Surface Pro 4 metal ring not at ground potential - Page 1
Forum section: electronics / projects

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: December 16, 2015, 09:13:48 am
content: Don't really know if I should put this here or in the "Other Equipment & Products section"I noticed this yesterday and this morning when writing on my new surface pro 4 (with charger connected) that my arm would get this slight yet sharp pinching feeling where it came into contact with the outer metal ring. I first thought this was just me being sensitive/having dry skin, but when it didn't go away I thought I might just check to make sure (I recognized the feeling from when I messed up the wiring in an active guitar, putting my strings at 20V to ground). Sure enough, I got the following result (see image).I measured a few dozen other points in the house just to make sure I wasn't measuring some other odd effect/had a broken multimeter. It's pretty much pure DC, as no AC voltage difference is measured.Is it just a matter of the surface charging like a capacitor with respect to ground? It's on wood tables and so on so there really isn't any place for potential charge buildup to go, but I'd think my body was a low enough resistance to allow for it?The charger doesn't have a grounding lug but that shouldn't be an excuse to have 20V DC offset... (or should it? What if I were to plug an audio cable into an old amplifier that had it's grounds tied to mains earth? Or a powered USB device? I think you get where I'm going)I haven't measured the impedance since I'm worried of breaking things inside.

author: Fungus
author_rank: Super Contributor
timestamp: December 16, 2015, 10:09:58 am
content: First: Your multimeter reading is meaningless.The two points you're connecting the multimeter to in that photo aren't related to each other because the charger is 'isolated'. There's no direct electrical connection between input and output.When two electrical systems are completely separated from each other we say they are "floating".You can connect two floating electrical systems together, no problem. When they're connected you can measure voltages and stuff because their values are now related to each other.And ... this is what's happening is that when you touch the tablet. You're making that connection between the two electrical systems (tablet and planet earth).Is it dangerous? No. No power can flow once a solid connection is made (if you grip the tablet firmly you won't feel anything at all).Can you do anything about it? Maybe. Try rotating the mains plug of the charger 180 degrees. This might change the voltage if the tablet's case from AC to DC. It's the fluctuating "AC" component that you're feeling.

author: dom0
author_rank: Super Contributor
timestamp: December 16, 2015, 10:26:47 am
content: Well they aren't completely separated, because there is a mutual capacitance in the power transformer of the charger and another (intentional) mutual capacitance in the Y capacitors from mains to output ground.QuoteDevices like audio gear can be destroyed by this, yes. The main issue is that cinch connectors are just piss poor connectors. The issue here is that their ground shell connects AFTER the center conductor. If it were the other way around like it is with almost every other coaxial connector there would be no issue at all, but this way the input has to withstand a high voltage (half mains) at a low current when plugging in. And some devices really don't like that ; insufficiently protected.That's were the old rule of "you should only plug/unplug cinch connectors when powered off" comes from.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: December 16, 2015, 10:54:05 am
content: Quote from: Fungus on December 16, 2015, 10:09:58 amAfter reading your comment (and feeling stupid for not realizing what you said), I tried measuring again, but this time I made sure my fingers made a firm connection on both the metal of the casing and the grounding lug. This changed the voltage to (only) 3-4 volts. If it were floating, wouldn't my body be enough to ground it?

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: December 16, 2015, 11:01:24 am
content: Quote from: dom0 on December 16, 2015, 10:26:47 amInteresting. I hadn't heard of that rule and never really thought of it when looking at the cinch connectors, but now I will keep it in mind!

author: amyk
author_rank: Super Contributor
timestamp: December 16, 2015, 11:37:41 am
content: http://www.aplomb.nl/SMPS_leakage/Doc_ie.htmlIt's the Y-capacitor, which forms a capacitive dropper between the mains and the secondary of the power supply. The capacitance is suitably low so you don't get a shock, but people vary in their sensitivity and low uA-range currents may be detected by some. 20V over 1M or 10M is ~2-20uA, which may be perceptible to you but not dangerous. Try the multimeter on the current range and see how much you get.The fact that it appears to be DC may just be due to some sort ofdiode effectbetween the (anodised?) metal case and the probe.

