Phone charger gives electric shocks, How to measure voltage. - Page 1
Forum section: electronics / beginners

author: aljowen
author_rank: Newbie
timestamp: January 06, 2016, 01:11:11 pm
content: -Backstory-I have a phone charger that was purchased from Maplins (made by intertek) and it gives very painful electric shocks whenever touching the ground connectors on the usb ports, The charger itself has no ground connection on the plug socket. This is the second charger of this model with this issue since i returned the first. I am now also living in a different county to when I had the previous charger so I can probably rule out the buildings power. I did electronic products in school but its been a long time and we never did ac, only dc, so any limited electronics knowledge that I may have had is now fairly rusty.-Issue-I want to correctly measure the voltage that is being output on the ground connector. From what i know about dc you measure across the component you want to check. But in this case it is a flying wire and touching it will shock you without any ground connection, so i presume that means it is AC but i could be wrong in assuming that. It also means there is no component to measure across.So I have tried a couple different methods all giving different results. I have no idea if any of them are correct.-Images-This is with the red connected and the black left flying.This was with red connected to the charger and black grounded on my computers usb port.This was with red on charger and the black grounded on my computer case.Thanks, Any help would be appreciated

author: Jeroen3
author_rank: Super Contributor
timestamp: January 06, 2016, 01:21:44 pm
content: Measure the USB Shield against mains earth in your socket. That can also be water pipe, assuming you have copper plumming.Or literally a metal rod in the ground.You are measuring AC voltage through the Y caps. Here you'll see why:https://en.wikipedia.org/wiki/Film_capacitor#Safety_and_EMI.2FRFI_suppression_film_capacitorsTry to measure the AC current (mA range) to earth (not water pipe!) should be less than 1mA. If the RCD (or GFCI) trips or the multimeter fuse blows, return or bin the device. It's a death trap.

author: rrinker
author_rank: Super Contributor
timestamp: January 06, 2016, 01:26:45 pm
content: Wow, not sure how you get almost 5 volts with only 1 lead of the meter even connected to anything - that appears to be a battery powered portable multimeter, correct?That last one though - BAD! Your computer case is likely connected to the power line ground, so between the power line ground and the ground side of the USB port on the charger you have over 100 volts! No wonder you get a shock! One way I can think this would happen would be that the USB charger had the hot and neutral lines crossed up internally - is the plug polarized in any way? In the US it is common for one blade to be wider than the other and the sockets should match, so that you can only plug a device in one way to prevent issues like this (now if the wiring from the outlet back to the breaker is poorly done, that's another story...). If the plug on the charger can fit either way, test it with the plug reversed and the 102V reading should go down - it should be 0, or close to it. Even if it does - this would appear to be a rather dangerous device and I wouldn't want to fool around with it much other than to take it apart and see how badly the built it. I would definitely not be plugging any of my phones or tablets into that thing.

author: wraper
author_rank: Unknown
timestamp: January 06, 2016, 01:31:46 pm
content: This kind of topic appears every few weeks here. You could try to search the forum for it.https://www.eevblog.com/forum/beginners/switching-power-supply-y-capacitor-between-primary-and-secondary-ground/https://www.eevblog.com/forum/beginners/ipad-electric-shock/https://www.eevblog.com/forum/projects/minor-electric-shocks-from-equipment-with-smps/https://www.eevblog.com/forum/beginners/ac-electrical-leakage-from-surface-pro-3!/

author: wraper
author_rank: Unknown
timestamp: January 06, 2016, 01:36:08 pm
content: Quote from: rrinker on January 06, 2016, 01:26:45 pm

author: rrinker
author_rank: Super Contributor
timestamp: January 06, 2016, 01:49:51 pm
content: You can say it's low current leakage all you want, but I have NEVER EVER been shocked by any one of dozens of computers I have built, all with switchmode power supplies, of course, nor by any of my iPhones or iPads. Or any other small metal devices that have switchmode power supplies. Not once. If it's strong enough to feel it's not right, period. If you feel it with a so-so ground, how much current will flow if you were well and truly grounded, say worst case, barefoot on a wet concrete floor?

author: wraper
author_rank: Unknown
timestamp: January 06, 2016, 02:01:46 pm
content: Quote from: rrinker on January 06, 2016, 01:49:51 pmThen put one hand on exposed metal part of your grounded PC and second hand on the macbook which has a double insulated PSU and see what happens.P.S. Some people can touch mains voltage and feel a small tickling at worst, others will receive a huge electric shock and pass out. There is a such thing as skin electric resistance and it's very dependent on particular person.

