9V Plug top power supply measuring 13.5 Volts!!!

Try loading the output. It should drop the the 9 volts when loaded down.

- Tim -

For devices that have their own internal regulation and that don't need a tightly regulated supply (e.g., one that can run from a battery), this kind of supply is simple and cheap. Think of it as a 13.5 VDC "pure" supply in series with a source impedance of about 9 ohms. When it is supplying a 500 mA load, that internal resistance will drop about 4.5 V, so at the terminals you'll see the expected 9 VDC.

Pretty well every power supply will read high if measured with no load. Even a regulated type has a small internal parasitic load to allow the regulator to work correctly.

What you've observed is perfectly normal for a basic non-regulated supply. With a load drawing the quoted 500mA it should drop to 9 volts. Or thereabouts.

That's normal. Note the rating is for 500 milliamps. Put a 500 mA load on the output and you will see 9 volts.