I felt a tingle! Surrounded by faulty electronics? - Page 1
Forum section: electronics / beginners

author: seanspotatobusiness
author_rank: Regular Contributor
timestamp: May 11, 2019, 06:19:53 pm
content: I was having a problem with my Raspberry Pi and wanted to check GPIO pins with an LED to check they're still functioning. I had my grounding strap on (1 Mohm) and my finger touched the resistor and I felt an unexpected tingle. When I checked with my multimeter in DC mode, I found 0 V between the resistor leg and the place where my ESD strap connects. When I switched to AC mode, I find ~200 V. I confirmed it by connecting a neon bulb which glows brightly. After some messing around, I found that the HDMI cable (the large metal part) to my BUSH flatscreen is providing the 230 V AC relative to ground. After further messing around, I find that three different USB power supplies are also giving 45-55 V on GND which is enough to make my neon bulb glow feintly.My ESD ground is the copper pipe of a central-heating radiator.If I connect the outside of the HDMI cable directly to ground via my multimeter, I measure ~1 uA but I would have expected much higher to get a good glow out of my neon bulb?This is probably a dumb question but could live and neutral being switched somewhere cause something like this to happen? I don't otherwise have any reason to expect live and neutral to have been switched somewhere but I thought I'd ask.

author: Yansi
author_rank: Super Contributor
timestamp: May 11, 2019, 06:24:12 pm
content: Thata because each power supply has some (small or more) capacity between primary and secondary side. This capacitance is either parasitic or deliberate (noise and EMI suppression).Such capacitance (typically in teh range of 0.1 to 10nF) usually allows a very minor safe current to flow when the secondary is connected to a grounded point.

author: ogden
author_rank: Super Contributor
timestamp: May 11, 2019, 06:49:17 pm
content: Right. Y capacitor it is.Using central heating radiator or any pipe for AC mains ground is dangerous and even potentially deadly. Don't do it. You can kill yourself or your relatives/neighbor. In case your neighbor have similar ideas of grounding, he can kill yourself. Chances of ground faults in piping are increasing in modern era of plastics (and simple negligence). Either install AC mains electricity for you lab with proper ground or simply live w/o such. You will be fine by connecting all ground terminals of your equipment, supplies and ESD mat/bracelet together without connecting it all to anything. Just learn simp[le rule - when you come to your table in your lab, touch ESD mat first before touching anything else. Problem with your ESD strap connected to ground which is not the same as supply ground - if you touch anything in your circuit, it receives voltages you noticed. In fact such "protection" does not protect but endanger your circuits.

author: seanspotatobusiness
author_rank: Regular Contributor
timestamp: May 15, 2019, 12:05:25 pm
content: Thanks for the responses. The television must be faulty though, to give the full mains voltage on exposed metal parts? It's not shorted to live because the current through a 910 ohm resistor is ~0.9 mA when connected between the chassis and ground. The moulded plug has an earth pin but it's not connected to anything. The cable only has two wires in it. Would this even meet UK regulations for consumer electrical goods? It was made by the presumably reputable Bush brand (model LY24M3). I thought if it has exposed metal that could become live then it has to be earthed. I thought the UK had pretty good electrical safety regulations.Is that screw in the corner intended to have some kind of earth connection, do you reckon?

author: dmills
author_rank: Super Contributor
timestamp: May 15, 2019, 12:20:37 pm
content: It is perfectly possible to build a class II (double insulated) product in a metal case, not easy but doable if the volume demands it, and in fact if you look you will see a sheet of insulating plastic forming the secondary insulation behind the power supply board.Does the case have the usual square within a square symbol indicating double insulation? For a consumer telly I would expect it to.900uA is on the high side of normal, but not necessarily a problem.Regards, Dan.

author: golden_labels
author_rank: Super Contributor
timestamp: May 15, 2019, 12:34:38 pm
content: seanspotatobusiness:“Q.C.passeds” in the upper right quadrant.That GD1 seems a bit far from the 250VAC input (CON102). On the other hand C123 and C124 look like Y capacitors and each of them has a leg close to GD1. Y capacitors are always connected to ground, so GD1 may indeed be a ground connection to the screw that holds PCB to the case. But if there is no grounding pin on the plug, Y capacitors will leak AC voltage to GD1. The current they can pass is very low, so it would not harm you, but may be enough to give you tingling.

author: wraper
author_rank: Unknown
timestamp: May 15, 2019, 12:50:24 pm
content: Quote from: dmills on May 15, 2019, 12:20:37 pmActually there is barely any difference with plastic case. Usually some conductive parts will be exposed to the outside anyway (say phone charger). With metal chassis you just need to place insulation sheet under PSU PCB if gap is small.

author: seanspotatobusiness
author_rank: Regular Contributor
timestamp: May 15, 2019, 12:53:47 pm
content: It does have a square within a square on the label.Even if this voltage isn't a threat to my health, I'd still like it gone because I think it's a threat to my Raspberry Pi and attached electronics which I want to connect via HDMI. I think I can replace the cable and connect the earth to that screw.

author: Zero999
author_rank: Super Contributor
timestamp: May 15, 2019, 01:00:07 pm
content: Quote from: ogden on May 11, 2019, 06:49:17 pmExcept he isn't using the radiator as an AC mains ground, but an ESD ground, which doesn't have to be perfect, hence the 1M resistor.At one place I worked, the ESD ground, was just a large piece of copper tape glued to the wall. A few MΩ between the ESD and mains grounds is more than acceptable.Quote from: seanspotatobusiness on May 15, 2019, 12:53:47 pmY-capacitors are fairly standard in switched mode power supplies. They ensure the secondary doesn't float at a high frequency, by shunting the capacitively coupled charge across the transformer, back to the primary side.You could earth the negative side of your Raspberry Pi. If you don't want to do it directly. Connect it to mains earth via a 100nF, 400V capacitor.