author: rrinker
author_rank: Super Contributor
timestamp: January 06, 2016, 02:08:10 pm
content: Just tried it with my iPad and my PC sitting next to my desk, all metal - also touched the slightly exposed metal ground of a USB cable plugged in to the desktop just in case somehow the case wasn't grounded, which is highly unlikely. No shock. Took my iPhone out of its case as well so I could touch the metal of it while plugged in to the charger, and also touched my computer case - also no shock. Don't have a meter handy to see if there is a potential there but too low to shock me - but I still say if it's great enough to feel a shock it isn't right.

author: wraper
author_rank: Unknown
timestamp: January 06, 2016, 02:18:15 pm
content: I'm not sure if ipad chargers have Y cap inside, small power chargers (<15-20W) often do not have them. Therefore I suggested macbook as I'm certain they do have them. Also, measuring voltage won't tell too much. Useful measurement would me measuring AC current from the GND from the output of the charger to the earth pin of the mains socket. If the current is more than about 0.5mA for low power equipment, it would be a serious concern.

author: rrinker
author_rank: Super Contributor
timestamp: January 06, 2016, 02:56:14 pm
content: I'll have to see what other sort of goodies I can check with, a MacBook is one thing I don't have - much as I love my iPad an iPhone, OSX just doesn't do it for me. And my Windows laptop has a grounded power supply. As does the external monitor connected to it (I'm now in my office and not at home).It's all relative, but I don't think I'm one of those high resistance people, I can get measurable readings with both a cheap crap multimeter and a Fluke, and while not exactly the same thing, I have no problem working capacitance touch devices. It's getting cold and dry here, and I can build up a quite substantial static charge in myself and get a good zap - bad time in my part of the US to work without a proper static strap, and the dogs and cats start avoiding people.

author: aljowen
author_rank: Newbie
timestamp: January 06, 2016, 03:34:57 pm
content: I am in the uk. So our plug sockets can only go in one way. The phone charger has a plastic ground pin on the plug socket and as such is not grounded. Since this is the second one with the exact same issue (bought many months apart too, so not just a single batch) I presume it is a design flaw of the device. Its painful enough that i have to remove my finger from the connector as soon as i touch it since it feels like its burning my skin.My computer case is screwed to the power supply which is a cooler master silent pro 80+ gold rated psu, the metal case of that power supply will be mains grounded within the power supply, so the computer case is mains grounded. The front usb hub that i used on the computer is a sata based hub i think but i cant remember, either way that will be grounded through my computers motherboard in some way or another i would presume. I have never received a shock from any of the computers ports.The multimeter that i am using is this one:http://www.amazon.co.uk/Crenova%C2%AE-Multimeter-Detector-Portable-Backlight/dp/B014IZF7HI/ref=sr_1_4?ie=UTF8&qid=1452093046&sr=8-4&keywords=multimeterIt is powered using a 9v battery.Thanks for the links to other articles. They did a good job of giving the gist as to what is going on.I have measured the current and it is about 110 microamps AC. The frequency is about 0.65Khz.

author: IanB
author_rank: Super Contributor
timestamp: January 06, 2016, 03:51:13 pm
content: If the current is in microamps then there should not be any danger. You are not normally expected to touch the metal parts of a USB plug or socket since they invariably have a plastic moulded outer shell. Make sure to hold the USB plugs by the plastic parts when you plug and unplug them.

author: aljowen
author_rank: Newbie
timestamp: January 06, 2016, 04:13:49 pm
content: Quote from: IanB on January 06, 2016, 03:51:13 pmWhile it may not be dangerous it does hurt quite a lot, so I think I will return it on that basis and find a charger that wont zap me whenever I touch a connector on it. Also any devices plugged into it then receive the same ground current, so my tablet gives me shocks since it has docking connectors on its chassis.The worst problem is if i am listening to music on my phone. My In Ear headphones have a metal chassis and are grounded to the phone through the 3.5mm jack. So if i connect my phone to the charger while listening to music it shocks me through my ears, which is incredibly painful and caused skin pain for a day or so after being shocked on the last charger i owned of the same model. At the time I had no idea what caused it. But was then immediately shocked through my hands when pulling them out of my ears too :p

author: Tomorokoshi
author_rank: Super Contributor
timestamp: January 06, 2016, 04:51:17 pm
content: Would it be possible to open it up and get some pictures of the circuit board?

