Power supply voltage?

Question

Arduino

Hello.
I want to assemble a cnc machine.
All components are there.
The board needs 12-36 volts. On the laptop power supply, the line is 19.5v 4.5a DC. But when I measure with a tester, there are 42.2 volts. (black-white-blue wire) shows 42 volts on black + white. WHAT?
+ How to find out which of the wires + which minus?
Black hung up on COM the second tested. Thank you.

Answer 1

Look closely at the tester and position (measured alternating current).

Answer 2

1. Why a laptop? 19 volts is a non-standard, 12 or 24 are much more often used in such devices, so it would be more correct to use a PSU from a desktop computer, which is also more powerful.
2. As a rule, minus on the outer braid, plus on the center wire. But very often laptop PSUs have not 2, but 3 contacts on the output plug, and the "native" PSU is identified through the third wire. For outside use, this is not important, but you must not confuse when connecting.