Mystery of the 50Hz artifact generated with a turned off smart plug and a laptop charger

Question

Here some short background: I am a PhD student doing research in Neuroscience and I am using a 50Hz artifact generated by a laptop charger in my research for the synchronization of different electrical data recording modalities. So I want to start by saying that the aim of this question is not to avoid a 50Hz artifact but to better understand it and to reliably be able to control it.

So in my setup I am using a laptop charger which to my understanding produces a 50Hz artifact by transforming the alternating current from an outlet into direct current suitable for a laptop. Plugging the laptop charger in and out produces a reliable 50Hz artifact I can record and directly see on a screen that looks similar to this:

Furthermore, in my setup I have a smart plug (tapo P110 Voltage AC, 100-240V, 50/60Hz,max load 3680W,16 A) with which I can automatically switch the power on and off which also works e.g. when connecting a lamp. Moreover I have a normal power strip with a manual switch (250V,50Hz,16.0A, max. 3680 W).

I tested different combinations of those three parts (charger, plug and power strip) and used a multimeter to measure the voltage and observed the following:

For conditions where the smart plug was switched off and connected to the wall outlet and the laptop charger I found unnormal behavior in the 50Hz artifact generated by the laptop charger: When the smart plug was switched off the laptop charger connected to it was still producing a 50Hz artifact which seemed even higher than when the smart plug was switched on. This is something I was really not expecting. It then looks something like this:

Interestingly however, if I did not connect the smart plug directly to the wall outlet but plugged it into the power strip which was connected to the wall outlet, the 50Hz artifact was as expected (very low when the smart plug was off and high when the smart plug is on, see picture 1).

I tried different conditions of the whole setup and also used a multimeter to measure the voltage and the results were as following:

Artifact (50Hz Art.) behavior:

1. wall outlet-- smart plug -- laptop charger
   --> smart plug on: 50Hz Art, off: 50Hz Art
2. wall outlet --smart plug --power strip (on)-- laptop charger

--> smart plug on: 50Hz Art, off: 50Hz Art.

3. wall outlet -- smart plug (on)-- power strip-- laptop charger

--> power strip on: 50Hz Art., off: no 50Hz Art

4. wall outlet -- power strip --smart plug -- (power strip) -- laptop charger

   --> smart plug on: 50Hz Art., off: no 50Hz Art.

5. wall outlet --power strip -- laptop charger

   --> power strip on: 50Hz Art., off: no 50Hz Art.

6. wall outlet --laptop charger

--> laptop charger plugged in: 50Hz Art., plugged out: no 50Hz Art.

Measured Voltages by multimeter:

1. wall outlet-- smart plug (off:0-0.001V)
2. wall outlet--smart plug (off) -- power strip(~950V)
3. wall outlet --smart plug (on) --power strip (~116V)
4. wall outlet --smart plug (off)-- power strip (0V) --laptop charger

I hope the explanation is understandable, but I am of course also happy to provide further details. All of this is really a mystery to me and I would be really grateful if someone has an explanation for this strange behavior.

Addition: Here are photos of all the elements in my setup

wall outlet:

power strip:

smart plug:

laptop charger:

Answer 1

You have grounded sockets/plugs so that's good.

You have Schuko plugs/sockets so basically you can swap live and neutral wires by inserting the plug in different orientation. So basically, you can never guarantee which wire is live and which is neutral after plugging a device in, so both mains wires should be treated with equal care as both may be the live conductor.

The power strip (likely) has a dual pole switch which means it will disconnect both live and neutral no matter which way you plugged the power strip into wall socket.

Then comes the problem. The smart plug teardown videos on Youtube indicate that there are several models but none of them had dual pole relay for mains wires, they all had a single pole relay so it will disconnect only one mains wire, which can be the live or the neutral as you can never know if the plug alwayd passes the neutral or the live and which it disconnects from downstream.

Now, the laptop adapter has likely standard inlet filter caps from live to earth and from neutral to earth, and additionally from live to neutral.

It means that basically live and neutral form a capacitive divider and the capacitive divider output is then basically live at half mains, but it is earthed. As neutral is also at earth potetial, the leakage current on earth wire should be quite small.

Another thing is to where the internal EMI cap connects the USB isolated output to mains side - generally to earth if there is one, but in two-pronged unearthed designs it must go to live or neutral wire.

If it happens that the smart plug disconnects the neutral from your laptop adapter, then you still have a live wire wigglig at 50 Hz sine wave with 325V amplitude going into the laptop supply, and no neutral, and then there are cap from live to earth, and from live to earth through two caps in series - basically more earth leakage current.

Essentially lack of neutral means everything has 0V potential difference but also compared to earth everything floats at 230VAC.

Also if the EMI Y cap between mains input and USB output side is not to earth but to something that is referenced to live/neutral, it will leak the AC through to the USB.

Also if you plug an ungrounded metal-cased device like a DVD player after the smart plug, I recommend not to touch it and anything else at the same time, or do plugging/unplugging of cables to it. Leaving live going into a device and disconnecting neutral is a hazard you need to be very careful.

Sort of same thing when you change a light bulb, you never know if the single pole switch cuts off neutral or live, and even with the switch off, it's not safe to poke fingers to the lamp contacts.

Answer 2

I think that this can be explained by realizing that internally, the laptop charger has a "Y" capacitor from the DC output to the AC input. i.e. to reduce high frequency switching noise on the DC output, the charger's incoming live terminal is used as a high-frequency ground.

This then means that you will see a moderate amount of 50 Hz superimposed on the DC output wires as a common-mode signal. However, if you reversed live and neutral at the AC plug, you might find that the 50 Hz present at the DC port is much reduced.

You can try this if you have a reversible AC plug/socket arrangement.