author: aljowen
author_rank: Newbie
timestamp: January 06, 2016, 05:09:11 pm
content: Quote from: Tomorokoshi on January 06, 2016, 04:51:17 pmNot the current one because i would like to be able to return it. The other old charger is at my parents house as well. I will see if I can get my dad to take it apart and send some photo's to me.I did take the old one apart though after they gave me the new one. But all I can remember is that they had 2 separate pcb's inside, one for the I/O and one for everything else. The I/O had two wires leading to it for +ive and -ive and then all 4 usb ports attached onto that. So they were powering all ports from a single charging circuit and I presume they had resistors to manage the current output for each port individually since 1 port is 2.1A and the others are 1.0A.

author: Simon
author_rank: Unknown
timestamp: January 06, 2016, 05:42:55 pm
content: Maplin sells garbage that make dodgy chinese product on ebay look like gems of quality technology..... Take it back or better contact trading standards (well consumer direct who will ignore you). I never buy from maplin, they just sell expensive garbage.Quote from: aljowen on January 06, 2016, 01:11:11 pm

author: mikerj
author_rank: Super Contributor
timestamp: January 06, 2016, 06:03:41 pm
content: Quote from: rrinker on January 06, 2016, 02:08:10 pmThis is very common on switch mode chargers, I'm surprised you've not felt this before. My original iPod and iPhone, both with a genuine Apple chargers used to show about 110v AC w.r.t ground on the case and felt quite unpleasant to touch though it wasn't exactly a shock as such.

author: _Andrew_
author_rank: Regular Contributor
timestamp: January 06, 2016, 06:26:05 pm
content: I have come across this with Mac Books before where your touching the case of the laptop and also happen to touch something else that is connected to the protective earth of the mains supply. Not painful but noticeable.If you look in the IFU that we all leave in the box and never look at for the phone there will be an instruction to say not to use the device while it is connected to a charger.My Mega PAT-420 has the facility to perform differential leakage current testing and it is quite surprising how often I see new appliances that have leakage currents near the permitted limits. Surge arresting gang plugs ect often exceed the limits on differential testing.Reference Page 109 of the IEE Code of Practice for In-service Inspection and Testing of Electrical Equipment ISBN 978-0-86341-833-4“V.3.6 Earth leakage testThis test is carried out at the rated mains input voltage and the maximum leakage should not exceed the following.> Class I products: 3.5mA (0.75mA for hand-held appliances)> Class II products: 0.25mA”

author: _Andrew_
author_rank: Regular Contributor
timestamp: January 06, 2016, 06:28:07 pm
content: Should also say that differential leakage current testing should be done with test equipment designed for the purpose.Don't go using a regular amp meter. If somthing is wrong things could go badly wrong.

author: electr_peter
author_rank: Unknown
timestamp: January 06, 2016, 06:41:37 pm
content: Quote from: aljowen on January 06, 2016, 04:13:49 pmWhile it is not dangerous as it is, people sensitivity to small currents vary a lot - some people feel it as a sensation, some as a pain under identical conditions. If you know that sensation really well, you can find this sensation (meaning small AC current leakage) in many places.That said, I still do not see anything that is really wrong with device function it self.This is what happening in my opinion:when powered from AC-DC charger, headphones transmit small leakage current though primary-secondary noise suppression capacitors -> headphones with metal parts -> grounded body to ground. This will happen with any charger (to a bigger or smaller effect) which is not grounded or has only two pins in mains plug.person in question is very sensitive to small currents. Especially, because metal partstouch inside of his/her ears.Again, described situation is pretty normal under circumstances, I see no real signs of failure. All symptoms are due to normal AC leakage through parasitic capacitances.To remove the problem, use one of these methods:change headphones to other ones with no exposed metal parts (especially inside ear) ORdo not listen to music while charging with ungrouded AC-DC adapter or one with two pin mains plug ORget grounded AC-DC adapter (with three pins mains plug) ORbuy bigger battery pack so you can use item for longer between recharges

author: Simon
author_rank: Unknown
timestamp: January 06, 2016, 06:52:23 pm
content: Err some men devised the earth pin for a fucking reason! lots of other men seem to think the laws of nature don't apply to them when designing this shit!

author: wraper
author_rank: Unknown
timestamp: January 06, 2016, 06:55:13 pm
content: Quote from: aljowen on January 06, 2016, 03:34:57 pmAbout an average of what you usually get from power supplies with similar power rating.

