seanspotatobusiness
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Danger of high voltage from HDMI cable

Thu May 16, 2019 2:48 am

I don't know how common this is but I recently noticed a tingle when I touched the ground of my Raspberry Pi while connected to earth for ESD protection. I ultimately determined that the HDMI connector was at 240 VAC and apparently this is normal, considered safe and is referred to as "leakage". If a 910 ohm resistor is placed between the metal television chassis and ground an RMS current of ~900 uA flows. Maybe this is safe for a human but could it be hazardous to the Raspberry Pi? If you short the HDMI cable directly to ground, you can make little sparks.

I'm not asking whether this is hazardous to health but is it hazardous to electronics? If so, I thought my post here might help make people aware of the risk of leakage-affected appliances harming their other electronics.
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Re: Danger of high voltage from HDMI cable

Thu May 16, 2019 8:34 am

900uA sounds a bit high, but not beyond the realms of possibility. Should still be safe. Note that it might not be the TV that is leaky, but anything also attached to the TV, e.g. Sky box or similar, as they all share a common ground plane.
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Re: Danger of high voltage from HDMI cable

Thu May 16, 2019 8:45 am

900uA through 910R implies about 820mV across the resistor. That sort of voltage level is not dangerous, nor are currents around 1mA. But the off-load voltage of 240V certainly is dangerous to a Pi, if there is a discharge path that hits something sensitive. As you found it will also give you a tingle, thoughnthe current is low enough not to cause a problem. Since the Pi power supply is almost isolated, iIf everything is connected before the monitor is powered, problems are unlikely. It's difficult to recommend mitigation measures without knowing the circuit that results in the stray potential -- it could be a reversed mains polarity (try reversing any two-pin mains connectors, if the design allows), or a faulty filter circuit. It is also impossible to know whether the condition is stable or not.
I am reluctant to give advice, but my feeling is that there should be a permanently connected discharge path (like your resistor) to earth at the monitor. Take this suggestion at your own risk, because I cannot see or investigate the full situation or monitor the consequences of such measures.
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Re: Danger of high voltage from HDMI cable

Thu May 16, 2019 11:16 am

I've never had a tingle ( :D ) from HDMI - I did have a Pi zero reset though when plugging it in so I now only connect cables (excluding USB) to it when the Pi is powered off.

An old friend of mine did blow himself across the room fixing our CRT TV though - and yes he was trained :lol:
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Re: Danger of high voltage from HDMI cable

Thu May 16, 2019 11:31 am

If you don't ground your TV, the filter caps in the supply will act as a capacitive divider for AC, which puts half mains voltage on the TV's chassis, its a common issue.

solution, ground your TV, as you should.

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Re: Danger of high voltage from HDMI cable

Thu May 16, 2019 11:45 am

mahjongg wrote:
Thu May 16, 2019 11:31 am
If you don't ground your TV, the filter caps in the supply will act as a capacitive divider for AC, which puts half mains voltage on the TV's chassis, its a common issue.

solution, ground your TV, as you should.
It was on and he touched the HT line :o
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Re: Danger of high voltage from HDMI cable

Fri May 17, 2019 8:15 pm

This thread is of considerable interest to me as in a prior life, I was a biomed tech in a hospital and one of my duties was to see that everything metallic within the patient's reach was at iso-potential ground. Most hospital rooms now have the TVs mounted high on the wall and out of reach but harder to control are the"goodies" brought in by family members to entertain the patient.

The problem in the old days was particularly severe and often an old "hot-chassis" TV would be brought in from home and set up on a bedside table. Invariably, the polarization of the power plug was defeated if present and the rabbit ears had long since been replaced by a wire coat-hanger shoved down into the antenna hole or an air vent until it contacted the chassis. I recall an ICU patient with more "hash" than ECG signal on the monitor trace and the nurse (who had made the trouble call) could not understand why it went away when the TV was unplugged: "You can't do that...it is their only source of entertainment." Might have lead to an early discharge as well :o !

Mahjonng's comments imply that the same problem may exist today with grounded inputs on the monitor/tv back being at 1/2 the mains potential.

Just for fun, I just put the DVM on the shield of my older ViewSonic (fluorescent back light) monitor and it had only 5-6 mv present but was connected to a grounded and operating computer and it is grounded through its mains plug. I will have to check our late model (LED back light) Samsung TV.

All the above not withstanding, I have often had the experience of an RPi rebooting when the HDMI plug is inserted.

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Re: Danger of high voltage from HDMI cable

Fri May 17, 2019 8:57 pm

I have a Microsoft Surface Pro. When plugged into it's charger I can feel that "electric vibration" as I gently run my fingers over it's metal chassis.

A while back I put a scope on it and looked. I could see it was flying up and own 100v at mains frequency with respect to ground.

Not dangerous and it disappears as soon you touch the chassis firmly.

I did wonder though how it might damage things like Raspberry Pi if connected some how.

I have seen RS485 transceivers blown repeatedly as the equipment at one end of the line was floating like that.

Properly grounding a Surface would be a tad inconvenient.

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Re: Danger of high voltage from HDMI cable

Sat May 18, 2019 12:06 am

Okay, it turned out that the live and neutral were switched on the extension cable I was using. Correcting this fixed the problem but I have another. The USB charger I'm using gives a negative which is 100 VAC relative to earth. When the live and neutral were the wrong way around this value was about 55 VAC. Could the USB charger have been damaged by the live and neutral reversal? Eventually my Pi will be powered by a PC PSU (which will be powering several other devices) whose ground will be connected to earth.

Could some of you guys check the voltage between earth (e.g. via a copper water/central heating pipe) and your USB charger ground (or Raspberry Pi GND pins? Remember to set your DMM to AC because DC might not show much.
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Re: Danger of high voltage from HDMI cable

Sat May 18, 2019 8:45 am

seanspotatobusiness wrote:
Sat May 18, 2019 12:06 am
The USB charger I'm using gives a negative which is 100 VAC relative to earth. When the live and neutral were the wrong way around this value was about 55 VAC. Could the USB charger have been damaged by the live and neutral reversal? Eventually my Pi will be powered by a PC PSU (which will be powering several other devices) whose ground will be connected to earth.
Polarity reversal is unlikely to have damaged the PSU.

The output of power supplies is frequently "floating" with regard to (mains) earth. This helps avoid hum from ground loops. Then there are mains filters built into the PSU or other equipment that will pull the output to somewhere around half mains, but as you found before the source impedance is high and very little current can flow (measure the voltage across a 1K resistor to earth, as before). The only concern is whether the initial connection of a cable with such a voltage can damage the Pi -- certainly there is evidence of causing resets. There should be no problems if connections are made before power is switched on.
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Re: Danger of high voltage from HDMI cable

Sat May 18, 2019 2:29 pm

I would not rely on getting live a neutral the right way around.

PSU output are generally isolated from the mains side. They just float around.

In a a lot of the world mains plugs an be plugged in either way around.

When I reverse the mains plug on my Surface Pro charger it makes no difference anyway, the "electric vibration" is still there and I can see it on my scope.

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Re: Danger of high voltage from HDMI cable

Sat May 18, 2019 3:00 pm

Most (switching) power supply have a filter to keep switching noise from reaching the mains cabling.
it consists of (seen from the PSU board towards the mains) of a common mode coil, followed by two (10nF high voltage) capacitors, one on each mains wire, connected to the earth wire (which is connected to the chassis/GND of the TV. if you don't connect the chassis to earth, half the mains voltage will appear on it from the capacitive divider consisting of: HOT-wire 10nF chassis 10nF Cold-wire.

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