplicates not alloeed 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 gtround 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-random;ly 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.

enter image description here

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    \$\begingroup\$ 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. \$\endgroup\$ Mar 9, 2017 at 13:15
  • \$\begingroup\$ 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? \$\endgroup\$
    – vrleboss
    Apr 9, 2017 at 17:26
  • \$\begingroup\$ @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. \$\endgroup\$
    – Russell McMahon
    Apr 10, 2017 at 9:24

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.

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    \$\begingroup\$ @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] \$\endgroup\$
    – Russell McMahon
    Mar 9, 2017 at 8:20
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    \$\begingroup\$ @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. ... \$\endgroup\$
    – Russell McMahon
    Mar 9, 2017 at 8:25
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    \$\begingroup\$ @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 ... \$\endgroup\$
    – Russell McMahon
    Mar 9, 2017 at 10:00
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    \$\begingroup\$ @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). \$\endgroup\$
    – Russell McMahon
    Mar 9, 2017 at 10:01
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    \$\begingroup\$ @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. \$\endgroup\$
    – Asmyldof
    Mar 9, 2017 at 10:41

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