author: electr_peter
author_rank: Unknown
timestamp: January 06, 2016, 06:59:45 pm
content: Quote from: Simon on January 06, 2016, 06:52:23 pmSo... I hope you understand the reasons you say here and contribute to forum topics with meaningful posts.Earthing/grounding is not trivial topic and not a holy grail by any means to put it shortly.

author: Simon
author_rank: Unknown
timestamp: January 06, 2016, 07:01:29 pm
content: Quote from: electr_peter on January 06, 2016, 06:59:45 pmThe output is isolated, if there is leakage then it makes sense to connect the negative of the output to earth, It's fairly simple actually.

author: rrinker
author_rank: Super Contributor
timestamp: January 06, 2016, 07:01:53 pm
content: Quote from: mikerj on January 06, 2016, 06:03:41 pmMine's a 4th gen iPad, but it is connected to a genuine Apple charger, the one it came with. I keep one permanently set up at my desk to charge both the iPad and iPhone, and I have another in my travel case in case I need to charge away from home. Never tried a non-Apple AC mains charger, though I did have a random one in my car. But yeah, never felt so much as a tingle while touching the metal back of the iPad connected to the charger and some other device connected to earth ground. Now I am curious since everyone seems to think this is pretty much normal, when I get home I plan to give it a test and see if there is measurable voltage there at all. I don't think there should be any difference other than line voltage, since I'm in the US it's 120V. My house is new enough that all outlets are equipped for a ground pin, and at least for the outlet when my computer and the phone charger are connected, one of those basic line testers shows the line, neutral, and ground all properly wired.Anyway, just curious for my own sake now, since I have never experienced an obvious shock of this sort, whereas the OP is getting what sounds like a rather nasty shock which to me just sounds dangerous and not merely an annoyance.

author: Simon
author_rank: Unknown
timestamp: January 06, 2016, 07:05:39 pm
content: It is not normal to get shocks or tingles from mains devices. If you do it means you are either misusing the product or it was designed badly and dangerously.

author: IanB
author_rank: Super Contributor
timestamp: January 06, 2016, 07:20:00 pm
content: Quote from: rrinker on January 06, 2016, 07:01:53 pmMy Apple chargers make a non-contact volt stick light up and beep. If I plug my phone in to charge the whole case of the phone activates the voltage detector. So there is a definitely a mains electric field getting through the USB output cable. I don't feel anything when I touch it though.

author: IanB
author_rank: Super Contributor
timestamp: January 06, 2016, 07:21:30 pm
content: Quote from: Simon on January 06, 2016, 07:05:39 pmMany actual products disprove this assertion. Just because it tingles doesn't mean it is dangerous.

author: electr_peter
author_rank: Unknown
timestamp: January 06, 2016, 07:39:17 pm
content: Quote from: Simon on January 06, 2016, 07:01:29 pmProper earth pin can be a solution, I agree. Other mentioned solutions with higher insulation can work as well.Grounding works only when it is properly implemented insystematic approachand no errors are made in the wiring (I guess 99.9% of population and many electricians do not have a clue how earthing/grounding should work and be wired. Have you seen examples ofprofessionallywired sockets? I make no assumptions for a while now.). One wire snap in wrong place can be a complete disaster for grounded approach depending on circumstances.My point is thatisolatedmeans high resistance path to AC mains andgroundedmeans your are holding one side of the transformerwhile thinking your are 100% safe. I think neither approach is inherently 100% safe - it depends on many factors and system level considerations.---Leakage current itself exists because "isolated" device is floating at ~half the mains AC voltage - thisunavoidable by defaultwith ungrounded items (all most all voltage sticks/pens and many multimeters can illustrate this). Small parasitic capacitances act as a current limiter which results in small varyingnon-harmfulcurrent. So people are very sensitive to this, so are not. And it depends on circumstances a lot (humidity, skin resistance, etc.).If I recall correctly, sensitivity limits of human senses are measured when ~50% of population starts to see/feel an effect. With huge variation in human sensitivity levels, some are very sensitive to this particular effect.

author: Nerull
author_rank: Frequent Contributor
timestamp: January 06, 2016, 07:56:52 pm
content: I've felt it with an iPod using included charger.A device CANNOT be both double insulated class II and have a grounded chassis. They're mutually exclusive. Demanding that a class II charger ground reference its output is demanding people violate electrical standards. Just sticking a ground wire on does not make it safe, and class II is generally considered safer than class I, as a wiring fault cannot make it dangerous.

