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I know about the ghost voltage concept. As far I as understood for example in an SMPS there is "Y" EMI filter and that little capacitance creates a RC voltage divider and we see the ghost voltage high if we measure with a device with high input impedance. I'm not going to write about details because there are many question on the mechanism of it which is basically caused by some sort of capacitive coupling.

But my question comes from my own experience in life. When I touch my laptop's surface sometimes I can feel that ghost voltage but not at every place's. So at home I don't have that but in another place the phenomenon is there.

So this made me think that if ghost voltage was caused by the capacitive coupling of the mains voltage why doesn't it happen everywhere? If the leakage capacitance is the same and we have the same AC at two different place why am I experiencing this at only some places? Can that be the mains at two different locations have some fundamental differences leading this issue?

user16307
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  • Do you have good grounds at all the places. Are they wired correctly? – Kevin White Sep 02 '17 at 17:26
  • *is "Y" EMI filter* Actually I would not call the Y-rated capacitor a filter as it does not actually filter. What is does is that it provides a path for the RF signals from the mains side switcher through the transformer to the secondary (low voltage) side and then back through that Y-rated capacitor to the mains side and the switcher. This closes the loop for RF signals so that they have a short path that they can follow. If the Y-cap was not there the RF signals would radiate from all the wires connected to the SMPS and cause EMI problems. – Bimpelrekkie Sep 02 '17 at 17:44
  • *Can that be the mains at two different locations have some fundamental differences leading this issue?* Yes that can be part of the explanation, if the neutral line is not at ground level but at say 100 Vac then this adds to the ghost voltage. Also your local environment, mains grounding present or not, how insulated is the floor, carpet vs stone floor or what shoes you're wearing (or not) makes a difference to your connection to ground and your ability to sense the voltage. – Bimpelrekkie Sep 02 '17 at 17:48
  • " if the neutral line is not at ground level but at say 100 Vac" Can that really happen??? I thought neutral is tied to earth at substation and only some little voltage might appear at neutral. – user16307 Sep 02 '17 at 17:52
  • In some countries you can plug the cord in two ways. So at the laptop power supply, you don't have a specific wire neutral or live any more, it will vary. And power supplies have an EMI capacitor between mains side and isolated side so it matters which way the plug goes. Also, if a device has a grounded plug, it must be connected to a grounded outlet, and some old buildings don't have grounded outlets (except in certain areas like kitchen or bathroom) – Justme Oct 26 '19 at 08:14

2 Answers2

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The laptop charger is grounded and has an AC line filter to earth ground.

The secondary is floating and the laptop is floating u less earthed by external connections such as VGA cable to an earthed LCD.

Since the earth line filter shunts common mode currents via a CMchoke thru the Y caps to earth gnd, there exists some stray capacitance across the ferrite transformer at the switching frequency which lowers the impedance of the small stray coupling capacitance.

When your body is earthed by socks and leather shoes on wet grass or touching a stove, you become the return current path for this tiny leakage.

Since the frequency is higher than line, you may feel more of a burning sensation than a 2f buzz if touched by a small contact area ( and thus high current density ) on the conductive corner of the laptop. Knees are very sensitive to this I have found. It is just an irritant and not a safety risk, unless you have a quality control failure.

When you disconnect the charger, the path is broken. If you grasp the laptop while charging with a larger surface area, the sensation usually goes away due to lower current density.

Almost as bad as mosquitoes.

Tony Stewart EE75
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  • What you describe is why the phenomenon occurs. I appreciate that. But why it happens when I plug it to an AC wall outlet in a cafe but not at home? Can it be related to harmonics in that particular AC network? – user16307 Sep 02 '17 at 17:49
  • It must be the coupling capacitance of the chairs to earth or insulation resistance that makes the difference. Was my explanation consistent with your experience? – Tony Stewart EE75 Sep 02 '17 at 17:54
  • We call it stray leakage, not ghost voltage or current, all E fields are usually invisible but in this case limited by design of core interwinding capacitance. – Tony Stewart EE75 Sep 02 '17 at 17:57
  • It can be a concern for secondary Hipot conduction failures from sloppy SMPS insulation on discrete parts in bigger designs. I have experienced this and forced major players to change their test process to ground floating secondaries to stress primary ac paths resulting in internal failures. I even had to modify Hipot tester to current limit failures to become non-destructive, – Tony Stewart EE75 Sep 02 '17 at 18:01
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Touch voltages are sometimes found in devices that are designed with the expectation that exposed metal parts will be grounded, but which, in fact, are not properly grounded. If such a device is used with an outlet which has a proper ground, no touch voltage is felt. However, if the device is used with an outlet which has a missing or broken ground connection, touch voltage may be felt. Some outlets, especially in older buildings, may be constructed for 3 prong plugs, yet are not actually wired with a ground connection, or the ground connection has been broken. Such may be the case in the cafe where you feel a touch voltage when using your laptop, while you do not feel a touch voltage when using your laptop in other locations.

Fortunately, such problems are easy to diagnose with an outlet tester. The image below from Amazon is not intended as a recommendation or endorsement of any particular brand of outlet tester. There are many different brands available to choose from. The image is only meant to serve as an aid in recognizing such a device.

enter image description here

Simply plug the AC outlet tester into the outlets of interest, and read the results by matching the energized lights with the key found on the device. You may well find that you feel touch voltage only when your laptop is plugged into outlets that have an open ground.

Math Keeps Me Busy
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