Minor electric shocks from equipment with SMPS

[zero_budget_tech]

Hi.

Like the title says, since equipment with switch mode supplies has become common, I have noticed that many of them give me minor electric shocks. I'm wondering if anybody knows why?

I don't really know what sort of info you need, but will try to list all the stuff I think is relevant, so this might be a bit long. I think it is unlikely to be a mains wiring problem, as I have moved house six or seven times since 2000, and it still keeps happening.

I think it must be something to do with the supplies themselves, I guess they are putting a voltage on the parts that are supposed to be grounded, and it must be quite significant, as I can feel it when I would expect to be well insulated from any alternate ground path.

For example, what is making me type this is that my e-cig keeps zapping my lips. It charges while in use from a USB connection to my laptop. The laptop is powered by a Delta branded PSU, which I believe is a competent brand, and as far as I know it is genuine, not a nasty fake. This is just an example though, have had this problem many times before.

The part I really don't understand, even if there is somehow a lot of volts on the supposed earth parts, there is no return path. I'm sat on my bed. At least an inch of fabric between my body and anything conductive. Even if I am conducting to the bed, it is sat on a wooden floor.

I asked this on some other forum ten years ago, and all I got was recommendations to get the mains tested. I hope someone here has something more well informed.

Thanks.

[wraper]

This is because there are few nF Y1 or Y2 safety rated capacitors between mains wires and ground wire. In case there is no earth connection in the socket, earth/case usually will be floating with half of the mains voltage (current is low). Or in case of double insulated SMPS (no earth connection in the mains plug) there can be safety capacitor between the mains side and negative wire of the secondary side.

[amyk]

There are several tens of uA leakage due to EMI suppression capacitors.

http://www.aplomb.nl/SMPS_leakage/Doc_ie.html

Perhaps you may be unusually sensitive to voltages, but if it is actually painful and not just somewhat noticeable you should measure the leakage current with a multimeter.

Also, e-cigs are usually not vaped while plugged in and charging.

[DanielS]

The part I really don't understand, even if there is somehow a lot of volts on the supposed earth parts, there is no return path. I'm sat on my bed. At least an inch of fabric between my body and anything conductive. Even if I am conducting to the bed, it is sat on a wooden floor.

Even if you were levitated in the middle of a vacuum chamber, any conductive mass, such as your body, is still capacitively coupled with any other conductive mass nearby, such as the Earth.

The current that gets to you through the EMI suppression capacitors does have a return path to ground: free air capacitive coupling between your body and all other grounded surfaces around you including Earth.

[StefanHamminga]

My Surface Pro 3 has a similar issue. Whenever it's connected to the charger and I slide my finger over the aluminium back of the chassis it feels like the surface vibrates. When I hold my finger still in one spot the sensation almost instantly disappears. Unplug the charger and nothing like this happens.

I took my scope and had a look. With the probe tip directly to the chassis basically nothing happened (grounded or not). Just putting my finger on the probe gave a slight voltage at AC frequency (picture 2). The third image was when I then touched the chassis with my fingertip, with the aluminium around 5mm from the probe tip (see picture 1). The voltage deltas were between 20 and 40 volts.

Not enough current to feel like a shock in any sense of the word, but maybe just enough to upset the local nerves a little when they are excited?

[zero_budget_tech]

Great answers, thanks.

This is because there are few nF Y1 or Y2 safety rated capacitors between mains wires and ground wire. In case there is no earth connection in the socket, earth/case usually will be floating with half of the mains voltage (current is low).

Makes sense. Kinda strange to put a 115v probe in my mouth...

Perhaps you may be unusually sensitive to voltages,

I don't think I am unusually sensitive, but I think I recognise things as electricity, that others would think was sand or bugs or something.

but if it is actually painful and not just somewhat noticeable you should measure the leakage current with a multimeter.

It's not painful. It feels like if I haven’t shaved my face for a while, and something pushes the stubble into my skin. Annoying, but not a problem.

Also, e-cigs are usually not vaped while plugged in and charging.

This one can be. That's why I bought it.

The current that gets to you through the EMI suppression capacitors does have a return path to ground: free air capacitive coupling between your body and all other grounded surfaces around you including Earth.

That makes sense, but I'm surprised I can feel the current through such a badly designed cap. Perhaps I could sell myself as an audiophool component?

The voltage deltas were between 20 and 40 volts.

Not enough current to feel like a shock in any sense of the word, but maybe just enough to upset the local nerves a little when they are excited?

I imagine it is more than enough. Did you never test a 9v battery by licking it? Slightly damp fingers will have higher resistance, but, your pad provides more voltage.

[DanielS]

That makes sense, but I'm surprised I can feel the current through such a badly designed cap.

The Y1/Y2 capacitors in your power adapter are most likely perfectly well designed in order to meet UL, CSA, VDE, etc. electrical safety requirements - safety is the main purpose of X/Y-rated capacitors. What you are feeling is the tens to hundreds of microamps of EMI jumping from the primary side to the secondary side through those ~1nF capacitors. If your adapter has a switching frequency of 100kHz and has a 150V ringing component there, you get up to 1mA going through a 1nF Y-capacitor. The capacitor is working perfectly fine but the adapter's DC-DC converter might be putting out excessive noise.

[zero_budget_tech]

That makes sense, but I'm surprised I can feel the current through such a badly designed cap.

The Y1/Y2 capacitors in your power adapter are most likely perfectly well designed in order to meet UL, CSA, VDE, etc. electrical safety requirements - safety is the main purpose of X/Y-rated capacitors. What you are feeling is the tens to hundreds of microamps of EMI jumping from the primary side to the secondary side through those ~1nF capacitors. If your adapter has a switching frequency of 100kHz and has a 150V ringing component there, you get up to 1mA going through a 1nF Y-capacitor. The capacitor is working perfectly fine but the adapter's DC-DC converter might be putting out excessive noise.

You misunderstand. I meant on the ground side, the cap that has the planet as one terminal, and my arse as the other. (I think terminal is wrong, but I don't know the proper word. Plate? Foil? Someone please inform.)

[Zero999]

It's a pretty standard configuration. The capacitor shorts the high frequency coupled to the secondary back to the primary. Unfortunately a small current from the mains flow but it's too low to cause any injuries. This is only an issue if many of these power supplies are connected together in parallel so the currents add together.

[mikerj]

The old iPods (gen 2/3?) with the metal backs are good for this, if you move your finger over the surface it feels like the surface is vibrating.