author: mikerj
author_rank: Super Contributor
timestamp: January 06, 2016, 10:26:15 pm
content: Quote from: Simon on January 06, 2016, 07:05:39 pmIt's quite normal on double insulated switch mode supplies. The manufacturers could leave out the EMI suppression cap to reduce the leakage, but then it almost certainly wouldn't meet any conducted emissions standards. How do you propose to fix this and maintain compliance to the various standards?

author: aljowen
author_rank: Newbie
timestamp: January 06, 2016, 10:42:33 pm
content: Couldn't they add a ground pin to the plug, then connect the ground connections on the usb port to that rather than connecting the ground connections on the usb port to something emitting ac current?I would better understand if i was touching powered terminals on the usb cables. Even if they hadn't of connected the ground pins to anything i would have thought that would prevent this issue (although perhaps cause some for connected usb devices that intend to use the ground connection.Out of interest would putting a diode and resistor on the ground to allow current to only flow from the connected devices to the charger itself have helped?Just tested this ZTE wall wart out of interest and it was only putting out 0.3v AC on the ground connector, it also has no ground pin on the plug socket. So there must be ways of not electrocuting people on cheap chargers.

author: pmbrunelle
author_rank: Regular Contributor
timestamp: January 07, 2016, 04:34:51 am
content: Quote from: mikerj on January 06, 2016, 10:26:15 pmWe have had the solution for over a century: mains frequency transformers.Funny how this 21stcentury stuff can give you an electric shock(while the old wall warts did not), yet this is considered progressIf your device has exposed metal parts that are connected to the internal circuitry(USB, audio jack, whatever), it's not really double-insulated.

author: Brumby
author_rank: Unknown
timestamp: January 07, 2016, 04:44:39 am
content: Quote from: pmbrunelle on January 07, 2016, 04:34:51 amThat thought had crossed my mind as well.The trouble is that SMPS are cheaper, lighter and usually smaller. All the manufacturers have to do is build them to meet the relevant standards - and that's what tends to happen.

author: Jeroen3
author_rank: Super Contributor
timestamp: January 07, 2016, 07:35:59 am
content: Quote from: aljowen on January 06, 2016, 10:42:33 pmI don't think you understand what is happening. Draw up a schematic first. From the transformer in your street/house, up to you being the (capacitor to) ground.

author: aljowen
author_rank: Newbie
timestamp: January 07, 2016, 01:44:54 pm
content: Quote from: blueskull on January 07, 2016, 05:13:07 amTo be honest that is probably the reasoning in full.

author: Zero999
author_rank: Super Contributor
timestamp: January 07, 2016, 02:10:51 pm
content: Quote from: mikerj on January 06, 2016, 10:26:15 pmAdd a screen between the primary and secondary and connect the Y capacitor to it.Quote from: pmbrunelle on January 07, 2016, 04:34:51 amUnfortunately mains frequency transformers don't comply with the minimum efficiency regulations, especially at very light loads such as standbyQuote.Exposed metal parts, connected to internal circuitry, still need to have double or reinforced insulation from the mains. Fortunately Y1 capacitors provide reinforced insulation and comply with the regulations, even if the tiny leakage current is enough to give someone a tingle.The real problem is when many appliances which use unearthed switched mode power supplies are connected together. The leakage currents add together and can exceed the regulatory requirements.

author: pmbrunelle
author_rank: Regular Contributor
timestamp: January 07, 2016, 05:49:19 pm
content: Perhaps by regulatory definitions a few nF counts as isolation, but I think a few (tens maybe) pF is more reasonable.As for standby losses, the real problem is that very few products come equipped with power switches anymore.Mostly, consumers are to blame for this whole mess.

author: Monkeh
author_rank: Super Contributor
timestamp: January 07, 2016, 06:19:01 pm
content: Quote from: rrinker on January 06, 2016, 02:08:10 pmYou'd get at most around 60VAC. Which you most likely would never feel.Quote from: Simon on January 06, 2016, 07:05:39 pmYes, it is. A HUGE number of devices are double insulated, and seeing potentially painful leakage currents on them is quite normal. Many if not most TVs, Bluray players, satellite tuners, and so on, have no earth connection, and unless they pick one up through external cabling (cable TV will provide one, some satellite installations, and often you sink enough through the boxes buried in the walls with terrestrial), they will bite you.I dealt with a >$3000 TV recently which was quite happy to give me a tingle and any analogue audio a very impressive hum.