Understanding proximity testers

[Seamus16]

I have an Extech 330 MM and amongst other things have been checking my various phone etc chargers and have noticed that the MM lights up on the low voltage side on some of the chargers. I have checked the continuity back to the mains pins and there is nothing shown.

I am not clear why this is happening so could someone enlighten me please.

[Connoiseur]

It is due to Y capacitor which is connected across the primary and secondary of the high frequency transformer to provide a low impedance path dor the rf noise to ground via the mains.

It has a very limited capacity, hence does not allow much mains frequency current to flow (you may feel pricking sensation on sensitive skin like back of hand or face but no shock). However NCV can be triggered because it detects electric fields, not current.

[Seamus16]

Thanks - that is a great help and gives me a better understanding. There is so much 'less than good' chargers around that I get suspicious.

[Brumby]

Your caution will serve you well.

There are so many chargers out there - and so many are sub-standard.  A lot of the time they will behave - but if there are going to be any failures, it will likely be one of these ... and some of the failure modes are quite scary.