Compare the voltage between the ground on the PSU and the ground on your laptop, without the Melzi connected to the laptop.
If there's a significant difference, that could be your issue.
I would suggest using a high-voltage AC scale first, then working downwards, then try DC. I would also recommend NOT touching the ground of both devices with anything but the probes till you're sure there isn't something weird.
eg: If somehow the PSU has live AC on what should be the ground and the laptop is grounded (or vice versa), current will flow through YOU if you touch both. Even if you wired it correctly, do not underestimate that the laptop may be incorrect or that a moulded plug, a power board, or even a household wall socket could be wired incorrectly, leading to situations where Ground/AC are reversed.
If there's a significant difference, that could be your issue.
I would suggest using a high-voltage AC scale first, then working downwards, then try DC. I would also recommend NOT touching the ground of both devices with anything but the probes till you're sure there isn't something weird.
eg: If somehow the PSU has live AC on what should be the ground and the laptop is grounded (or vice versa), current will flow through YOU if you touch both. Even if you wired it correctly, do not underestimate that the laptop may be incorrect or that a moulded plug, a power board, or even a household wall socket could be wired incorrectly, leading to situations where Ground/AC are reversed.