author: Fungus
author_rank: Super Contributor
timestamp: December 16, 2015, 12:29:04 pm
content: Quote from: TheUnnamedNewbie on December 16, 2015, 10:54:05 amYour body won't completely eliminate it because it has a resistance.There's two paths from tablet to earth: One through you and one through the multimeter.The multimeter has a resistance of 5-10 megaohms (usually).You can measure your own resistance with the multimeter, put one probe in each hand with the meter in megaohms range (I just got 2 megaohms when I tried it, grabbing something like a tablet with more surface area probably gives less resistance).So...Path (a) Multimeter, 5-10 megaohmsPath (b) Person, 2 megaohms.That's a ratio of about 4:1. Going from 20 volts to 3 volts? It's in the right ballpark, perfectly believable.Edit: The signal is AC and your meter is set to DC...

author: Fungus
author_rank: Super Contributor
timestamp: December 16, 2015, 12:45:58 pm
content: Quote from: amyk on December 16, 2015, 11:37:41 amYep.There's a capacitor connected from primary to secondary on the isolation transformer. This will reference the transformer's output to one of the mains AC wires. ie. It's not truly floating. There's a small capacitance between tablet and earth.If the capacitor goes to the live wire then you'll feel the 50/60Hz tingle of mains AC when you touch the tablet. If the capacitor goes to the neutral wire, you won't (or it will be greatly reduced).This is why reversing the mains plug can make the tingle go away.Edit: This raises a question. I know that capacitor is there, but... why? What purpose does it serve? Is it so there's never more than 220/100V difference between two separate mains powered devices?

author: mikerj
author_rank: Super Contributor
timestamp: December 16, 2015, 02:17:53 pm
content: Quote from: Fungus on December 16, 2015, 12:45:58 pmIt helps reduce the common mode noise being conducted back into the line connections.

author: mich41
author_rank: Regular Contributor
timestamp: December 16, 2015, 09:58:35 pm
content: There is significant variation between chargers. The worst USB charger I have delivers about 10mA while some others easily stay under 1mA. Of course grounded one would be best.Quote from: Fungus on December 16, 2015, 12:45:58 pmIt's connected to negative output of a bridge rectifier. You'll get your tingle either way.

author: amyk
author_rank: Super Contributor
timestamp: December 17, 2015, 11:51:09 am
content: 10mA!? Are you sure that's not uA? The safety regulations limit it to 300uA (0.3mA) if I remember correctly. 10mA is high enough to be dangerous...This video shows how the effects of 5-6mA are already really, really painful:https://www.youtube.com/embed/wR6g38Pxwog

author: dom0
author_rank: Super Contributor
timestamp: December 17, 2015, 12:02:56 pm
content: 10 mA (continuously) should trip most 30 mA RCDs.

author: tooki
author_rank: Super Contributor
timestamp: December 17, 2015, 02:18:21 pm
content: Quote from: TheUnnamedNewbie on December 16, 2015, 09:13:48 amThis effect isn't limited to that device. I had the same thing with my old Apple PowerBook G4, which had an aluminum housing. Using the 2-prong AC plug into the charger resulted in slight tingling on the metal skin of the machine. Using the 3-prong AC cord grounded the whole thing and the tingle went away. It was quite common, I hear. (Not sure if modern models are any different -- I am still using an all-plastic MacBook at the moment.)

author: mich41
author_rank: Regular Contributor
timestamp: December 17, 2015, 02:39:24 pm
content: 10mA peak, sorry. RMS was probably only few milliamps or less as it was a highly distorted sine with narrow peaks.And yes, it was easily felt. Why do you think I ended up measuring it?

author: Fungus
author_rank: Super Contributor
timestamp: December 17, 2015, 02:52:29 pm
content: Quote from: tooki on December 17, 2015, 02:18:21 pmYep. It seems common. Google search gives a lot of hits.And it's not really surprising. Power suppliesaredesigned that way. Anything with exposed metal that's connected to the device's ground will tingle.