spiffy_malice
Posts: 2
Joined: Wed Oct 17, 2018 9:04 am

USB electrical heat/zap

Wed Oct 17, 2018 9:08 am

I’m having some curious heat issues with my new 3B+ USB ports.. IE, when I have one or no controllers plugged in, if I run my finger along the USB ports I get a hot zap, like an electrical tingle that’s a small bit painful, like it was very hot, but not gradual-computer-heat-hot, more like zappy hot.

Curiously, if the power adapter cable is pulled out and only the HDMI is plugged in, it still occurs for a little bit. If I pull the HDMI cable out (plugged to the TV and nothing else), and run my finger over the exposed plug end, I get the same feeling.

When multiple USB plugs are in, and I jam my fingers around the cracks, I don’t really feel it. Maybe because all the plugs block the ‘current’ (though putting in two or three plugs, and touching the third-fourth, does not cause the zap, maybe all the current is diverting to the right places?)

So, questions…

a) HDMI cables carry current? Should I feel the zap at the end like that, is my cable faulty? Is it the tv?
b) should the PI be carrying a zap all the way to the USB ports to the point where I feel it from the outside the case? Is this normal PI electrical conductivity?
c) is this zap going to burn something if it’s pressed up against fabric, ie, sofa, cushion…
d) my supplier is happy to return and exchange, but I’d rather not if it’s 1-normal ? 2- the cable rather than the pi… what’s the course of action here? Can some kind people jam their fingers around the USB ports when the HDMI / power and 1 or 0 USB controller(s) are plugged in and tell me what's normal?

User avatar
mahjongg
Forum Moderator
Forum Moderator
Posts: 13238
Joined: Sun Mar 11, 2012 12:19 am
Location: South Holland, The Netherlands

Re: USB electrical heat/zap

Wed Oct 17, 2018 9:12 am

Ground your monitor/TV, without grounding it, the power supply filter in your TV will leak about half the mains voltage to the ground wiring of your TV.
Because your PI has a double isolated supply, the voltage on the ground coming over the HDMI cable is not shorted to earth.

spiffy_malice
Posts: 2
Joined: Wed Oct 17, 2018 9:04 am

Re: USB electrical heat/zap

Wed Oct 17, 2018 9:57 am

Hmm.. that sounds like a reasonable possible explanation. But doesn't the third prong of my tv cable provide the grounding? I'm in the UK, the third prong is standard. But if for some reason this isn't the case...

a) how do I go about grounding my TV? Running a wire from something to something? Right now the tv is on a wooden bookcase cabinet above a floating victorian wooden floor. What needs to go where? Here is a photo.. the TV has now been installed on the shelf above the little girl, running cables I've cut through the back of the panel to the outlets (via a surge protector) https://photos.app.goo.gl/jkGGyEtJFHy8JpWz5

b) If i don't ground it... is there any danger / potential of harm to the Pi / TV / my kids, my home etc.?

lbdroidman
Posts: 13
Joined: Wed Oct 17, 2018 7:46 pm

Re: USB electrical heat/zap

Fri Oct 19, 2018 7:30 pm

Wow, you've got A/C driving into the ground plane of your exposed and sensitive electronics and you're asking if something could get fried?

Let me be perfectly blunt: Until you get that grounding sorted all out, unplug EVERYTHING. If you can feel the A/C by touching the USB jacks, then just imagine what the CPU will feel if you touched an I/O pin.

drgeoff
Posts: 10962
Joined: Wed Jan 25, 2012 6:39 pm

Re: USB electrical heat/zap

Fri Oct 19, 2018 7:49 pm

lbdroidman wrote:
Fri Oct 19, 2018 7:30 pm
Wow, you've got A/C driving into the ground plane of your exposed and sensitive electronics and you're asking if something could get fried?

Let me be perfectly blunt: Until you get that grounding sorted all out, unplug EVERYTHING. If you can feel the A/C by touching the USB jacks, then just imagine what the CPU will feel if you touched an I/O pin.
That is probably OTT.

It is quite common to have a small amount of AC leakage from mains input to the groundy side of external connectors and under the right circumstances it can certainly be felt. One of the reasons why instruction manuals invariably tell you to connect everything up while everything is unplugged from the power sockets.
Quis custodiet ipsos custodes?

User avatar
mahjongg
Forum Moderator
Forum Moderator
Posts: 13238
Joined: Sun Mar 11, 2012 12:19 am
Location: South Holland, The Netherlands

Re: USB electrical heat/zap

Sat Oct 20, 2018 11:27 pm

the "leak" is typically caused by two, (typically 10nF capacitors) connected between both (hot and cold) mains leads on one end, and GND on the other, as a "capacitive ac divider" it will thus divide the mains voltage typically in two, if the GDN node is isolated from Earth.

a sensitive person might feel this voltage, but as it has 10nF in series it won't provide much current flow.

still if that voltage happens to be distributed across some semiconductor, then it will probably damage it, but that will only happen when one end of that semiconductor has a path to earth for the current to flow to.

coltrane
Posts: 12
Joined: Fri May 15, 2020 6:44 am
Location: Portugal

Re: USB electrical heat/zap

Mon May 18, 2020 8:57 pm

I just bought the 3 amp Pi 4 power supply (had started getting more and more "thunderbolt" alarms with the 2.5 amp from the Pi 3B+, I guess due to warmer weather) so I could finally play with an external drive. The external drive is USB powered and has aluminium casing, and for the first time I felt the tingling when lightly touching the drive casing.
In Portugal mains plugs are not polarized, so I tried reversing the TV plug and the Pi supply but the tingling remained on every combination.
Further investigation showed that it's enough to put a firm glow on a pole-detector screwdriver rated at 200-250 V (but disappears as soon as I "ground" the metal case with my finger).
Disconecting the TV proved it comes from the Pi supply and flows both via the USB and HDMI ground. A quick test with the old power supply showed the same problem. It happens that TV plugs are not grounded around here (that's only found on kitchen appliances) and I understand it's a minute current, but I plan to work with audio and worry about mains "hum".
I'm thinking of running a suitable resistor from the Pi ground to the ground socket on the wall, to simulate the finger "grounding".

Out of curiosity I tested two phone chargers and none showed the leak (with the pole-detector).

Return to “Troubleshooting”