Hi 
My actual project is a graphical equalizer. The Arduino is powered with a 5V 4A Power supply.
When I connect the Board with an audio cable to my Smartphone (Ground connected directly to GND) everything is fine.
But when I use my PC or CD-Player I measure 125V AC at the cable. The Arduino works fine too, but when you touch the cable you feel an unpleasant prickling.
Semes not to be healthy for the Arduino.
Is this normal or not? Is there a way to avoid this?
Hi and welcome.
This is normal.
The power supply of your PC does this.
As soon as you put some load (your body) to this, the voltage will drop, but still this is not very good.
This can be a problem, because you now only need a single fault to get into real trouble, where with other types of power supplies you'd need at least 2 faults to get into the same trouble.
You could isolate your PC's power supply by coupling it with the grid through a 1:1 safety transformer.
They do exist, but i don't know if that's actually what they're called.
You could also try to find an USB isolator.
Or use an USB to serial adapter that has an internal isolation.
Those can be had for less than 30 USD, but you'd have to wait a bit for it to arrive from China.
It's a bit of overkill (because it offers 4 different output levels or protocols) and bulky, but i think it's the best and cheapest way to do this.
The receptacle your PC is plugged into may be wired incorrectly (putting voltage on the neutral or even ground)!
Could be a very dangerous situation. Check it out ASAP!
Thanks for the quick answers
The receptacle your PC is plugged into may be wired incorrectly (putting voltage on the neutral or even ground)!
could this be a problem? Thaught the voltage is relative. My PCs audio positive and ground output as 125V in relation to neutral
If he means the wall socket, then it is very unlikely, but also very dangerous if that were to be true.
When switch mode power supplies are nominally isolated from the mains they
actually have a mains connected winding as the primary of a transformer, the secondary
of which is connected to the supply output. Due to the high frequencies of switch mode
supplies (100kHz or in that region), there is significant capacitive coupling between
primary and secondaries of the transformer due to their proximity, leading to a current of
up to a few mA flowing.
A quality supply with an earth connection and inter-winding screen will behave much
better.