AC / DC adaptor, unexpected voltmeter reading

Question

I'm planning on using an AC/DC power adaptor to power an Arduino and various devices.

Before actually powering up the Arduino with the adaptor I wanted to play with the AC/DC adaptor, as it has variable voltage setting (1.5, 3.0, 4.5, 6, 7.5,9 and 12 V), by using a voltmeter to check the voltage at different setting. To my surprise, the voltage readings were not 1.5, 3.0 etc..., but instead larger (5, 15, etc ...).

Is this normal behavior for an adaptor, do electrical devices labeled at 1.5 Volts initially require 5 Volts?

Answer 1

That's normal behavior for an unregulated power supply. An unloaded transformer will give a higher voltage than nominal. Putting some load on it will drop the voltage quickly.

For the Arduino I would suggest to use the 9 V setting, so that the Arduino's voltage regulator doesn't get too hot when you would power several things (like a lot of LEDs) from the 5 V supply.