I’m measuring the voltage on my adapter, and it says 18 volts. Is this normal?

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This is correct. Normal DC adapters are unregulated and there is an inverse relationship between voltage and current.

Here is how to interpret the rating on an adapter. Let’s use the Handy Board's 12 volt, 500 mA (milliamp) DC adapter standard as an example.

This rating means that when a load is drawing 500 mA of current, the adapter voltage will be 12 volts.

If the adapter is plugged into the wall but its output is not connected to anything—in other words, there is no load—then the current is zero and the voltage measured will higher than the adapter's specification. For the Handy Board’s “12 volt” adapter, a reading of 18 volts is normal if there is no load.

If there is load that draws more than 500 mA, then the output voltage would be less than 12 volts. Note that it is possible to draw more than 500 mA even though an adapter might only be rated for 500 mA. The effect is that the output voltage will be less than the adapter's specified voltage, and also this will overtax the adapter and potentially cause it to fail.