john1 said:
Please bear in mind that a lot of these 'in-line' supplies are not
isolated from the mains.
Some of them actually have their output connections connected to
the electric mains.
I hope not!
We are talking about a supply for a laptop computer here.
The low voltage connector probably has exposed pins.
The laptop will have external connectors, video, RS232, USB etc etc.
If the low voltage supply for the laptop is connected to the mains, the manufacturer is leaving himself open to all kinds of legal problems.
If you have firm knowledge of a PSU which does this, please share it with us, we want to know.
However, having said that, I have a couple of Gateway laptops and if you measure from the DC output connector to earth with a high impedance voltmeter, it reads 120volts. This is due to capacitive coupling between the primary and secondary of the transformer in the PSU. There is no earth connection to the PSU and so there is no earthed faraday screen between primary and secondary.
The 120volts has a high impedance source and is quite safe, at least to a human. Electronic components, CMOS especially, may not agree with that!
If I do the same measurement on my Dell laptop PSUs, I get 0volts. The thing is built properly and is earthed.
JimB